For Professor Tommy

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Chapter 4 Sensation and Perception

Gateway Theme

We actively construct our perceptions out of the information provided by our senses and our past experiences. The resulting perceptions are not always accurate representations of events.

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How do we take in and process information from the world around us? What is the difference between sensing and perceiving a given stimulus? The study of these questions in psychology is called psychophysics, and you may find interesting how complex these processes are that we take for granted on a minute-by-minute basis.

Gateway Questions

In general, how do sensory systems function?

Why are we more aware of some sensations than others?

How does the visual system function?

What are the mechanisms of hearing?

How do the chemical senses operate?

What are the somesthetic senses?

In general, how do we construct our perceptions?

How is it possible to see depth and judge distance?

How do expectations, motives, emotions, and learning alter perception?

How can I perceive events more accurately?

How can effective communication help me in my personal and professional life?

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Our sensory systems work to receive information from our environment and convert them into an electrochemical signal that gets sent to the brain, a process called transduction. Once the information is received in the brain, the process of perception occurs, where we select, organize, and interpret those stimuli.

Psychophysics (1 of 2)

An artificial visual system

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Have you ever had a hearing test where the audiologist asks you to raise your hand whenever you hear something? She was measuring your absolute threshold for hearing. If, instead, she were to play a tone and ask you to indicate when you hear it change, she would be measuring your difference threshold. The standard for these thresholds is a 50 percent hit rate, or detecting the stimulus (or stimulus change) 50 percent of the time.

Our sensory systems are not simply passive receptors for information, but rather select which information is attended to and sent off to the brain for further processing.

In terms of unusual sensory systems, called extrasensory perception (ESP), the majority of the available data investigating its existence has found…. (wait for it)… nothing! So if ESP exists, we’ve been unable to demonstrate it scientifically.

Psychophysics (2 of 2)

Visual pop-out

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Because our senses can select which stimuli to attend to, they can also reduce their response to ongoing, unchanging stimuli in a process called sensory adaptation. This is different than habituation discussed in other chapters, as this is an actual weakening of the physical receptors versus simply reducing our behavioral response to a constant stimulus.

Cells in the brain called feature detectors are specifically designed to identify certain types of sensory input (shapes, colors, lines, etc.). These individual cells work together to help us construct “wholes” from the individual components of a sensory stimulus.

Seeing (or Not Seeing) What Is There

Focusing attention

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When we make the conscious (or something unconscious) decision to focus on a narrow set of data from our surroundings, this is called selective attention. Imagine what life would be like if you could not selectively “tune out” the noise from our environment and you had to attend equally to the sound of the cash register, the flavor of your coffee, and the sound of the espresso machine making the next beverage. If you could not filter out this irrelevant data, you might quickly lose your sanity!

Related to this is inattentional blindness, or a failure to notice a stimulus because we are focusing elsewhere. How might this be relevant when you are driving and also talking or texting on your cell phone? When stimuli change, inattentional blindness can be reduced. So you see that many different concepts all work together to influence what we sense and perceive (in this case sensory adaptation, selective attention, and inattentional blindness).

Structure of the Eye

Anatomy of the eye

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The light that we see falls within the visible range of the electromagnetic spectrum, and the nature of the light waves determines how we perceive them. The wavelength corresponds to huge, the narrower the band of wavelengths the greater the saturation, and the amplitude of the waves indicates the brightness of the color.

There are many different structures of the eye, each of which plays its own role in the visual system. Study carefully the following parts:

Cornea

Iris and Pupil

Lens (and the process of accommodation)

Retina

There are other ocular structures, but these are the ones that are most immediately salient in the process of vision.

When the structures do not work quite right, we may suffer visual distortions. When light images are focused in front of or behind the retina, we may experience myopia or hyperopia respectively (you may know these as near-sightedness and far-sightedness). These often result from incorrect curvature of the cornea, a condition called astigmatism. As we age, the natural flexibility of the lens reduces, leading to an age-related reduction of near vision called presbyopia.

Rods and Cones

Anatomy of the retina

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The photoreceptors in your retina, the rods and cones, are each responsible for specific types of visual experiences. Cones allow us to see when there is plenty of light around, and are sensitive to colors. Rods work well in dimmer situations, but only receive black and white data. When these receptors come together they form the optic nerve, which departs the retina leaving us with a blind spot in each eye.

The cones are also concentrated in the center of the retina, called the fovea, and this is where we have our greatest visual acuity (or clarity). On the outsides of the retina we have more rods, and this fact explains why peripheral vision is less focused and clear than central vision. At night if you want to see something a little more sharply (when there is less light) try moving the image to the left or right of your center visual field and the rods may help you focus on it. During the day, however, this will not work!

Color Vision

Firing rates of blue, green and red cones in response to different colors

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The trichromatic theory of color vision notes that we have three different kinds of cones that are receptive to different color ranges in the spectrum. The opponent process theory suggests that our visual receptors process colors in “either/or” pairs (red/green, yellow/blue, and black/white). Together these two theories explain how color perception as well as opposing afterimages occur. What are the strengths and weaknesses of each theory?

When color receptors fail to work properly we may experience what is erroneously called color blindness, but is really color weakness. It is more common in men than in women, and most common in white men.

Imagine how your life might be different if you had never seen colors and did not even understand the concept of a color!

Seeing in the Dark

Typical time course of dark adaptation

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When you move into a darker situation, the process of dark adaptation begins. The colored ring of muscle (the iris) dilates the opening in the eye (the pupil) so that more light can get in, and the retinal cells adapt in a chemical process. Review the text to see exactly how this process occurs, and how light sensitivity is restored when we move out of the darkness.

How We Hear Sounds

Characteristics of sound waves

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Just like light waves, sound waves have various properties that determine what we hear. The frequency of the wave determines the pitch and the amplitude reveals the loudness or intensity.

There are several structures of the ear, each of which is important to understand in order to comprehend the process from sound wave reception to recognition of the auditory stimulus. Study carefully how the following structures work:

pinna

auditory canal

eardrum (tympanic membrane)

ossicles (malleus/hammer, incus/anvil, and stapes/stirrup)

oval window

cochlea

basilar membrane

organ of Corti

hair cells

While this may feel like a lot of information, note how each structure works with the one before it and the one after it in order for you to have an auditory experience.

The two primary theories of pitch perception are named for the principles they forward—the place theory suggests that stimulation of different areas (or places) in the cochlea correspond with higher or lower pitch reception, while the frequency theory posits that the hair cells fire at a certain rate depending on the frequency of the sound wave that stimulates them.

Hearing Loss

Decibel scale

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A common question is “what causes a person to lose their hearing?” There is no simple answer, as hearing loss can have many causes. Conduction hearing loss is a result of problems with the structures of the middle ear, which decreases vibrations as they make their way to the inner ear. Sensorineural hearing loss occurs when the inner ear’s hair cells or the auditory nerve are damaged, and cannot transmit signals effectively to the brain. This is often caused by exposure to very loud noise over an extended time period.

How are different types of hearing loss treated? What sort of technology can help restore hearing?

The Sense of Smell

Receptors for the sense of smell (olfaction)

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Our sense of smell is far more important than you may realize, and is closely connected to emotional centers in the brain. This is why scents evoke such visceral, emotional memories.

We have the genes for many different types of scent reception, but only 400 of them are expressed in the average human being. Chemicals in the air fit in a lock and key fashion so that we have an olfactory experience. The scents that we find pleasant or aversive are based on many factors, including cultural background and our own experiences.

What must it be like to suffer from anosmia, or “smell blindness?” Do you think you’d be bothered by it?

Taste and Flavors

Receptors for the sense of taste

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Another chemical system is our sense of gustation, or taste. There are five identified taste sensations—sweet, salty, bitter, sour, and umami—and each is precipitated by specific chemicals that stimulate taste buds. The sense of taste is rather simple when compared to other sensory systems, as chemicals dissolve in our mouths and stimulate those same taste buds, which in turn send signals to the brain where we have a gustatory experience.

As with smell, the things that we find pleasant to taste are determined by a number of factors, including culture, genetics, and experience.

The Skin Senses

Receptors for the skin senses

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Although the skin senses tend to get relegated to “second class” status in the study of psychophysics, they really shouldn’t! Our sense of touch—or the haptic system—is an important part of our lives. A hot shower, a significant other stroking your arm, sexual experiences—these are all due to the ability to feel touch. Our skin is the largest organ on our body, so make sure to focus on how the receptors for touch allow you to have these experiences.

Pain

A sensory gate of pain

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Although we may not like the experience of pain, it is important that we be able to feel it. This is how the body informs us that there is something wrong that needs our attention and, in some cases, intervention. The large nerve fibers that send sharp messages comprise the body’s warning system, while the small nerve fibers that carry achy, chronic pain are the body’s reminding system. The first tells us that something bad is happening, while the second tells us that something bad has happened.

The gate control theory of pains suggests that the pain pathways in the spinal cord have “gates” that can be opened or closed. When they are closed, pain messages cannot pass through and make it to the brain. Many medications that are regarded as pain killers are really pain blockers, in as much as they close the gates so that pain messages cannot be received (and thus perceived) in the brain.

What methods of pain control exist beyond using medications? Can we truly stop pain without chemical intervention?

The experience of phantom limb pain shows us that sometimes we can experience pain that is caused by something other than the stimulation of the pain site. This represents a “misfiring” of nerves that leave us with that pain experience.

The Vestibular System

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What causes motion sickness when we ride a roller coaster, fly in a helicopter, or travel in a car or train? This is a result of the vestibular system, which is primarily informed by structures in the inner ear (the otolith organs and the semicircular canals). This is the system that informs us of the effects of gravity and acceleration and gives us a sense of balance.

When we move (or stop moving) fluid in these organs continues moving (or stops moving) to give us information about that motion. If the fluid movement is in conflict with what our other senses (mostly vision) tell us is happening, we can develop the motion sickness that we associate with different activities. So if you think floating in space is fun, you might not be so impressed when you get very nauseated!

Interpreting Sensory Input

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It may seem very straightforward that we sense information, the data are sent to the brain, and then we interpret it. How hard can that be, right? Well don’t be fooled! There are many ways our sensory and perceptual systems can be misled.

Perceptual illusions occur when there is a disconnect between what we perceive and what is actually happening in the world around us. You may try to search for examples of optical illusions (also called visual or perceptual illusions) on the internet to see a number of examples. What is interesting about them is that the awareness that what we are seeing is not what is actually there is not enough to cause us to see the stimulus differently.

How does an illusion differ from a hallucination?

When we are uncertain of the accuracy of our perception, we may seek out additional sources of information to confirm it. This is called reality testing, and it is an important component of some mental illnesses where the defining feature is psychosis (or loss of contact with reality).

Bottom-Up and Top-Down Processing

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When do our expectations guide and influence our perceptions, and under what circumstances are perceptions a unique result of the sensory information available? This is the crux of how we distinguish between bottom-up and top-down processing. Make sure that you remember that they are not necessarily independent of each other. In many situations, we use both processes at the same time. Refer to the text for some examples of each type of processing.

Gestalt Organizing Principles

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The Gestalt principles of perception refer to a set of organizing processes that allow our brains to look for and find relationships between sensory information in our surroundings. Though susceptible to errors, the Gestalt principles help us to make sense of the world around us, reducing an enormous amount of data to more manageable levels. Focus on the following organizing principles:

Nearness (or proximity)

Similarity

Continuation (or continuity)

Closure

Common region

Contiguity

Take time to appreciate how these principles utilize the basics of the figure-ground relationship.

Perceptual Constancies

Shape constancy

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You should be in awe of how hard it is to actually fool your own brain. One example of this difficulty lies in the existence of perceptual constancies—including size, shape, and brightness constancy, among others. These phenomena show us that our brains can distinguish between real change and apparent change in our surroundings. As an example, when something moves farther away from you it appears to get smaller, but your brain knows that it is not actually getting smaller.

Binocular Depth Cues

Convergence

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When we are trying to determine how far something is from us—three dimensional perception—we are using depth cues. And when we use both eyes, they are called binocular cues. Two examples include:

Retinal disparity (the difference between how images are cast on our left and right retinas), and;

Convergence—how our eyes move together as something that we are looking at gets closer to us.

Both eyes are needed to use this type of information, but in some cases we only need information from one eye or the other.

Monocular Depth Cues

Pictorial depth cues

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These cues are monocular (or one-eye) cues. Refer to your text for a description of each:

Linear perspective

Relative size

Height in the plane

Light and shadow

Overlapping (or interposition)

Texture gradients

Aerial perspective

Relative motion (motion parallax)

How are these cues used in different situations, including artwork and architecture?

Motives, Emotions, and Perception

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Have you ever heard that it is a bad idea to go grocery shopping when you are hungry? It is true, but if you happen to find yourself in such a store, make sure to stay away from the prepared foods area. Why?

We are affected in the way we sense and perceive by the internal states we are experiencing. As such, when we’re very hungry a piece of hot pizza or a pan of macaroni and cheese might appear, smell, and even taste far better than they would when we are not hungry. This is not just a function of being full in the stomach, but rather our perceptual processes change as a result of different internal states.

How might this affect other areas of functioning? For example, do you find your significant other as physically attractive when you are angry at him or her?

Perceptual Expectancies

Creating perceptual expectancies

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A perceptual set (or expectancy) refers to the tendency to perceive things in a specific way based largely on our expectations of what we are going to perceive.

This is something that is used in the world around us, often without us knowing about it. For example, at a restaurant do you order the “special of the day” thinking that it is some amazing preparation by a world class chef? Does it taste better if you’re told that it is a “special?” Would you feel differently if you knew that most of the time a “special of the day” is really just a way of getting rid of the leftover food from the previous day?

Perceptual Learning: Do They See What We See?

The effects of prior experience on perception

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The fact that our perception can be altered by expectation is really a consequence of learning, which as you may remember can lead to an actual change in the brain. Can this change filter down (or up) to our perceptual systems? This is the basis of perceptual learning.

This also helps us understand how cultural differences can exist in perception. Refer to the text for a discussion of why the Müller-Lyer illusion works in some cultures but not in others based on the “carpentered world” hypothesis.

How To Become a Better “Eyewitness” to Life

Table 4.2 Factors Affecting the Accuracy of Eyewitness Perceptions

Sources of Error	Summary of Findings
Wording of questions	An eyewitness’s testimony about an event can be affected by the wording of questions that the witness is asked.
Postevent information	Eyewitness technology about an event often reflects not only what was actually seen, but also information obtained later on.
Attitudes, expectations	An eyewitness’s perception and memory for an event may be affected by his or her attitudes and expectations.
Drug intoxication	Drug(including alcohol) intoxication impairs witness’ ability to recall events after they occur.
Cross-racial perceptions	Eyewitnesses are better at identifying members of their own race than they are at identifying people of other races.
Weapon focus	The presence of a weapon impairs an eyewitness’s ability to identity the culprit’s face and other personal details .
Accuracy-confidence	An eyewitness’s confidence is not a good predictor of his or her accuracy.
Exposure time	The less time that an eyewitness has to observe an event, the less correctly that she or he will perceive and remember it.
Unconscious transference	Eyewitnesses sometimes identify as a culprit someone whom they have seen in another situation or content.
Color perception	Judgements of color made under monochromatic light (such as an orange streetlight ) are highly unreliable.
Stress	High levels of stress impair the accuracy of eyewitness perceptions.

Source: Adapted from Wells & Olson, 2003: Yarmey, 2010

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Take a look at these recommendations (and study them in the textbook). Can you think of ways in which they apply to you? It’s amazing how such simple suggestions can make such a big difference!