PSY 350 - 5 psych discussions due in 30 hours

Learning Objectives

After completing this chapter, you should be able to:

• List at least five differences between rods and cones. • Draw a wave and explain its characteristics. • Draw an eyeball and trace the path of light through the eyeball, labeling structures through which light

passes on its way to the retina. • Name at least one disorder associated with each part of the eye. • Differentiate between the Trichromatic Color Theory and the Opponent-Process Theory. • Explain how information about light is transmitted from the retina to the occipital lobe. • Describe how color, form, and movement are processed by the visual system. • Identify visual problems associated with damage to the occipital, temporal, and parietal lobes. • Draw the three main divisions of the ear, including the outer, middle, and inner ear, labeling the parts of each. • Trace the path that information about sound travels from the inner ear to the temporal lobe. • Identify five kinds of deafness. • Explain where the brain produces speech and contrast that to how the brain receives and interprets speech. • Differentiate between music perception in musicians versus nonmusicians. • List disorders associated with speech and music perception.

6

Vision and Audition

Jupiterimages/Creatas/Thinkstock

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CHAPTER 6Section 6.1 Sensory Stimuli and Receptors

At the age of 19 months, Helen Keller became deaf and blind following an illness that damaged her ner- vous system. With the help of a skilled, devoted teacher named Anne Sullivan, Keller learned to communicate with other people by using the manual alphabet of the deaf, a system of hand signals used to spell out words. She also learned to read Braille, using fingertips to feel the raised bumps of the Braille symbols. At the age of 10, she learned how to speak after only 1 month of les- sons. Keller was such an intelligent, diligent student that she was accepted into Radcliffe College and gradu- ated from Radcliffe with honors in 1904.

Although she was unable to process visual and audi- tory information, Keller learned at a very young age to identify objects in her environment by their smell, taste, or feel. This tells us that different sensations reach the brain by different pathways. In Keller’s case the path- ways for vision and audition were not functional, and information about light and sound could not be trans- mitted to her brain. All of her other pathways were intact, however, which enabled her to experience all the other sensations that we feel every day.

What are these sensations? They include taste, smell, touch, pain, warm and cold, dizziness, a full stomach, a sore muscle, and movement of our limbs. We have specialized sensory cells, called receptors, that respond to specific stimuli and send information about those stimuli to the central nervous system. In this chapter we will examine the receptors associated with the visual and audi- tory systems and discuss how these cells encode information about stimuli. We will also explore the areas of the brain that process visual and auditory information.

6.1 Sensory Stimuli and Receptors

Receptors are activated by stimuli. These stimuli can come from outside the body (external stimuli) or from within the body (internal stimuli). Examples of external stimuli that stimulate our receptors are lights, sounds, tastes, odors, pressure, and changes in temperature. Internal stimuli occur within the body and include stretching of muscles or tendons, blood pressure, and chemicals carried in the blood, such as glucose or hormones.

Stimuli generally take one of two forms: chemical or mechanical. Chemical stimuli are molecules that bind with receptor sites on receptors. For example, molecules that are dissolved in water (such as salt or NaCl) excite receptors on the tongue. Chemicals located inside the body, like blood glucose, stimulate receptors located deep within the body. Receptors that are stimulated by chemical stimuli are called chemoreceptors.

Mechanical stimuli exert physical force, such as pushing, pulling, or vibrating. Like chemical stim- uli, mechanical stimuli can be external or internal. A tap on the skin is an example of an external mechanical stimulus, whereas stretching of a muscle is an internal mechanical stimulus. Mecha- noreceptors are receptors that respond to mechanical stimuli.

Mary Evans/Science Source

Photo 6.1 This image was taken on the day Helen Keller graduated from Radcliffe College in 1904.

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CHAPTER 6Section 6.1 Sensory Stimuli and Receptors

Characteristics of Waves

Many external stimuli are transmitted through the environment as waves. For example, vibra- tions and sound travel in waves. Electromagnetic radiation, which comes from the sun and other sources of radiant energy, also travels in waves. Figure 6.1 illustrates the many forms of electro- magnetic radiation, including visible light.

Figure 6.1: Various forms of electromagnetic radiation

Where would visible light fit on the electromagnetic spectrum?

Waves have a number of characteristics that you should know about in order to understand the material in this chapter. Wavelength refers to the distance between corresponding parts of two consecutive waves (Figure 6.2). Like other forms of electromagnetic radiation, light is charac- terized by its wavelength. As you can see in Figure 6.1, each color of the rainbow (red, orange, yellow, green, blue, indigo, and violet) is associated with a particular wavelength. Red light, which has a wavelength of approximately 700 nanometers (or 10–9 meters), is considered long- wavelength light. In contrast, blue light, with a wavelength of about 500 nanometers, is called short-wavelength light. Visible light makes up a very small proportion of the electromagnetic spectrum, which ranges from a wavelength of 10–14 meters for cosmic rays to a wavelength of 10–6 meters for radio waves (Figure 6.1).

We can also characterize waves by their frequency, which is the number of waves that occur in a certain period of time. One complete wave is called a cycle (Figure 6.2). Thus, the frequency of a wave is expressed as cycles per second (cycles/second). The International System of Units uses the term Hertz (named for a German scientist, Heinrich Hertz, who studied waves in the late 19th century) as the unit for frequency. Sound waves are usually described by their frequency. For example, women’s voices typically have a frequency between 200 and 1,000 Hz, and men’s voices typically have a frequency between 60 and 500 Hz. (Hz is the abbreviation for Hertz.) This

10–14 10–12 10–10 10–8 10–6 10–4 10–2 1 102 104 106 108

Wavelength in meters (m)

Gamma rays

X rays Ultra- violet rays

Infrared rays

Radar FM TV Short- wave

AM AC electricity

Wavelength in nanometers (nm)

400 500 600 700

Violet Blue Green Yellow Red

Short wavelength Long wavelength

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CHAPTER 6Section 6.1 Sensory Stimuli and Receptors

demonstrates that higher frequency sounds are perceived as having a higher pitch than lower frequency sounds. Frequency is a physical characteristic of waves, and pitch is the corresponding perceptual experience of frequency.

Figure 6.2: Characteristics of a wave

This figure illustrates one complete sound wave cycle. A sound wave’s frequency is defined as cycles per second.

Light can also be described in terms of frequency. Blue light, for example, has a frequency of approximately 660 trillion Hz, or 660 THz 1012 Hz. More commonly, however, we think of blue light in terms of wavelength. The opposite is true for sound. We could easily use wavelength to characterize sound, but we don’t. According to convention, light is described in wavelength units (meters) and sound in Hertz.

Another characteristic of waves is amplitude (Figure 6.2). The amplitude is the height of the wave from the lowest point to the highest point. The amplitude of a sound wave corresponds to the psychological phenomenon known as loudness. A large amplitude is associated with a loud sound, and a small amplitude is associated with a softer, less loud sound. We use the unit decibel (dB) (named after Alexander Graham Bell, who studied the transmission of sound and invented the telephone) to measure loudness. The decibel scale ranges from 0 (least perceptibility) to about 160 (extremely loud, causing extreme pain).

Receptors

All receptors function as transducers. That is, they transduce or convert one form of energy, such as chemical energy or mechanical energy, to an electrochemical form of energy, such as an action potential. As you know, neurons communicate by means of action potentials, which initiate the release of neurotransmitters into synapses. The function of receptors, therefore, is to provide information about specific stimuli to neurons in the central nervous system by translating informa- tion about the stimuli into patterns of action potentials. Receptors are sensitive to the quality of stimuli, and they send quantitative information to the brain about the quality of stimuli.

A b o ve

n o rm

a l

B e lo

w n o rm

a l

Diagram of sound wave

P re

s s u

re

Time

(Phase angle)

Amplitude

Wavelength (λ)

0° 0°

180° 270°90° 90° 180° 270°360°

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CHAPTER 6Section 6.1 Sensory Stimuli and Receptors

How do receptors translate information into patterns of action potentials? You should recall from Chapter 2 that, according to the all-or-none law, action potentials do not vary in amplitude. This means that, after an action potential is activated, it always goes to completion, reaching the same positively charged peak each time. Receptors use action potentials to transmit information about stimuli to the brain. When the stimulus is intense, the action potential does not increase in size. Rather, the frequency of the action potential increases with an increase in stimulus intensity. That is, with a more intense stimulus, more action potentials per second are sent to the brain; and with a less intense stimulus, fewer action potentials per second are transmitted to the brain (Figure 6.3). Thus, intensity of the stimulus is encoded as frequency of action potentials.

When a receptor is continuously stimulated by the same stimulus, the receptor will decrease its firing in response to that stimulus (Figure 6.3). This decrease in responsivity by the receptor to constant or repeated stimulation is called adaptation. All types of receptors show adaptation, even pain receptors. For example, imagine that you walk into your apartment after your spouse has spent the entire afternoon baking brownies. At first, the smell of chocolate is very intense and pleasant. But after a few seconds, you can no longer smell the brownies because of adaptation of your olfactory receptors. We will examine other examples of adaptation when we discuss other sensory systems.

Figure 6.3: Coding information about stimuli

The strength of a stimulus is encoded by the frequency of the action potential. A strong stimulus produces more action potentials per second than a weaker stimulus does. Notice that the size of the action potential does not change. During adaptation, a neuron decreases its rate of firing in response to repeated or prolonged stimulation.

We have various receptors that respond to specific types of stimuli: taste receptors for taste stim- uli, mechanoreceptors in the skin for touch, and so forth. The receptors communicate with neu- rons that, in turn, communicate with other neurons that process information about the stimuli and relay it to the cerebral cortex, where conscious processing of the stimuli takes place. This network of neurons that form a pathway from the receptor to the cerebrum is known as a sen- sory system. In this chapter we will examine two sensory systems, vision and audition. In the next chapter you will learn about the systems that process information about skin and muscle sensa- tions, odors, taste, and head position.

Weak stimulus

Action potentials

Action potentials

Stimulus

On Off

On Off

Stimulus

Time

Strong stimulus

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CHAPTER 6Section 6.2 The Visual System

6.2 The Visual System

Compared to the other sensory systems, the visual system has received the most scientific investigation. This means that our knowledge about the visual system is quite sophisticated. However, we are still a long way from understanding how visual images are processed and identi- fied in the brain. In this section we will examine how a visual image is encoded by visual receptors and then processed by the brain.

Anatomy and Function of the Eye

The visual receptors are located in the retina, a flat, multilayered tissue situated in the back of the eye (Figure 6.4). The human eyeball is attached to the skull and held in the eye socket by means of six tiny muscles, which permit movement of the eye. The tough, white outer layer of the eye is called the sclera. Toward the front of the eye, the sclera disappears and is replaced by a transpar- ent covering, called the cornea (Figure 6.4). Contact lens wearers should be quite familiar with the cornea because that is the tissue on which the contact lenses are placed.

Figure 6.4: Anatomy of the eye

Many parts work together to allow the eye to function properly.

Human eye

Sclera

Ciliary muscle

Iris

Pupil Cornea

Aqueous humor

Vitreous humor

Muscle controlling eye movement

Pigment epithelium

Fovea

Retina

Lens

Visual axis

Optical axis

Optic disk (blind spot)

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CHAPTER 6Section 6.2 The Visual System

To reach the visual receptors in the retina, light must pass through the cornea. Behind the cornea is a watery pocket called the aqueous humor, through which light must pass. Next, light must enter the eyeball through the pupil, which is the hole in the center of the iris, the circular, colored structure of the eye. The iris contains smooth muscle, which regulates the diameter of the pupil.

After light passes through the pupil and enters the eye, it encounters the lens, a transparent structure that focuses the visual image on the retina. The shape of the lens is controlled by spe- cialized smooth muscles, which alter the shape of the lens to compensate for the distance of the distal stimulus to the eye. When a stimulus is far away from the eye, the lens becomes flattened in shape in order to produce a sharp image of the object on the retina. On the other hand, the lens becomes rounder and fatter when a stimulus is located close to the eye. This process in which the lens changes shape to focus the image on the retina is called accommodation.

As we age, the lens loses its elasticity, and accommodation becomes impaired. This condition is called presbyopia. You may have noticed that your parents, or you yourself, started wearing glasses while reading or switched to bifocal lenses in their forties—a sure sign of presbyopia.

The Retina Light must pass through the lens to reach the visual receptors in the retina. The visual receptors receive nourishment from blood vessels that lie in front of the retina. The root of the word retina comes from ret-, which means “net” or “network” in Latin, referring to the fact that a network of blood vessels covers the front of the retina. Photo 6.2 is a photograph of the inside of the eyeball as seen through an instru- ment called an ophthalmoscope that is used to view the interior of the eye. You can see the multitude of blood vessels that crisscross in front of the retina. Why don’t you see these blood vessels in your own eyes? Certainly, as light passes through these blood vessels to stimulate visual receptors on the retina, they produce shadows of these blood vessels on the retina. However, due to adaptation, the visual receptors do not respond to the constant stimulation pro- duced by the blood vessels situ- ated in front of the retina. Hence, we are normally unaware of their presence.

Visual Receptors There are two types of visual receptors: rods and cones. These structures derive their names from their appearance. That is, rods are rod shaped, and cones have a tapered appearance. There are a number of other differences between rods and cones. In the human eye there is only one type of rod and three types of cones. However, the rods outnumber the cones: approximately 120 million

Don Wong/Science Source

Photo 6.2 In this opthalmoscope image of a normal retina, you can see the complex network of blood vessels inside the eye that cover the retina.

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CHAPTER 6Section 6.2 The Visual System

rods to 5 million cones. Cones are concentrated in the center of the retina in the area centralis (cen- tral area), particularly in the region of a tiny dimple in the retina known as the fovea. In contrast, the rods are dispersed throughout the retina, with a very low concentration in the area centralis.

The fovea is located along the optic axis, an imaginary straight line that runs from the center of the distal stimulus through the pupil and lens to the retina (Figure 6.4). Light that follows this imaginary line into the eye stimulates cone receptors in the fovea. These cone receptors permit us to see fine detail, whereas rods are insensitive to detail. Rods, on the other hand, provide us with peripheral vision, and cones do not, because there are few cones located in the periphery of the retina. In addition, rods respond better to movement than do cones.

Most importantly, rods and cones differ in their sensitivity to light (Figure 6.5). Rods are extremely sensitive to light and work best in dim light. In contrast, cones are less sensitive to light and work best in bright light. Cones are also sensitive to the wavelength of light and permit us to perceive color, as you will learn later in this section. Rods are not sensitive to wavelength of light and, thus, cannot provide information about color. Table 6.1 summarizes the differences between rods and cones.

Table 6.1: Differences between rods and cones

Attribute Cones Rods

Shape Cone shaped Rod shaped

Number About 5 million About 120 million

Location Around the fovea In the periphery of the retina

Information provided Detail Peripheral vision

Color Movement

Work best in Bright light Dim light

Figure 6.5: Rods and cones in the human eye

Rods provide information related to peripheral vision and movement, and work best in dim light. Cones, however, pick up details and color and work best in bright light conditions.

Lens

LIGHT

Retina

Rods

Cones

Optic nerve

Rods

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CHAPTER 6Section 6.2 The Visual System

When light enters the eye and strikes the visual receptors, photopigment molecules located inside the rods and cones absorb it. The photopigment molecule is composed of a derivative of vitamin A called retinal. (This is why vitamin A is so important for vision.) When the photopigment absorbs light, it changes shape and becomes bleached. After a photopigment is bleached, it becomes inac- tive and cannot absorb any more light until the photopigment reverts to its original shape. The photopigment becomes unbleached in the dark, after the light has been removed.

Color Vision

The ability to see color comes from the fact that we have three types of cones, each sensitive to different wavelengths of light. Working independently, Thomas Young (1773–1829) and Hermann von Helmholtz (1821–1894) were the first to propose that as few as three different receptors are needed in order for people to see all the 200 shades of color that we perceive. They based their theory, now called the Young-Helmholtz or Trichromatic Color Theory, on the results of experi- ments that demonstrated that people can match any color stimulus using three primary colors of light: red, green, and blue.

However, not until the late 1970s, with the development of the microspectrophotometer, a device that measures the wavelengths absorbed by cones, did investigators know for certain that people actually have three types of cones. Indeed, it turned out that Young and Helmholtz were quite correct! Humans have three types of cones: S cones that absorb short wavelength or blue light, M cones that absorb medium wavelength or green light, and L cones that absorb long wavelength or red light. Only primates have three types of cones. Other mammals have only two types of cones, S cones and LM cones, which are an intermediate type of cone between L and M cones, respond- ing to yellow light (Rodieck, 1998).

The ability to see color requires three different types of cones. If an individual has fewer than three functional types of cones, then color vision will be impaired. Trichromats are individuals who have all three functional cone types (S, M, and L) and have normal color vision. Dichromats have only two func- tional cone types. As you’ve already learned, nonprimate mammals are dichromats. People who have a non- functional M or L cone type are also dichromats. To human dichromats the rainbow appears blue at the inside of the arc, turns to a color- less gray in the middle, and changes to yellow at the outside of the arc. Monochromats have only one func-

tional cone type and, thus, have impaired color vision because at least two cone types are needed in order to make wavelength comparisons and perceive color. To them, a rainbow looks like an arc of bright light (Rodieck, 1998).

Polka Dot Images/Thinkstock

Photo 6.3 The ability to see color requires three different types of cones.

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CHAPTER 6Section 6.2 The Visual System

People with impaired color vision are often referred to as color-blind. Color blindness is typically an inherited disorder because the cones we possess are controlled by gene expression. The genes for cones are found on the X chromosome, which normally contain genes for S, M, and L cones (McClements et al., 2013). However, people who are color-blind inherit X chromosomes that carry defective or missing genes for one or more cones. In order for a woman to be color-blind, she must inherit two X chromosomes (one from her mother and one from her father) that carry defective genes for cones. Therefore, women are unlikely to be color-blind unless they inherit defective genes from both parents, because the normal genes on one X chromosome will compensate for the abnormal genes on the other.

If a man inherits an X chromosome with defective genes for cones, he will have impaired color vision because he does not have another X chromosome to offset the X chromosome with the defective gene. You probably know someone who is color-blind. Nearly 10% of all men possess an X chromosome that carries genes for defective or missing cones.

Opponent-Process Theory of Color Vision The Trichromatic Color Theory cannot explain all perceptual experiences involving color. For exam- ple, the theory proposed by Young and Helmholtz cannot explain why staring at the green, black, and yellow flag in Figure 6.6 produces a red, white, and blue afterimage, called a negative afterim- age. In 1878 Ewald Hering proposed another theory of color vision, called the Opponent-Process Theory, which could account for phenomena that the Trichromatic Color Theory could not explain.

Figure 6.6: Negative afterimage

For 30 seconds, focus your eyes on the star in the bottom right corner of the yellow area on the flag. Then move your eyes to the blank rectangle to the right. What do you see? (You should see a negative afterimage as described in the text.)

According to the Opponent-Process Theory, three opponent processes (red-green, blue-yellow, and black-white) code for color in the nervous system. One neuron, called a red-green oppo- nent process cell, increases its firing in response to the presence of red and decreases its firing in response to green. Another neuron, called a blue-yellow opponent process cell, shows a differen- tial response to blue versus yellow. The third type of neuron, the black-white opponent process cell, responds to changes in brightness, increasing its firing in response to white and decreasing in response to black.

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CHAPTER 6Section 6.2 The Visual System

Therefore, both Young-Helmholtz’s Trichromatic Color Theory and Hering’s Opponent-Process Color Theory are correct. The Trichromatic Color Theory explains the function of the cones in producing color vision, whereas the Opponent-Process Theory reflects the activities of neurons further along the visual pathway. Let’s examine the visual pathway in the nervous system.

Transmission of Visual Information from the Retina to the Brain

Remember cranial nerve II from Chapter 4? Cranial nerve II, or the optic nerve, exits the retina at a spot called the optic disk. Another name for the optic disk is the blind spot because no receptors are present at the place where the ganglion axons leave the retina. Thus, light striking the blind spot does not register an image. You are not normally aware of this blind spot because the other eye and the cerebral cortex fill in the missing information. The optic nerve carries information from the retina to the brain. Just in front of the pituitary gland and below the hypothalamus, the optic nerves from the left and right eyes converge to form the optic chiasm (Figure 6.7). In humans, action potentials coming from the half of the retina nearest to the nose cross over to the other side of the brain. Action potentials coming from the half of the retina nearest to the side of the head do not cross over but rather continue to the same side of the brain. Study Figure 6.7 closely.

Figure 6.7: Ventral view of the anatomy of the visual pathway to the brain

Images of objects in the right visual field are projected to the left side of the brain. The opposite is true for visual stimuli in the left visual field.

LEFT

RIGHT

Visual fields

Optic tract

Visual cortex

Nasal hemiretina

Lateral hemiretina

Optic nerve

Optic chiasm Lateral geniculate nucleus

Lateral hemiretina

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CHAPTER 6Section 6.3 Visual Processing in the Brain

The optic tract emerges from the optic chiasm and enters the brain (Figure 6.7). This tract actu- ally splits into two branches: a larger branch that goes to the lateral geniculate nucleus (LGN) of the thalamus and a much smaller branch that goes to the superior colliculus in the midbrain. The superior colliculus processes information about the location of the visual stimulus in the visual field and assists the eyes in moving and tracking visual stimuli. In contrast, information that goes to the lateral geniculate nucleus of the thalamus is sent on to the occipital lobe of the cerebrum for conscious processing.

6.3 Visual Processing in the Brain

Visual perception involves a number of functions: (1) processing color, (2) processing shape or form, (3) locating objects in three-dimensional space, and (4) processing motion. In this sec- tion we will examine how each of these perceptual functions is processed in the brain.

You have just learned the important structures in the pathway from the rods and cones to the visual cortex in the occipital lobe. Information from the receptors in the retina is relayed to the neurons in the lateral geniculate nucleus of the thalamus. From the LGN, axons carry visual infor- mation directly to the primary visual cortex, which is also known as area V1, in the occipital lobe of the cerebrum. In summary, the optic nerve carries visual information from the retina to the LGN of the thalamus, and then visual information goes from the thalamus to the occipital lobe.

Let’s retrace this pathway because it is actually two pathways carrying information about color, form, motion, and spatial relationships. The two pathways begin with the rods and cones. The first pathway, called the ventral stream, ends in a brain region called the inferior temporal lobe and is concerned with processing information about color and form (Figure 6.8). The second pathway, called the dorsal stream, terminates in the posterior parietal lobe and is involved in detecting motion and the location of objects in the visual field (Goodale & Milner, 1992; Mishkin & Unger- leider, 1982). Due to their functions, the ventral stream is often called the “What” system because it tells us what we are looking at, and the dorsal stream is called the “Where” system because it tells us where the visual stimulus is located (Leavitt, Molholm, Gomez-Ramirez, & Foxe, 2011). Table 6.2 summarizes the differences between the “What” and the “Where” systems.

Recall that cones are sensitive to wavelength, which corresponds to the psychological attribute that we call color, and they also permit us to see detail. Rods, on the other hand, respond best to motion. Information from the rods and cones is transmitted to separate layers in the LGN.

Altogether, the LGN is made up of two types of neurons: parvocellular neurons and magnocellular neurons (Livingstone & Hubel, 1988). These terms get their names from Latin: magno- means “large” and parvo- means “small.” Information from cones is relayed to the parvocellular neurons of the LGN, and information from rods is carried to the magnocellular neurons of the LGN. Thus, the parvocellular layers process information about color, form, and detail, whereas the magnocel- lular neurons receive information about spatial locations and motion.

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CHAPTER 6Section 6.3 Visual Processing in the Brain

Figure 6.8: Anatomy of the visual system Figure A illustrates the internal view of the brain while Figure B demonstrates the external view of the brain.

Thalamus

A.

B.

Dorsal pathway (“where” system)

V3

V2

V1

V5

V4

Ventral pathway (“what” system)

Inferior temporal

cortex

Temporal lobe

Posterior parietal

lobe

Frontal lobe

Temporal lobe

Posterior parietal

lobe

Frontal lobe

Optic nerve

Optic chiasm

Lateral geniculate nucleus

Superior colliculus

Visual cortex (occipital lobe)

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CHAPTER 6Section 6.3 Visual Processing in the Brain

Table 6.2: Comparison of the “What” and “Where” pathways

“What” “Where”

Type of information processed Color, form/detail Motion, spatial location

Source of information Cones Rods

Type of associated LGN cell Parvocellular Magnocellular

Final target in occipital lobe V3 (form), V4 (color) V5

Final destination Inferior temporal lobe Posterior parietal lobe

Axons from the magnocellular and parvocellular layers of the LGN project to different regions of area V1 in the occipital lobe. Information from neurons in V1 is then passed on to area V2 of the occipital lobe. V2 functions like a mail server, delivering messages to the correct electronic addresses in the occipital lobe. In general, information in V2 is sent to one of the major visual processing centers, called V3, V4, or V5, in the occipital lobe.

In summary, the “What” and the “Where” systems shoulder the two basic tasks of vision: object recognition and object location. These two systems can (and often do) function independently. The “What” system permits us to recognize objects in the environment regardless of their loca- tion. In contrast, the “Where” system allows us to move around in an environment in which noth- ing is identifiable and enables us to pick up unfamiliar objects.

Binding

Before we turn to a consideration of human visual disorders, I want to stress that the pathways you have just learned about do not act independently and are not isolated from the other path- ways. One unsolved mystery concerning visual perception is called the binding problem: how our brains combine information from the “What” system and the “Where” system to produce a uni- fied perception (Crick, 1984; Damasio, 1989; Friedman-Hill, Robertson, & Treisman, 1995). Visual binding has been shown to involve not only the visual pathways that you have just learned about, but also forebrain areas associated with attention and memory (Baddeley, Allen, & Hitch, 2011).

Research in humans, monkeys, and cats helps us understand how visual binding may occur (Elliott & Muller, 2000; Funk & Epstein, 2004; Seymour, Clifford, Logothetis, & Bartels, 2010; Singer & Gray, 1995; Tallon-Baudry, Bertrand, Delpuech, & Pernier, 1996). Some neurons in the cerebral cortex fire together at the same time, producing synchronous firing patterns called oscillations. Bursts of synchronous neural activity in the occipital lobe, the right temporal cortex, and the left frontal cor- tex that occur at a rate of 40 times per second (40 Hz) are associated with visual binding. The 40-Hz oscillations are observed in the brains of 8-month-old human infants but are absent in the brains of 6-month-old infants (Csibra, Davis, Spratling, & Johnson, 2000). Infants who are older than 7 months are able to perceive visual illusions, whereas younger infants cannot. For example, infants older than 7 months can perceive the illusory Kanizsa square shown in Figure 6.9, and younger infants can- not. Visual binding is required to perceive this illusion. Thus, the presence of 40-Hz oscillations in cortical neural activity appears to be necessary for visual binding.

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CHAPTER 6Section 6.3 Visual Processing in the Brain

Figure 6.9: Kanizsa square

The small black square in the center of this figure is a visual illusion. Binding is necessary to perceive such an illusion.

Disorders of the Visual System

Visual disorders are the result of damage to or disease of the eye or the brain. In this section we will focus on problems that impair vision.

Cornea The cornea must be smooth and transparent in order for light to enter the eye without distortions. An injury to the cornea that causes scarring will interfere with the transmission of light through the cornea, causing blurring. When the surface of the cornea is irregular, light rays are scattered as they pass through the cornea. This problem is known as an astigmatism. A person with an astig- matism can see perfectly well in some orientations but will experience blurring in others.

Aqueous Humor The eyeball maintains its shape due to the pressure of fluid within the eye. To keep the pressure inside the eye from getting too high, the aqueous humor drains off through tiny ducts in the eye. When a blockage prevents drainage of the aqueous humor, the pressure inside the eyeball increases, producing a condition known as glaucoma. In glaucoma the pressure inside the eyeball becomes so high that pressure on the optic nerve causes damage to the nerve (Nicolela et al., 2001). Glaucoma is the leading cause of blindness in the United States today.

Lens For proper transmission and focusing of light on the retina, the lens must be elastic and transpar- ent. As we age, the lens loses its elasticity and its transparency. We discussed presbyopia earlier in this chapter. At age 16 the lens begins to lose its elasticity, and by age 50 accommodation is seriously compromised, meaning that the lens can no longer attain the spherical shape needed to bring near objects into focus. People with presbyopia have trouble seeing close objects clearly and require corrective lenses.

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CHAPTER 6Section 6.3 Visual Processing in the Brain

Years of exposure to bright sunlight and ultraviolet radiation cause the lens to darken and lose its transparency. When this happens, images become blurred and indistinct. In some people the lens becomes opaque and vision is totally obstructed. This condition is known as a cataract. Treatment for a cataract involves surgical removal of the damaged lens. Of course, after the lens is removed, the patient’s ability to focus light on the retina is eliminated. To cor- rect this problem, a plastic artificial lens is sewn in place of the old lens (Hwang & Olson, 2001). One way to decrease the risk of develop- ing cataracts is to wear sunglasses with a good ultraviolet (UV) block that shields the eyes from the sun’s harmful radiation.

Sometimes the shape of a person’s eyes makes it impossible for the lens to bring light into focus on the retina. For example, when the eye- ball is too long along the optic axis, the lens brings the light to focus in front of the retina. This problem is called myopia, or nearsightedness. A person with myopia is unable to see objects in the distance clearly, although vision for near objects is unimpaired. In contrast, in hyperopia, when the eyeball is too short, the lens cannot bring light coming from near objects into focus on the retina. Another name for hyperopia is farsightedness. A person who is farsighted cannot see near objects clearly but can bring far objects into focus. Corrective lenses are used to treat myopia and hyperopia.

Retina Damage to neurons on the retina will impair vision. Most commonly, retinal disease involves the rods and cones. Macular degeneration is a leading cause of blindness in the elderly. In this disor- der the cones in the fovea die, leaving the person unable to read or see detail. Eventually, all of the receptors on the retina are affected, producing blindness (Sun & Nathans, 2001).

Retinitis pigmentosa is a genetic disorder that affects the rods, which make up 95% of human photoreceptors (Davidson et al., 2013). This disorder often strikes in childhood, although some forms of the disease affect only adults. The first symptoms are night blindness and tunnel vision, in which peripheral vision is lost and the visual field is limited to central or cone vision. These symptoms indicate that rods are impaired or nonfunctional. In most forms of the disorder, loss of vision spreads to the cones, and total blindness ensues.

Diabetes, which you learned about in Chapter 2, can also produce blindness. People with diabetes have very fragile capillary blood vessels that rupture easily and heal slowly. This is a problem in the eye because there are so many blood vessels lining the retina. Some diabetic individuals expe- rience rupturing of these blood vessels, which spills blood into the eyeball, obscures vision, and deprives the neurons on the retina of oxygen and other nutrients. Laser surgery is used to stop the bleeding inside the eye.

Fuse/Thinkstock

Photo 6.4 Corrective lenses are used to treat myopia and hyperopia.

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CHAPTER 6Section 6.3 Visual Processing in the Brain

Blindness Blindness typically results from complete destruction of V1 on both sides of the cerebrum, leav- ing the individual unable to see and recognize objects. However, some individuals who have total, bilateral damage to V1 show a phenomenon called blindsight, in which the individual cannot see or describe an object but will reach accurately toward the object (Milner, 1995; Weiskrantz, Cowey, & Hodinott-Hill, 2002). In addition, individuals with blindsight can move through the environment easily, avoiding obstacles, and their eyes can accurately follow a moving target. The explanation for this amazing phenomenon, in which the individual is blind but appears to be able to know the location of objects in space, is that the posterior parietal lobe continues to receive visual informa- tion even after the total destruction of V1. Information from the superior colliculus is transmitted directly to V5, and this information is then relayed to the posterior parietal cortex, where informa- tion about location and movement is integrated even in the absence of input from V1.

Akinetopsia Recall that information about motion is processed in V5. Therefore, damage to V5 causes the affected person to be unable to perceive movement or moving objects, a disorder known as aki- netopsia. An individual with akinetopsia can see an object when it is stationary, but not when it is moving. For some people with akinetopsia, visual images of moving objects cannot be integrated. For example, when one person with akinetopsia poured tea into a cup, the stream of tea looked like a frozen column to her (Corballis, 1994). If you’ve ever been in a darkened room lit by a strobe light, you might be able to imagine what it is like to have damage to area V5. Under strobe lighting, movement appears to disappear, and the visual experience of movement is replaced by a series of discrete images in which the object is located in a different place in successive images.

Damage to the Inferior Temporal Lobe Damage to the inferior temporal lobe produces a number of disorders, collectively called visual agnosia, that involve the inability to recognize familiar objects in the visual field. People with visual object agnosia cannot identify or name objects but can describe the color, motion, and details of those objects. In severe cases, called apperceptive agnosia, affected individuals cannot identify or report knowledge of even the simplest shapes and forms (Milner, 1995).

Another form of visual agnosia is prosopagnosia, in which people with damage to the right inferior temporal lobe cannot recognize faces of people who are familiar to them. People with prosopag- nosia sometimes cannot recognize other natural objects such as animals or vegetables. In con- trast, damage to the left inferior temporal lobe results in a disorder called pure alexia. Individuals with pure alexia can often identify individual letters but cannot put the letters together to read them as whole words (Corballis, 1994; De Renzi, 2000; Jones & Tranel, 2001; Mendez, 2001).

Let’s turn to a discussion of the auditory system next and consider how speech and music are processed in the cerebral cortex.

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CHAPTER 6Section 6.4 The Auditory System

6.4 The Auditory System

Helen Keller could not hear, but she could feel vibrations. Although she was deaf, Keller could tell when people were entering or leaving the house, identify people by their footfall, and distinguish musical instruments being played in an orchestra. Vibrations stimulated receptors in her skin, her bones, and other structures in her body, and she was able to use this information the same way we use auditory information. That is, auditory information comes to us in the form of vibrations or oscillations of air molecules. Those of us who are not deaf have functional auditory receptors, called hair cells, that relay information about vibrations to the brain.

The Nature of Auditory Stimuli

At the beginning of this chapter, you learned about the characteristics of waves. We think of sounds as traveling in waves. Vibrating objects create a pattern of air flow in which a band of compressed air molecules alternates with decompression, in which few air molecules are found. Thus, sound waves are composed of peaks, which represent the band of compressed air molecules, followed by troughs, where there are few air molecules. When we talk about sound waves, we are usu- ally referring to their frequency and amplitude. The human auditory system is sensitive to sound waves that have a frequency between 20 and 20,000 Hz. Any vibrations that have a frequency less than 20 Hz or greater than 20,000 Hz cannot be detected by our auditory systems. Other animals have different ranges of auditory sensitivity. Have you ever blown a dog whistle? You can’t hear a thing when you blow it, except for your breath coming out of your mouth. But dogs can hear it. That’s because the whistle puts out air vibrations with a frequency of about 50,000 Hz, which is way above our upper sensitivity limit. Dogs can hear frequencies over 50,000 Hz, and rats have an even higher range, over 100,000 Hz.

We perceive the amplitude of sound waves as loudness, which as you’ve learned is measured in decibels (abbreviated dB). Our ability to detect a sound at a particular dB level depends on the frequency of the sound. That is, the human auditory system is constructed in such a way that we hear sounds with frequencies between 1,000 and 5,000 Hz best. Typically, humans with normal hearing can detect these sounds at around 0 to 10 dB. However, for low-frequency sounds (less than 200 Hz), the sound must be at least 20 to 60 dB before we can hear it (Betke, 1991; Sivian & White, 1933).

Anatomy and Function of the Auditory System

The peripheral portion of the auditory system can be divided into three parts: the outer ear, the middle ear, and the inner ear.

The Outer Ear The outer ear consists of the pinna and the ear canal. The pinna is the outer, fleshy part of the ear that acts as a sound collector. Some animals have the ability to move their pinna in different direc- tions to catch sound waves better. Humans, however, typically have immobile pinna, although some people can wiggle their pinna slightly. The external auditory meatus is the ear canal or opening into the temporal bone, where the middle and inner ears are located (Figure 6.13). This canal is shaped in such a way that it amplifies sound waves with a frequency of 2,000 to 5,000 Hz.

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CHAPTER 6Section 6.4 The Auditory System

The Middle Ear Vibrating air is collected by the pinna and is guided into the external auditory meatus toward the middle ear, which begins at the tympanic membrane, or eardrum. The eardrum is a thin, flex- ible membrane that is stretched across the ear canal (Figure 6.10). Attached to the eardrum is a chain of three tiny bones, or ossicles. The first ossicle, which is directly connected to the eardrum, is called the hammer or malleus (malleus means “hammer” in Latin). Vibration of the eardrum causes the hammer to vibrate, which in turn produces vibration of the second ossicle, called the anvil or incus (incus means “anvil” in Latin), and then the third ossicle, the stirrup or stapes (stapes means “stirrup” in Latin).

Figure 6.10: Anatomy of the ear

Figure A shows the parts of the outer, middle, and inner ear; Figure B shows the three canals of the cochlea; Figure C shows the cochlear duct.

The middle ear, therefore, comprises the eardrum, the ossicles, and the eustachian tube. The eustachian tube is a canal that runs from the middle ear to the throat. Although it plays no role in the transmission of vibrations from the air to the auditory receptors, the eustachian tube assists in hearing by allowing air to enter or escape from the middle ear in order to equalize the pressure on both sides of the eardrum. Have you ever ridden in a car going through the mountains and felt your ears “pop”? Or perhaps you’ve felt your ears “pop” while seated in a jet that is landing. This popping sensation is the sound of air rushing in or out of your middle ear.

Pinna

Skin

External auditory meatus

Skin

External auditory meatus

Eardrum StapesEardrum

Eardrum

200 Hz

400 Hz 600 Hz

1,000 Hz

800 Hz 4,000 Hz

2,000 Hz

1,500 Hz

3,000 Hz

Vestibular canal

Vestibular canal

Tympanic canal

Tympanic canal

Basilar membrane

Cochlear duct

Cochlear duct Tectorial

membrane

Hair cells

Basilar membrane

Reissner’s membrane

Oval window

Eustachian tube

Cochlea

Auditory nerve

Auditory nerve

Semicircular canals

Incus Malleus

Bone Semicircular canals

Stapes

Stapes

Incus

IncusIncus

Malleus

Malleus

Sound

Bone

C.

A.

B.

Eustachian tube

20,000 Hz

7,000 Hz 5,000 Hz

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CHAPTER 6Section 6.4 The Auditory System

The Inner Ear Vibrating air in the ear canal causes the eardrum to vibrate, which produces vibration of the ossi- cles. Thus, vibrating air is translated into vibrating membrane and then vibrating bone. In the inner ear, vibrations of the stirrup are translated into vibrating membrane again, as the stirrup vibrates against the oval window, a thin membrane that covers the opening to the inner ear. The inner ear itself is located in the cochlea, a snail-shaped canal located in the temporal bone.

The cochlea contains three chambers: the scala vestibuli (or vestibular canal), the scala media (cochlear duct), and the scala tympani (tympanic canal). The oval window opens into the scala vestibuli, and vibration of the oval window causes the fluid in the scala vestibuli to vibrate. The scala vestibuli and scala tympani are connected at the apex of the cochlea (Figure 6.10b). When the fluid in the scala vestibuli vibrates, these vibrations stimulate movement of the fluid in the scala tympani.

The scala media is located between the scala vestibuli and the scala tympani. Between the scala tympani and the scala media is a thin, flexible membrane called the basilar membrane. The recep- tors, or hair cells, are found on top of the basilar membrane. These receptors get their name from the tufts of hairs that protrude from the tops of these cells. The hairs of the receptor cells are embedded in a stiff membrane, called the tectorial membrane, that lies on top of the hair cells (Figure 6.10b).

Vibration of the oval window causes the fluid in the scala vestibuli and scala tympani to vibrate, which makes the basilar membrane vibrate. When the basilar membrane moves, the hair cells on top of it move, causing their hairs to be pulled and bent. Remember that auditory receptors are mechanoreceptors, which means that they are excited by mechanical forces like tugging or pull- ing. The hair cells are located between a very elastic, movable membrane (the basilar membrane) and a stiff, inflexible membrane (the tectorial membrane). Therefore, movement of the basilar membrane puts mechanical force on the hairs of the hair cells, and this mechanical force is trans- lated into an action potential in the hair cell.

Auditory Pathway to the Brain

Axons from sensory neurons in the cochlea relay information from the hair cells to the brain. These axons come together to form the auditory branch of cranial nerve VIII, the auditory nerve. The auditory nerve enters the medulla in the hindbrain and synapses with neurons in the cochlear nucleus (Figure 6.11). From the cochlear nucleus, axons go to the superior olive, a structure located in the hindbrain, which processes information about pitch. Two pathways emerge from the superior olive: One goes to the reticular formation and on to the cerebellum, and the other goes to the inferior colliculus in the midbrain. From the inferior colliculus, axons go to a nucleus in the thalamus called the medial geniculate nucleus, which processes auditory information and sends it on to the primary auditory cortex in the temporal lobe.

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CHAPTER 6Section 6.4 The Auditory System

Figure 6.11: Auditory pathway to the cerebrum

Auditory system disorders may be caused by disruptions to one or more of several structures including the outer, middle, or inner ear, as well as the auditory nerve, or within the brain on the pathway to the auditory cortex.

Disorders of the Auditory System

A diminished ability to hear, called hearing loss, occurs when transmission of auditory information along the auditory system is impaired. Conductive hearing loss is a hearing disorder in which vibra- tions are not effectively transmitted from the eardrum to the hair cells of the cochlea. Deafness is the extreme form of hearing loss in which the individual cannot perceive auditory stimuli. In general, there are five kinds of deafness: (1) outer ear deafness, (2) middle ear deafness, (3) inner ear deafness, (4) nerve deafness, and (5) central deafness.

Outer Ear Deafness Blockage of the auditory canal will cause hearing loss or deafness. For example, a waxy substance, which acts as a natural bug repellent, is produced in the walls of the auditory canal. Excess buildup of this waxy substance can clog the ear canal and interfere with the passage of sound waves down the canal. A tumor or other obstruction, like a foreign object placed in the ear, can also impede hearing. Some individuals, such as those born to mothers with syphilis or German measles, are born with no external auditory meatus. Obviously, the absence of an ear canal makes it impossible for auditory stimuli to excite the nervous system, and these individuals are deaf.

Auditory cortex (temporal lobe)

Medial geniculate nucleus (thalamus)

Inferior colliculus

Auditory nerve from left ear

Cochlear nucleus

Superior olive

Auditory nerve from right ear

External auditory meatus External auditory meatus

Auditory nerve

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CHAPTER 6Section 6.4 The Auditory System

Middle Ear Deafness Damage to any of the structures in the middle ear will interfere with transmission of auditory information and produce middle ear deafness. For example, a ruptured or torn eardrum cannot vibrate effectively. Fortunately, eardrums often heal following rupture. If healing is not possible, the damaged eardrum can be replaced with an artificial eardrum constructed from the patient’s own body tissue. As we age, our joints become stiff due to arthritis and rheumatism, and move- ment is slowed. This is true of the joints between our ossicles, too. That is, in old age, vibration of the hammer produces slower, diminished vibrations of the other ossicles, impairing conduction of auditory information. This conduction problem is usually corrected with a hearing aid. The “For Further Thought” box discusses how a hearing aid works.

For Further Thought: Conductive Deafness and Hearing Aids

Nearly 1 out of every 10 people have a hearing loss that can be corrected by a hearing aid. A hearing aid is a deceptively simple device that collects sound waves, amplifies them, and transmits the amplified waves directly into the ear canal close to the eardrum. Thus, a hearing aid has three components, which correspond to each of its functions: a microphone that picks up sound waves, an amplifier that boosts the sound, and a receiver that delivers the amplified sound into the ear, plus tiny replaceable batteries.

There are many varieties of hearing aids. Behind-the-ear (BTE) hearing devices fit behind the pinna and have a plastic tube that is inserted into the ear canal to deliver the amplified sound directly to the middle ear. Some people who wear eyeglasses have special frames that contain a hearing aid in the earpiece that goes behind the ear. As with stan- dard BTE aids, these hearing aids have a plastic tube that is placed in the ear canal. The in-the-ear (ITE) device is a popular type of hearing aid developed in the 1970s that is placed directly in the ear canal. However, this type of aid is quite difficult to use with a telephone because the wearer gets a feedback noise whenever anything is held too close to the microphone of the hearing aid.

A newer kind of hearing aid is the completely-in-the-canal (CIC) device. The CIC hearing aids are custom- made to fit completely in the ear canal. The microcanal aid is even smaller and fits deeper in the ear canal. A nylon string is attached to the microcanal hearing aid to enable the wearer to remove it from the canal. These two styles are the most popular aids available today because they are nearly invis- ible to the casual observer. Because they are located deep in the ear canal, these hearing aids do not produce any feedback noise when the wearer is using a telephone.

The eustachian tube, too, can be the source of middle ear deafness. Infections that begin in the throat can travel up the eustachian tube to the middle ear, which impedes vibration of the eardrum and ossicles. A child who suffers from chronic middle ear and throat infections can develop scarring and blockage of the eustachian tube. A blocked eustachian tube interferes with air pressure balance in the middle ear, making it difficult for the eardrum to vibrate effectively. To treat a scarred, blocked eusta- chian tube, a tiny tube is inserted into the eardrum. The tube allows air to enter or leave the middle ear to equalize air pressure on both sides of the eardrum and improves hearing ability by allowing the eardrum and ossicles to vibrate freely.

Radius/SuperStock

Photo 6.5 Behind-the-ear hearing devices fit behind the pinna and have a plastic tube that is inserted into the ear canal.

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CHAPTER 6Section 6.5 Auditory Processing

Inner Ear Deafness Damage to the hair cells in the cochlea produces inner ear deafness. This type of deafness is seen in people who are exposed to loud sounds over a long period of time, such as rock musicians, dentists, farmers, and other workers who operate loud machinery. As you learned at the begin- ning of the chapter, extremely loud sounds are transmitted in the form of large-amplitude sound waves. These sound waves are translated into large-amplitude movements of the basilar mem- brane, which causes violent rocking of the hair cells that tears and damages the hairs of the hair cells. A damaged hair cell cannot be repaired or replaced. Another form of inner ear deafness is progressive hearing loss, which involves degeneration of hair cells. Progressive hearing loss is an inherited disorder that is characterized by genetic heterogeneity (Mencía et al., 2009).

In the past, experts believed that inner ear deafness could not be corrected. However, the development of cochlear implants, tiny electrodes surgically placed in the cochlea, where they can stimulate the audi- tory nerve, has helped a number of people with inner ear deafness to hear again (Rauschecker & Shannon, 2002). A tiny microphone is tucked behind the ear of the hearing-impaired person and picks up sound, which is sent to a processor. The processor, which is about the size of a wallet and is worn under clothing, digitalizes the sound. The digitalized signals are trans- mitted through the skin to the cochlear implant, which stimulates the auditory nerve.

Nerve Deafness Damage to the auditory nerve can occur as a result of trauma or infection. In some people an inherited degenerative disease causes the auditory nerve to waste away, causing progressive deafness. No treat- ment is currently available for this type of deafness.

Central Deafness Damage to any of the brain structures in the auditory pathway to the auditory cortex will produce central deafness. Trauma or disease such as a brain tumor can cause central deafness. In the next sections we will examine the effect of damage to brain areas associated with audition.

6.5 Auditory Processing

I am sitting in my office on an unusually quiet afternoon. One of my students is taking a makeup exam in a room down the hall from me to the left. From time to time, I can hear the rustling of papers as she turns a page in the exam. I can also hear footsteps and muffled voices farther down the hall to my right. These auditory stimuli provide a good deal of information to my brain about my environment. The frequency and temporal patterns of the stimuli enable me to identify the nature of the sound, much like the “What” visual system. In addition, I am able to localize

age fotostock/SuperStock

Photo 6.6 Cochlear implants involve surgi- cal placement of tiny electrodes in the cochlea, where they stimulate the auditory nerve.

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CHAPTER 6Section 6.5 Auditory Processing

the sound, based on the loudness and echo qualities of the auditory stimuli, comparable to the “Where” visual system. Thus, it appears that my brain, like yours, contains auditory “What” and “Where” systems.

Anatomical studies of the brains of rhesus monkeys have demonstrated that the auditory areas of the primate cerebral cortex are indeed connected to two pathways or streams (Rauschecker & Scott, 2009; Romanski et al., 1999). One pathway, the auditory “What” stream, begins in the forward section of the primary auditory cortex in the temporal lobe and ends in the prefrontal cor- tex (Figure 6.12). In contrast, the auditory “Where” stream originates in the back portions of the primary auditory cortex and terminates in a different area of the prefrontal cortex. However, it is important to remember that, as in the visual system, a great deal of interaction and sharing of infor- mation takes place between the auditory “What” and “Where” streams (Kaas & Hackett, 1999).

Figure 6.12: The “what” and “where” streams of the auditory system

The proposed auditory “what” stream begins in the front end of the primary auditory cortex in the tem- poral lobe and ends in the prefrontal cortex. The proposed auditory “where” stream begins in the back area of the primary auditory cortex and ends in an upper area of the prefrontal cortex.

Dorsolateral prefrontal

cortex

Frontal lobe

Ventrolateral prefrontal

cortex

Temporal lobe

Primary auditory cortex

Secondary auditory cortex

Proposed “Where” stream

Proposed “Where” stream

Proposed “What” stre

am Proposed “What”

stre am

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CHAPTER 6Section 6.5 Auditory Processing

Identifying a sound and determining the location of its source are the functions of the auditory “What” and “Where” streams, respectively. Analysis of the frequencies of the sound as well as the timing patterns within the sound permits us to identify a sound. Frequency and timing are impor- tant components of speech and music perception.

Processing of Speech

Speech is one of the several ways in which humans communicate. We also use body language, including facial expressions and hand gestures, and written language to communicate. Language is a formalized system of symbolic representations that we use to communicate ideas, questions, and commands. Most languages have both a spoken form and a written form, although some languages of native peoples do not have a writ- ten form. American Sign Language (ASL) is a standard system of hand gestures used by deaf people to communicate and has neither a spoken nor a written form.

The basic units of spoken language are called phonemes. For example, the American dialect of the English language contains about 40 differ- ent phonemes. We combine pho- nemes to produce morphemes, which are meaningful units of sounds. To form the morpheme “cape,” the phonemes k, long a, and p are combined. To create complex strings of morphemes (such as sentences), we follow rules of grammar that stipulate how morphemes are to be com- bined. Phonemes, morphemes, and grammatical rules vary for each spoken language. You should keep in mind, too, that speech involves two processes: production and comprehension. As you will learn in this section, different areas of the brain are associated with the production and compre- hension of speech.

The Effects of Brain Damage on Speech Processing Because speech is a uniquely human capacity, research with animal subjects is not possible when studying the neural basis of speech. Our knowledge about speech processing in the brain is the result of research conducted in the 19th and 20th centuries. The earliest research involved stud- ies of individuals with communication disorders who had suffered damage to specific areas of the brain (Goodglass, 1993). Today, brain-imaging studies are confirming the findings of earlier research of brain-damaged individuals. Let’s review these findings.

Damage to Broca’s Area Recall from Chapter 1 that Broca’s area was one of the first areas of the cerebral cortex to be asso- ciated with a specific function, namely the production of speech (Figure 6.13). Remember Tan, the

Huntstock/Thinkstock

Photo 6.7 American Sign Language is a standard system of hand gestures used by deaf people to communicate and has neither a spoken nor a written form.

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CHAPTER 6Section 6.5 Auditory Processing

man who could utter only one syllable (“tan”) after a stroke damaged the inferior portion of the frontal lobe? Since Broca’s original research on Tan, many studies of brain-injured individuals have confirmed Broca’s finding that speech production is associated with a specific area in the inferior frontal lobe. Today this area is called Broca’s area, in honor of its discoverer (Buckner, Corbetta, Schatz, Raichle, & Petersen, 1996; Whitaker & Kahn, 1994). Damage to Broca’s area produces dis- orders of language production known collectively as forms of expressive aphasia.

Figure 6.13: Classic speech centers

Each part of the brain performs a different function in the body. Broca’s area is associated with speech. What other sections do you think contribute to speech?

Depending on the extent and location of damage to Broca’s area, any of a number of symptoms are observed. In extreme cases people with expressive aphasia are unable to speak at all or can speak only one word or a phrase. I remember working with an elderly woman who could utter only a curse, “son of a bitch,” following a stroke that damaged Broca’s area. I’d ask her, “How are you feeling today?” And she would answer, “Son of a bitch, son of a bitch, son of a bitch.” Another young man could say only “no.” When asked how he was feeling, he answered, “No-o-o-o.”

Some stroke patients lose their ability to speak or write one part of speech. For example, Caramazza and Hillis (1991) studied two patients: one who could not say verbs and one who could not write verbs, although they had no problem with nouns, adjectives, adverbs, and prepositions. Both patients could use the word crack as a noun but could not produce it when it was used as a verb in a sentence. Research with these and similar patients suggests that the brain sorts words by grammatical class. In addition, written words and spoken words appear to be stored in differ- ent locations in the cortex (Caramazza & Hillis, 1991; Cohen, Jobert, Le Bihan, & Dehaene, 2004; Petersen, Fox, Posner, Mintun, & Raichle, 1988; Shapiro, Moo, & Caramazza, 2006).

Broca’s area (on left side)

Frontal lobe

Prefrontal area

Premotor cortex

Primary auditory cortex

Angular gyrus

Temporal lobe

Auditory association cortex (including Wernicke’s area on left side)

Occipital lobe

Parietal lobe

Motor cortex

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CHAPTER 6Section 6.5 Auditory Processing

Many other people with damage to Broca’s area exhibit symptoms of Broca’s aphasia, which is also referred to as verbal aphasia. Symptoms of Broca’s aphasia include (1) nonfluent speech that is effortful, with long pauses inserted between short phrases; (2) grammatical errors in which words are jumbled and function words like prepositions are missing; and (3) errors involving rep- etition, omission, or addition of sounds. As an example, one patient described his presurgical preparation in the following way: “When they had me to get ready that is they shaved off all my hair was and a few odd parts of pencil they quive me in the fanny” (Penfield & Roberts, 1959). People with Broca’s aphasia often exhibit anomia (also called nominal aphasia), in which they cannot name familiar objects. However, they typically have no problem comprehending spoken language or reading written material, and they also are aware of their impairment. The “Case Study” describes the case of a college-aged woman with verbal aphasia.

Case Study: Verbal Aphasia

LaDonna was a college freshman when she was involved in a car accident in which she was flung from the car through the wind- shield. She lay in a coma for 3 months before slowly regaining con- sciousness. As a result of the accident, LaDonna sustained damage to her left frontal cortex, including the motor cortex and Broca’s area. This brain damage caused temporary paralysis of her right arm and leg, which eventually developed into permanent weakness of those limbs. More devastating was the damage to Broca’s area because it left LaDonna unable to express herself clearly.

As is typical with Broca’s aphasia, LaDonna’s speech was slow and halting. She found she had to search for words, and when she tried to say them, often a different word or sound would pop out. She was constantly surprised and dismayed at the words that came out of her mouth.

A good example of this difficulty was an event that occurred a short while after LaDonna had awakened from her coma. One day LaDonna needed to use the toilet and decided that, rather than ring for a nurse to help her, she would hop on her good leg to the bathroom. She got herself to the edge of the bed. But when she tried to stand up on her good left leg, she immediately fell to the floor. She had no idea how weak she was or how heavy the useless right side of her body was.

As she lay on the floor, LaDonna tried to reach the bell to call for the nurse but couldn’t quite get her hand on it. She was forced to remain on the floor, helpless, until someone discovered her. To add to her humiliation, she had urinated on herself during her fall.

A nurse came into the room about 10 minutes later and was horrified to find LaDonna on the floor. She asked LaDonna what happened. LaDonna looked up sheepishly and replied, “I’m taking a walk in the rain.” That’s not what she had meant to say at all. She had meant to tell the nurse about her need to use the toilet and about how she fell, but her words did not match her intention. This is the kind of communication problem that LaDonna had to learn to deal with following her accident.

iStockphoto/Thinkstock

Photo 6.8 Verbal aphasia does not prevent those affected from understanding language, but it does make it very difficult for them to communicate with others.

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CHAPTER 6Section 6.5 Auditory Processing

Damage to Wernicke’s Area Wernicke’s area is located close to the primary auditory cortex in the temporal lobe. Damage to Wernicke’s area produces a disturbance in language comprehension called receptive aphasia, which is characterized by an inability to understand what is being said or to follow simple com- mands. An individual with receptive aphasia typically speaks fluently, using long phrases with little hesitation. Some people who have damage to Wernicke’s area exhibit a phenomenon called word deafness, in which they cannot understand spoken words, although they can perceive nonverbal sounds in the environment and can read, write, and talk normally (Corballis, 1994).

Associated with receptive aphasia are characteristic speech problems, including anomia, para- phasia, and jargon aphasia. Paraphasia involves the substitution of one phoneme for another phoneme or one word for another word. For example, a person with paraphasia might say, “I wand to hatch mobile,” when he meant to say “I want to have more.” With jargon aphasia, the person speaks fluently, using expansive, grammatically correct phrases. However, the content of the phrases is nonsensical in jargon aphasia. One person, when asked to explain the function of a key, replied, “Indication of measurement of piece of apparatus or intimating the cost of apparatus in various forms” (Brain, 1961). Unlike people with Broca’s aphasia, those with receptive aphasia are often unaware of their impairment.

Damage to the Arcuate Fasciculus Broca’s and Wernicke’s areas communicate through a bundle of axons known as the arcuate fas- ciculus. You can imagine that damage to the arcuate fasciculus would result in a lack of commu- nication between the cortical area responsible for speech production and the area responsible for language comprehension. This disruption in communication produces a disorder called con- duction aphasia, in which affected individuals have relatively good language production and comprehension but cannot repeat what is said to them. Individuals with conduction aphasia can sometimes accurately repeat meaningful words but not meaningless “nonsense” syllables, indi- cating that these people have an impaired ability to parrot word sounds.

Damage to the Cerebellum Damage to the cerebellum also produces impairment in speech production (Gordon, 1996). Recall from Chapters 2 and 5 that the cerebellum is involved in coordinating muscle contractions to produce smooth, accurate movements. The cerebellum receives information from the cerebrum and participates in the planning and execution of speech sounds. The most common impairment observed following cerebellar damage is dysarthria, a disorder in which people cannot control the rate, volume, or rhythm of their speech. In addition, articulation of phonemes is usually disturbed in people with dysarthria. Removal of tumors from the cerebellum sometimes results in cerebel- lar mutism, in which the affected individual cannot speak following surgery. Young children who develop mutism after cerebellar surgery typically recover within several months, and they exhibit severe dysarthria when they begin speaking again (Gordon, 1996).

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CHAPTER 6Section 6.5 Auditory Processing

Findings from Brain-Imaging Studies of Speech A number of brain-imaging studies have revealed that the area of the human temporal lobe con- taining Wernicke’s area is activated when speech sounds are heard. In Wernicke’s area, speech sounds produce stronger activation than do tones, indicating that this area responds best to spo- ken language (Binder et al., 1994; Calvert et al., 1997; Demonet et al., 1992; Petersen et al., 1988; Zatorre, Evans, Meyer, & Gjedde, 1992).

Helen Neville and her colleagues (1998) conducted a study using functional MRI to determine areas of brain activation in normally hearing and congenitally deaf (deaf since birth) people pro- cessing sentences in English and American Sign Language. They compared brain activation records of three groups of subjects: (1) normally hearing people who did not know ASL and who were native English speakers, (2) congenitally deaf people whose native language was ASL and who learned English later in life, and (3) normally hearing people who were born to deaf parents and had both ASL and English as native languages. All subjects were presented with English sentences and nonsense consonant strings that they had to read and a video of a native deaf signer who produced sentences in ASL and nonsign gestures. For all subjects functional MRI scans were con- ducted for both hemispheres.

Neville and her colleagues (1998) found that Broca’s area and Wernicke’s area became activated in deaf and hearing subjects when they were exposed to their native language, English versus Ameri- can Sign Language. Normally hearing subjects, including those with deaf parents for whom ASL was a native language, displayed significant activation of Broca’s area and Wernicke’s area in the left hemisphere when they read English sentences, with very weak activation of the right hemi- sphere. Deaf subjects did not show left hemisphere activation when they read English sentences, but they showed activation of corresponding areas of the right hemisphere instead.

When processing ASL (their native language), deaf subjects showed significant activation of Broca’s and Wernicke’s areas in the left hemisphere, as well as corresponding areas in the right hemisphere. Normally hearing individuals who had ASL as a native language showed the same activation pattern as deaf participants. Thus, all subjects who processed sentences in their native language displayed activation of Broca’s and Wernicke’s areas in the left hemisphere, regardless of whether the language was auditory (English) or visual (ASL).

In addition, Neville’s lab discovered that another area of the cortex, called the angular gyrus, was also activated when English or American Sign Language was processed (Neville et al., 1998). The angular gyrus is located at the junction of the temporal, parietal, and occipital lobes and is believed to integrate visual, auditory, and somatosensory information. Damage to this cortical area has been implicated in reading disorders as well as a writing disorder known as agraphia. People with agraphia often have trouble forming letters when using a pen or pencil, have spelling errors, and cannot space words and letters uniformly across a page when writing. However, peo- ple with agraphia typically have no difficulty speaking or understanding spoken language (Gross- man et al., 2001; Levine, Mani, & Calvanio, 1988; Whitaker & Kahn, 1994).

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CHAPTER 6Section 6.5 Auditory Processing

Language: Left or Right? Paul Broca was the first investigator to report that language functions were localized to the left cerebral hemisphere (Broca, 1861). Since then, a large number of investigations, including case studies of stroke patients and other brain-injured individuals as well as brain-imaging studies, have confirmed Broca’s finding that the left hemisphere is specialized for language (Muller et al., 1999; Whitaker & Kahn, 1994). For example, 95% of all people with aphasia have damage in the left hemisphere (Provins, 1997). American Sign Language also appears to be processed predomi- nantly in the left hemisphere in native signers (Corina, Vaid, & Bellugi, 1992; Neville et al., 1998). Left hemispheric damage in deaf individuals produces sign language aphasia, in which signing is disturbed (Poizner, Klima, & Bellugi, 1987).

In further support of Broca’s finding, Geschwind and Levitsky (1968) examined 100 brains of peo- ple who had recently died and discovered that the area of the temporal lobe that contains Wer- nicke’s area was significantly larger on the left side of the brain, compared to the right, in 65% of the brains. Geschwind’s research team went on to describe other asymmetries in the size of language centers, with those on the left being significantly larger (Galaburda, LeMay, Kemper, & Geschwind, 1978). However, Broca’s and Wernicke’s areas are not confined to the left hemisphere in all individuals. A small percentage of people have these language centers on both sides of the brain, and an even smaller percentage have language centers only on the right side of the brain.

No one knows why left hemispheric lateralization of language occurs in most, but not all, people (Provins, 1997). The term hemispheric refers to cerebral hemisphere, and lateralization comes from lateralis (“side” in Latin). Thus, neuroscientists use hemispheric lateralization to indicate that a particular function, such as language, is processed primarily in only one side (or hemisphere) of the cerebrum. Language appears to be lateralized to the left hemisphere for most people.

When the normal left lateralization for language is disrupted, speech abnormalities are produced. For example, stuttering is a disturbance of speech production characterized by disfluency, abnor- mal timing (such as long pauses or drawn-out syllables), and uncontrolled repetition of phonemes. The biological basis for this disorder is unknown, although brain-imaging studies are helping us understand the brain mechanisms involved. Peter Fox and his colleagues (1996) compared PET scans of 10 men who stuttered since early childhood with 10 male normal controls. PET imaging was conducted while both groups of subjects read a paragraph by themselves or as part of a cho- rus. When chorus reading, stutterers do not stutter and read normally, with no disfluency.

PET scans of the stutterers indicated overactivation of the right cerebral hemisphere and the cer- ebellum when they were stuttering. In addition, left lateralization of speech was absent during stuttering. However, when the chronic stutterers read fluently during the chorus reading condi- tion, the speech and motor areas in the left hemisphere became dominant, indicating that disrup- tion of the normal lateralization of speech is associated with speech disfluency.

Processing of Music

Music is a complex auditory phenomenon that has various components, including melody, rhythm, and sometimes verbal lyrics. Melody consists of a sequence or pattern of various fre- quencies, whereas rhythm is the temporal component that dictates the length of time that the individual frequencies are heard as well as the length of the pauses. Musical compositions can be distinguished on the basis of their melody, rhythm, and lyrics (the words that are sung). For example, “Mary Had a Little Lamb” and “Merrily We Roll Along” have identical melodies but can

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CHAPTER 6Section 6.6 Chapter Summary

be distinguished by their rhythm and lyrics. Different musical instruments can be distinguished by the pattern of frequencies that each produces, with each instrument emitting a characteristic pat- tern of frequencies. From the cochlea to the cerebral cortex, neurons in the auditory system are organized in such a way that they respond only to specific frequencies (Merzenich, Kaas, & Roth, 1976; Peretz & Zatorre, 2005; Rauschecker, Tian, & Hauser, 1995).

As with the perception of language, the perception of music involves many different parts of the brain and appears to be lateralized (Ricker et al., 2000; Zatorre, 1998). However, the localization of music functions in the brain appears to be different for musicians and nonmusicians. For non- musicians, music is divided between the left and right hemispheres, with melody being processed on the right and rhythm on the left (Bever & Chiarello, 1974; Boucher & Bryden, 1997; Corballis, 1994; Liegeois-Chauvel, Peretez, Babai, Laguitton, & Chauvel, 1998; Maess, Koelsch, Gunter, & Friederici, 2001; Mattheis, Silvestrini, Troisi, Cupini, & Caltagirone, 1997). Musicians tend to use the left hemisphere almost exclusively when playing or listening to music.

In musicians large areas of the cerebral cortex are involved in processing music (Hirata, Kuriki, & Pantev, 1999; Pantev et al., 1998). In general, the earlier the age at which musical training began, the larger the music-processing areas in the brain (Pantev et al., 1998). For example, MRI studies have revealed that the corpus callosum is 10% to 15% thicker in musicians who began studying music before age 7, compared to nonmusicians and people who study music later in life. A thicker corpus callosum would allow abundant communication between the two hemispheres to coordi- nate movements that produce intricate musical compositions (Schlaug, Knorr, & Sietz, 1994).

Damage to brain areas that are associated with music perception produces impairments in music perception, known as amusia. Damage to the frontal lobe results in expressive amusia, the inability to produce music, and damage to the temporal lobe results in receptive amusia (Corballis, 1994). The impairments associated with receptive amusia vary depending on the hemisphere involved. Damage to the right hemisphere produces an overall impairment of music perception that inter- feres with the recognition of melodies. Damage to the left temporal cortex results in an inability to process rhythm (Liegeois-Chauvel et al., 1998; Schuppert, Munte, Wieringa, & Altenmuller, 2000).

6.6 Chapter Summary Sensory Stimuli and Receptors

• Specialized sensory neurons called receptors respond to specific stimuli. • Chemoreceptors respond to chemical stimuli, and mechanoreceptors respond to mechan-

ical stimuli. • Receptors act as transducers to convert chemical or mechanical energy into action

potentials. • Many external stimuli are transmitted through the environment as waves, which possess

special characteristics, including wavelength, frequency, and amplitude. Wavelength is the distance between corresponding parts of two consecutive waves. Frequency refers to the number of waves that occur in a given period of time. The amplitude of a wave is the height of a wave from its lowest to highest points.

• Receptors show a number of special characteristics, including adaptation. Adaptation is the decrease in responsiveness of a receptor to a repeated stimulus.

• The network of neurons that form a pathway from the receptor to the cerebrum is called a sensory system.

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CHAPTER 6Section 6.6 Chapter Summary

The Visual System • Visual receptors, called rods and cones, are stimulated by light, which passes through the

cornea, aqueous humor, pupil, lens, and vitreous humor to reach the retina. • According to the Trichromatic Color Theory, the ability to see color comes from the fact

that we have three different types of cone (S cones, M cones, and L cones), each sensitive to different wavelengths of light.

• According to the Opponent-Process Theory, three opponent processes (red-green, blue- yellow, and black-white) code for color in the nervous system.

Visual Processing • Information from rods and cones is relayed to neurons in the lateral geniculate nucleus

(LGN) of the thalamus. • From the LGN, visual information is relayed to the primary visual cortex, also called area

V1, in the occipital lobe and then on to V2, V3, V4, and V5. • The parvocellular neurons in the LGN receive information from cones on the retina and

transmit that information to blob regions of V1. The magnocellular neurons in the LGN receive information from the rods and relay this information to the interblob regions of V1.

• Visual processing takes place along two pathways, the ventral (“What”) stream that pro- cesses information about color and form and a dorsal stream (the “Where” system) that processes information about motion and the location of objects in the visual field.

• The processing of color involves area V4, which sends information on to the inferior tem- poral cortex and the prefrontal cortex. Information about form is relayed to area V3 and then to the inferior temporal cortex. Information about movement and spatial relations is relayed to area V5 and then to the posterior parietal lobe.

• It is uncertain how the brain combines information from the “What” and “Where” sys- tems in a theoretical process called binding.

Disorders of the Visual System • Damage to various structures in the visual system will impair vision, producing blind-

ness, astigmatism, glaucoma, presbyopia, cataracts, retinitis pigmentosa, and macular degeneration.

• An astigmatism results from an irregularly shaped cornea. • The leading cause of blindness is glaucoma, which is caused by increased pressure in the

eyeball. • Disorders associated with the lens of the eye include presbyopia and cataracts. • Macular degeneration causes blindness beginning with the death of cones in the fovea,

whereas retinitis pigmentosa causes blindness beginning with the death of rods in the periphery of the retina.

• Visual perceptual disorders are associated with damage to particular areas of the cerebral cortex, including blindsight, achromatopsia, akinetopsia, and visual agnosia.

• People with blindsight cannot see but can move through the environment easily, avoiding obstacles.

• Damage to the inferior temporal cortex can produce visual agnosia, whereas damage to the posterior parietal lobe results in an inability to accurately locate and reach for objects in the environment.

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CHAPTER 6Section 6.6 Chapter Summary

The Auditory System • Auditory receptors are called hair cells. These hair cells are located on the basilar mem-

brane of the cochlea and respond to vibrations that have a frequency between 20 and 20,000 Hz.

The Nature of Auditory Stimuli • The frequency of sound waves is measured in Hertz (Hz) units, whereas loudness is mea-

sured in decibels (dB).

Anatomy and Function of the Auditory System • The outer ear consists of the pinna and auditory canal. • The middle ear consists of the eardrum, three ossicles, and eustachian tube. • The inner ear is found in the cochlea, which contains three chambers: the scala vestibuli,

scala tympani, and scala media. • Vibrations pass through the outer and middle ear to reach the cochlea in the inner ear.

Auditory Pathway to the Brain • The auditory nerve (cranial nerve VIII) transmits information from the cochlea to the

cochlear nucleus. • From the cochlear nucleus, auditory information is transmitted to the superior olive, on to

the inferior colliculus, and then to the medial geniculate nucleus in the thalamus before terminating in the auditory cortex in the temporal lobe.

Disorders of the Auditory System • Damage to specific structures in the auditory system produce different types of deafness

or hearing loss, including outer ear deafness, middle ear deafness, inner ear deafness, nerve deafness, and central deafness.

• Hearing aids are used to correct a conductive hearing loss associated with middle ear deafness. Cochlear implants are used to correct inner ear deafness.

Auditory Processing • Auditory perceptual processing also takes place along “What” and “Where” streams that

begin in the primary auditory cortex in the temporal lobe and end in the prefrontal cortex.

Processing of Speech • Spoken language comprises phonemes (the basic units of language) that are combined to

form meaningful units of sounds called morphemes. • Damage to Broca’s area causes a number of expressive aphasias, including Broca’s

(or verbal) aphasia. Anomia is the inability to name familiar objects. • Damage to Wernicke’s area produces receptive aphasia. Paraphasia involves the substitu-

tion of one phoneme for another. • Damage to the arcuate fasciculus produces a disorder called conduction aphasia. • The most common disorder following damage to the cerebellum is dysarthria. • When deaf individuals are exposed to their native language (American Sign Language or

English), both Broca’s area and Wernicke’s area become activated in the left hemisphere. • PET scans of stutterers show overactivation of the right hemisphere and cerebellum when

they are stuttering.

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CHAPTER 6Key Terms

Processing of Music • For nonmusicians, melody is processed in the right hemisphere and rhythm is processed

in the left, whereas musicians tend to use the left hemisphere exclusively when playing or listening to music.

• Impairments in music perception are referred to as amusia.

Questions for Thought

1. Give an example of sensory adaptation that you’ve experienced. 2. Why do cats see better at night than humans do? 3. Imagine what it would be like if you suffered damage to your “What” visual system. How

would damage to the “What” visual system differ from damage to the “What” auditory stream?

4. What similarities, if any, do brain-imaging studies indicate exist between spoken lan- guage and American Sign Language?

Web Links

This site, provided by the National Institute on Deafness and Other Communication Disorders, offers extensive information on audition, deafness, and other communication disorders. http://www.nidcd.nih.gov

For more information about aphasias, visit the National Aphasia Association’s website. This page presents facts about aphasias, tips on how to communicate with someone afflicted with aphasia, and many other helpful pieces of information to assist in understanding aphasia. http://www.aphasia.org/

The website of the American Foundation for the Blind offers invaluable information to further your knowledge about blindness. The site provides lists of programs and services for the blind, useful publications, and helpful tips for those living with vision loss. http://www.afb.org/

The Medline Plus website, a service of the U.S. National Library of Medicine and the National Institutes of Health, supplies information on hearing disorders and deafness. This includes an interactive tutorial on hearing loss, information on the latest news in the field, advice for disease management, and research information on the subject. http://www.nlm.nih.gov/medlineplus/

Key Terms

accommodation The process by which the lens of the eye changes shape in order to focus light.

adaptation A decrease in the firing of a recep- tor to repeated stimulation.

agraphia A writing disorder characterized by difficulty forming letters when using a pen or pencil, spelling errors, and inability to space words and letters uniformly across a page when writing.

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CHAPTER 6Key Terms

akinetopsia A visual disorder associated with damage to V5 in which the affected individual cannot perceive movement or moving objects.

American Sign Language (ASL) A standard system of hand gestures used by deaf individu- als to communicate phonemes, the basic units of spoken language.

amplitude The height of a sound wave from the lowest point to the highest point.

amusia An impairment in music perception.

angular gyrus An area of the cortex located at the junction of the temporal, parietal, and occipital lobes that integrates visual, auditory, and somatosensory information.

anomia A language disorder in which the affected individual cannot name familiar objects.

anvil The anvil-shaped small bone, or ossicle, in the middle ear, responsible for transmitting sound vibrations.

area V1 The primary visual cortex located in the occipital lobe.

astigmatism A blurring of vision in some ori- entations due to irregularities in the shape of the cornea.

auditory nerve Cranial nerve VIII, which carries information from the inner ear to the brain.

auditory “What” stream An auditory pathway that begins in the front area of the primary auditory cortex and ends in the prefrontal cortex.

auditory “Where” stream An auditory path- way that begins in the back area of the primary auditory cortex and ends in an upper area of the prefrontal cortex.

basilar membrane The thin, flexible mem- brane in the cochlea that supports the hair cells.

blindness A disorder in which a person is unable to see and recognize objects visually.

blindsight A phenomenon in which a person cannot see an object but will reach accurately toward the object.

Broca’s aphasia A disorder of language pro- duction associated with damage to Broca’s area, also called verbal aphasia.

Broca’s area An area of the inferior frontal cortex associated with the production of speech.

cataract A visual disorder in which the lens becomes opaque and vision is obstructed.

chemoreceptors Receptors that respond to chemical stimuli.

cochlea A snail-shaped canal that contains the inner ear.

cochlear implants Devices that transmit audi- tory signals to the auditory nerve.

cochlear nucleus A group of neurons in the medulla that receives auditory information from the auditory nerve.

color blindness An inherited disorder that produces impaired color vision.

conduction aphasia A communication disor- der in which affected individuals have good language production and comprehension but cannot repeat what is said to them.

cones Visual receptors that respond best in bright light and are sensitive to color and detail.

cornea The transparent covering on the front of the eye that aids in focusing light as it enters the eye.

deafness A condition in which an individual cannot perceive auditory stimuli.

dichromats Individuals with two functional cone types who have impaired color vision.

dorsal stream A visual pathway that is involved in detecting motion and the loca- tion of objects in the visual field; the “Where” system.

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CHAPTER 6Key Terms

dysarthria A disorder in which affected persons cannot control the rate, volume, or rhythm of their speech.

ear canal The opening into the temporal bone where the middle and inner ears are located.

eardrum The tympanic membrane, which is stretched across the end of the ear canal.

eustachian tube A canal that runs from the middle ear to the throat.

expressive aphasia Disorders of language production.

fovea A tiny dimple in the center of the retina where cones are concentrated.

frequency The number of sound waves that occur in a certain period of time.

glaucoma A condition in which the intraocular pressure increases, producing damage to the optic nerve.

hair cells Specialized receptors in the auditory system, located in the cochlea.

hammer The hammer-shaped small bone, or ossicle, in the middle ear, responsible for transmitting sound vibrations.

Hertz The unit for frequency of waves that is used by the International System of Units; it is named for a German scientist, Heinrich Hertz, who studied waves in the late 19th century.

hyperopia Farsightedness, in which a person cannot bring near objects into focus.

inferior temporal lobe The area of the brain in which the ventral steam ends.

inner ear The innermost part of the ear, con- sisting of the cochlea.

iris the circular, colored structure in the front of the eye that regulates the diameter of the pupil.

jargon aphasia A speech disturbance in which the person speaks fluently, using expansive, grammatically correct phrases, but the content of the phrases is nonsensical.

language A formalized system of symbolic representations that we use to communicate ideas, questions, and commands.

lateral geniculate nucleus (LGN) A nucleus in the thalamus that receives information directly from the ganglion cells in the retina and relays it on the visual cortex.

lens A yellowish, transparent structure located inside the eyeball that focuses light on the retina.

loudness A psychological phenomenon that corresponds to the amplitude of a sound wave; a large amplitude is associated with a loud sound, and a small amplitude is associ- ated with a softer, less loud sound.

macular degeneration A disorder in which cones in the fovea die, causing blindness.

mechanoreceptors Receptors that respond to mechanical stimulation, such as pulling, stretching, or vibrating.

medial geniculate nucleus A thalamic nucleus that receives auditory information from the midbrain and sends it to the auditory cortex in the temporal lobe.

middle ear The part of the ear that contains the eardrum, three ossicles, and the eusta- chian tube.

monochromats Individuals who possess only one functional cone type and see only black, white, and gray.

morphemes Meaningful units of sounds that are produced by combinations of phonemes, the basic units of spoken language.

myopia Nearsightedness, in which an indi- vidual cannot see far objects clearly.

Opponent-Process Theory A theory of color vision that states that three opponent pro- cesses (red-green, blue-yellow, black-white) code for color in the nervous system; it reflects the activities of neurons further along the visual pathway.

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CHAPTER 6Key Terms

optic chiasm The structure formed by the merging of the left and right optic nerves.

optic disk The blind spot on the retina where the axons that form the optic nerve exit the retina.

optic nerve Cranial nerve II, which carries information about vision from the eye to the brain.

optic tract A part of the visual system in the brain that splits into two branches: a larger branch that goes to the lateral geniculate nucleus of the thalamus and a much smaller branch that goes to the superior colliculus in the midbrain.

outer ear The outermost part of the ear, con- sisting of the pinna and auditory canal.

oval window The opening to the inner ear covered by a thin membrane that vibrates in response to vibrations by the stapes bone.

paraphasia A language disorder that involves the substitution of one phoneme for another phoneme or one word for another word.

parvocellular layers The four top layers of the lateral geniculate nucleus that process infor- mation about color, form, and detail.

phonemes The basic units of spoken language.

photopigment A chemical that absorbs light entering the eye.

pinna The outer, fleshy part of the ear that acts as a sound collector.

posterior parietal lobe The area of the brain in which the dorsal stream ends.

presbyopia A condition found in older people in which the lens loses its ability to accommodate.

primary auditory cortex An area of the tem- poral cortex that receives information directly from the medial geniculate nucleus of the thalamus.

prosopagnosia A form of visual agnosia in which affected individuals cannot recognize faces of people familiar to them.

pupil The hole in the center of the iris through which light passes to stimulate receptors in the retina.

pure alexia A form of visual agnosia associ- ated with damage to the left inferior temporal lobe in which affected individuals can iden- tify individual letters but cannot put letters together to read them as a whole word.

receptive aphasia A disturbance in language comprehension.

receptors Specialized sensory cells that respond to a specific stimulus.

retina A flat, multilayered tissue at the back of the eyes that contains the visual receptors.

retinitis pigmentosa A genetic disorder that first destroys rods and eventually spreads to the cones, causing total blindness.

rods One of the retina’s two types of photo- receptors, rods are very numerous and respon- sible for vision in low light levels.

sclera The tough, white outer layer of the eye.

sensory system A network of neurons that form a pathway from the receptor to the cerebrum.

stimuli The plural of stimulus; anything that comes from outside or within the body and activates the body’s receptors.

stirrup The stirrup-shaped small bone, or ossicle, in the middle ear, responsible for transmitting sound vibrations.

stuttering A disturbance of speech charac- terized by disfluency, abnormal timing, and uncontrolled repetition of phonemes.

superior olive A hindbrain structure that pro- cesses auditory information before sending it on to the midbrain.

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CHAPTER 6Key Terms

tectorial membrane The stiff membrane that lies on top of the hair cells and in which the hairs of the receptor cells are embedded.

transducers Sensory cells that transduce or convert one form of energy, such as chemical energy or mechanical energy, to an electro- chemical form of energy, such as an action potential.

Trichromatic Color Theory A theory of color vision that proposes that as few as three differ- ent receptors are needed in order for people to see all the 200 shades of color that we perceive; it explains the function of the cones in producing color vision.

trichromats Individuals with three functional cone types who have normal color vision.

ventral stream A visual pathway that is concerned with processing information about color and form; the “What” system.

visual agnosia A visual disorder caused by damage to the inferior temporal lobe in which the affected individual cannot recognize famil- iar objects in the visual field.

Wernicke’s area An area in the temporal lobe associated with language comprehension.

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