Sensation and Perception

CHAPTER 4—SENSATION AND PERCEPTION I. Introduction A. Synesthesia – the perceptual experience of one sense that is evoked by another sense 1. Example: Experiencing specific tastes when certain sounds are heard

II. Our Senses Encode the Information Our Brains Perceive A. Sensation – simple stimulation of a sense organ 1. Example: Seeing patterns of ink on a page

B. Perception – the organization, identification, and interpretation of a sensation in order to form a

mental representation

1. Example: Transforming the patterns of ink into a coherent mental representation of words and

concepts C. Sensation and perception are related, but separate, events

D. Transduction – what takes place when many sensors in the body convert physical signals from the

environment into neural signals sent to the central nervous system

E. Psychophysics – methods that measure the strength of a stimulus and the observer’s sensitivity to

that stimulus

1. Gustav Fechner, a German physicist whose interests turned toward psychology and philosophy,

developed a method for relating a measured stimulus to an observer’s yes-or-no response F. Measuring Thresholds (or “boundaries”)

1. Begin by measuring a single sensory signal to determine how much physical energy is required to

evoke a sensation in an observer 2. Absolute Threshold – the minimal intensity needed to just barely detect a stimulus a. Example: A candle flame 30 miles away on a clear, dark night

b. Typically defined as the intensity (brightness) required for a person to say she or he has

perceived the stimulus on 50% of the trials 3. Difference Thresholds

a. Human perceptual systems excel at detecting changes in stimulation, rather than the simple

onset or offset of stimulation

b. Just Noticeable Difference (JND) – the minimal change in a stimulus that can just barely be

detected

i. Compare a standard (S), or fixed intensity stimulus, to other intensities of the stimulus

(bright light vs. slightly brighter light)

ii. The JND can be calculated for each sense and is roughly proportional to the magnitude of the

standard stimulus

iii. Weber’s Law – the just noticeable difference of a stimulus is a constant proportion despite

variations in intensity

(a) Example: The difference between a one-ounce weight and a two-ounce weight is easier to

detect than the difference between a ten-pound weight and a ten-pound, one-ounce weight

G. Signal Detection

1. Sensory signals face a lot of competition, or noise, which refers to all of the other stimuli coming

from the internal and external environment

a. Sensory systems are noisy and when the signals are very small, senses provide only a “fuzzy”

indicator of the state of the world

2. Signal Detection Theory – observation that response to stimulus depends both on a person’s

sensitivity to stimulus in the presence of noise and on person’s response criterion

a. Theory proposes a way to measure perceptual sensitivity––how effectively the perceptual

system represents sensory events––separately from observer’s decision-making strategy

H. Sensory Adaptation – sensitivity to prolonged stimulation tends to decline over time as an organism

adapts to current conditions 1. Our perceptual systems emphasize change in responding to sensory events because changes often

call for action

2. If adaptation did not occur, things that you consider mundane would be constant distracters

throughout the day (e.g., constantly being aware of how your tongue is situated in your mouth)

III. Vision I: How the Eyes and the Brain Convert Light Waves to Neural Signals A. Visual Acuity – ability to see fine detail

1. The Snellen chart is used to measure visual acuity, where normal vision is reported as 20/20 or

the ability to read line #8 at a distance of 20 feet from the chart a. An eagle’s vision is more like 20/2 b. 20/200, or barely seeing the big “E” on line #1, is considered legally blind B. Sensing Light

1. Visible light is the portion of the electromagnetic spectrum that humans can see (measured in

nanometers, or billionths of a meter, between waves) a. Light waves can be measured by length, amplitude and wavelength (distance between peaks) i. Hue, or color, is determined by the length of a light wave ii. Brightness, or intensity, is determined by the amplitude (height) of a light wave

iii. Purity, or saturation (color richness), is determined by the number of wavelengths

(frequency) that make up the light 2. The Human Eye a. Cornea – clear, smooth outer tissue of the eye that slightly bends light b. Pupil – a hole in the colored part (iris) of the eye

c. Iris – translucent, doughnut-shaped muscle that controls the size of the pupil and hence the

amount of light entering the rest of the eye d. Lens – clear, double-convex structure that focuses light onto the retina e. Retina – light-sensitive tissue lining the back of the eyeball f. Accommodation – the process by which the eye maintains a clear image on the retina

i. If the eyeball is too long, relative to where the lens focuses light toward the retina,

nearsightedness (“myopia“) is the result

ii. If the eyeball is too short, relative to where the lens focuses light toward the retina,

farsightedness (“hyperopia”) is the result (a) Eyeglasses and contact lenses provide another lens to refocus the light onto the retina

(b) LASIK surgery physically reshapes the existing lens in the eye to appropriately focus light onto the retina

3. Phototransduction in the Retina a. Light adaptation – vision adjusting as you go from low light to bright light b. Dark adaptation – vision adjusting as you go from bright light to low light

c. There are two types of photoreceptor cells in the retina that contain light-sensitive pigments

that transduce light into neural impulses

i. Cones – photoreceptors that detect color, operate under normal daylight conditions, and

allow us to focus on fine detail (a) About 6 million per retina, mostly in the fovea (b) Take approximately 8 minutes to adapt to low light levels ii. Rods – photoreceptors that become active only under low-light conditions for night vision > (a) About 120 million distributed around each retina, except in the fovea (b) Take approximately 30 minutes to adapt to low light levels iii. Fovea – an area of the retina where vision is the clearest and there are no rods at all

(a) Objects in your peripheral vision are more blurry because they are detected by mostly rods rather than cones

d. Retina contains three layers of cells

i. Rods and cones are in the innermost layer, receiving from bipolar cells, and sending signals

to the middle layer

ii. Bipolar cells are in the middle layer, receiving from rods and cones, and sending signals to the outer layer

iii. Retinal ganglion cells (RGCs) are in the outer layer, receiving from bipolar cells, and sending

signals to the brain via the optic nerve

(a) Blind Spot – a location in the retina that contains no rods or cones where the optic nerve leaves the retina; therefore it has no mechanism to detect light

4. Receptive Fields

a. Receptive Field – the region of the sensory surface that, when stimulated, causes a change in

the firing rate of that neuron

b. Lateral Inhibition – opposing responses of neighboring photoreceptors interacting to result in

the signals sent to the bipolar cells, then on to the RGCs

c. Most receptive fields that represent an RGC contain either a central excitatory zone surrounded

by a doughnut-shaped inhibitory zone (on-center cell) or a central inhibitory zone surrounded by a doughnut-shaped excitatory zone (off-center cell)

i. RGC is a kind of “spot detector” encoding differences in brightness or color by recording

differences in excitation and inhibition of receptive fields C. Perceiving Color 1. Color often offers fundamental clues to an object’s identity and whether or not to be alarmed 2. Seeing Color

a. The shortest wavelengths of visible color are deep purple, then blue, green, yellow, orange,

and the longest waves are seen as red

b. Although rods contain only one type of photopigment, cones can contain one of three

photopigments sensitive to Red (Long-wavelength; L-cones), Green (Medium-wavelength; M- cones), or Blue (Short-wavelength; S-cones) that are the primary colors of light

i.

Thomas Young, and later Hermann von Helmholtz, determined how all colors can result from mixing the three primary colors

(a) Additive Color Mixing – increasing light to create color (e.g., combine red and green spotlights to create yellow light)

(1) White is the reflection of all colors (or lights)

(b) Subtractive Color Mixing – removing light from the mix (e.g., combine yellow and red paints to create orange paint)

(1) Black is the reflection of no color (or light)

3. Trichromatic Color Representation in the Cones – the pattern of responding across the three

types of cones that provides a unique code for each color

a. Color Deficiency (often called Color Blindness) – a sex-linked genetic disorder in which one or

more of the cone types is missing 4. Color-Opponent Representation into the Brain

a. Cones can fatigue if you stare at one color for too long, resulting in a form of sensory

adaptation called color afterimage

b. Color-Opponent System – pairs of visual neurons that work in opposition (red vs. green and

blue vs. yellow) i. Red-Green cells are excited by red wavelengths and inhibited by green wavelengths ii. Blue-Yellow cells are excited by blue wavelengths and inhibited by yellow wavelengths c. Trichromatic and color-opponent systems begin color perception D. The Visual Brain

1. Half of the axons in the optic nerve that leave each eye come from the RGCs representing the left

visual field; other half come from the RGCs representing the right visual field 2. Optic nerve from each eye travels to the lateral geniculate nucleus (LGN) in the thalamus

3. Information leaves the thalamus and travels to the primary visual cortex (area V1) in the

occipital lobe 4. Neural Systems for Perceiving Shape a. Area V1 is specialized for detecting the location and orientation of edges

i. There are cells to detect vertical edges, horizontal edges, 45° edges, and everything in- between

5. Pathways for What, Where, and How

a. Two functionally distinct pathways, or visual streams, project from the occipital cortex to visual

areas in other parts of the brain

i.

The ventral stream, or “what pathway,” projects to the lower levels of the temporal lobes and represents an object’s shape and identity

ii.

The dorsal stream, or “where pathway,” projects to the parietal lobes and dorsal temporal lobes and identifies the location and motion of an object

b. Visual-Form Agnosia – the inability to recognize objects by sight c. Optic Ataxia – difficulty using vision to guide reaching and grasping movements

IV. Vision II: Recognizing What We Perceive A. Attention: The “Glue” that Binds Individual Features into a Whole

1. Binding Problem – how features are linked together so that we see unified objects in our visual

world rather than free-floating or miscombined features

2. Illusory Conjunctions – perceptual mistake where features from multiple objects are incorrectly

combined

3. Feature Integration Theory – Treisman and colleagues proposed theory holds that focused

attention not required to detect individual features that comprise a stimulus such as color, shape, size, and location of letters, but required to bind those individual features together

4. The Role of the Parietal Lobe

a. Patient R.M. suffered strokes that destroyed both left and right parietal lobes

b. Observed abnormally large number of illusory correlations, even when given as long as 10

seconds to look at displays

c. Fit with transcranial magnetic stimulation (TMS) studies where researchers attempted to

temporarily “turn off” posterior parietal lobe i. When TMS was applied to occipital lobe, it had no effect on illusory conjunctions ii. Helped to refine idea that feature binding depends critically on attentional processes 5. Binding and Attention in Synesthesia

a. Recent research shows some of same processes involved in normal feature binding also occur in

synesthesia b. FMRI studies of synthetic individuals revealed that parietal lobe regions implicated in normal

binding of color and shape become active during experience of letter-color synesthesia B. Recognizing Objects by Sight

1. We are able to recognize the underlying features of objects and people even if some parts or

details change from one encounter to the next

a. Modular View – specialized brain areas, or modules, detect and represent faces or houses or

even body parts

b. Distributed Representation – the pattern of activity across multiple brain regions that identifies

any viewed object, including faces

c. Recent fMRI studies indicate that some brain regions in the occipital and temporal lobes do

respond selectively to specific object categories

d. Recent electrophysiology studies indicate neurons in the temporal lobe that respond to objects

and people viewed from multiple angles, and to the words that represent the objects

e. Perceptual Constancy – a perceptual principle stating that even as aspects of sensory signals

change, perception remains consistent 2. Principles of Perceptual Organization

a. Before recognition can occur the perceptual system must group the image regions that belong

together into a representation of an object i. Gestalt perceptual grouping rules

(a) Simplicity (Pragnanz): when confronted with two or more possible interpretations of an object’s shape, the visual system tends to select the simplest or most likely interpretation

(b) Closure: we tend to fill in missing elements of a scene, allowing us to perceive edges that are separated by gaps as belonging to complete objects

(c) Continuity: edges that have the same orientation tend to group together perceptually

(d) Similarity: regions that are similar in color, lightness, shape, or texture are perceived as belonging to the same object

(e) Proximity: objects that are close together tend to be grouped together

(f) Common fate:– elements of a visual image that move together are perceived as parts of a single moving object

3. Separating Figure from Ground

a. Our perceptual systems focus attention on some objects as distinct from their environments;

that is, we tend to identify a figure apart from the (back)ground in which it resides i. Size: smaller regions are likely to be figures ii. Movement: figures tend to move more than their surroundings iii. Edge Assignment: identifying whether an edge belongs to the figure or the ground

(a) FMRI studies show that the “face regions” of the temporal lobe are more active when viewing the Rubin “face-vase” as a face versus a vase.

4. Theories of Object Recognition

a. Image-based object recognition theories state that an image seen previously is stored in

memory as a template, then compared to the current image for recognition purposes

i. Template – a mental representation that can be directly compared to a viewed shape in the

retinal image ii. Problems (a) You would need multiple templates for different orientations of an object

b. Parts-based object recognition theories propose that the brain deconstructs viewed objects into a collection of parts, stored as structural descriptions or mental inventories, that act as sort of an “alphabet” of geometric elements (called geons) that can be combined to make objects

i. Problems

(a) Only allows for recognition at the level of categories, not individual objects (e.g., explains recognizing a face, but not Grandma’s face)

C. Perceiving Depth and Size 1. Knowing what is around you and where things are located are important a. The world is three dimensional and the retina is two dimensional

i. Three dimensions are represented on the retina through depth cues (monocular, binocular)

that change as you move through space

2. Monocular (or Pictorial) Depth Cues – aspects of a scene that yield information about depth when

viewed with only one eye a. These cues rely on the relationship between distance and size

i. Relative Size – the retinal image of an object you’re focused on grows smaller as that object

moves farther away, and larger as it moves closer

ii. Familiar Size – many common objects (e.g., adult people) fall within a familiar range of

heights (i.e., 5-7 feet tall), so the retinal image size alone is usually a reliable cue for distance

iii. Linear Perspective – phenomenon that parallel lines seem to converge as they recede into

the distance

iv. Texture Gradient – when viewing a uniform pattern, the size of the pattern elements, as well

as the distance between them, grows smaller as the surface recedes from the observer

v. Interposition – when one object partially blocks another object, the one doing the blocking is

usually closer than the one being blocked

vi. Relative Height in the Image – objects lower in your field of vision are closer to you; higher

objects farther away 3. Binocular Depth Cues

a. Stereoscopic Vision – having space between our eyes means that each eye registers a slightly

different view of the world

b. Binocular Disparity – the difference in the retinal images of the two eyes that provides

information about depth 4. Motion-Based Depth Cues

a. Motion Parallax – a depth cue based on the movement of the head over time (when moving, far

away objects don’t tend to move much whereas closer objects move by quickly)

5. Illusions of Depth and Size

a. Ames Room – a trapezoidal room that makes two identically-sized people appear to be grossly

different in size based on the room cues when viewed with one eye

b. Moon Illusion – the full moon appears larger when it is at the horizon than high in the sky,

because other objects (hills, trees, etc.) intervene between the moon and the viewer D. Perceiving Motion and Change 6. Motion Perception

a. Neural circuits detect changes in the stimulation location on the retina of an object (as it moves

through a stationary visual field) as different speeds and directions of motion

b. The brain also detects your head and eye movements and subtracts those movements from the

motion of the retinal image when you are moving

c. Waterfall Illusion – staring at the downward flow of a waterfall for several seconds results in the

upward motion of surrounding trees and rocks when you look away from the waterfall (similar to color afterimages)

i. If motion-sensitive neurons (in area MT of the temporal lobe) are fatigued through

adaptation, the opposing sensor will take over

d. Apparent Motion – the perception of movement as a result of alternating signals appearing in

rapid succession in different locations

i. Motion picture sampling rate is 24 frames per second (fps) to reproduce the continuous

motion of the original scene 7. Change Blindness and Inattentional Blindness a. Change Blindness – failure to detect changes to the visual details of a scene i. Walk by same store window every day and notice new suit or dress b. Inattentional Blindness – failure to perceive objects that are not focus of attention

V. Audition: More Than Meets the Ear A. Hearing can be measured via sound waves––changes in air pressure unfolding over time B. Sensing Sound

1. Pure Tone – a simple sound wave that first increases air pressure and then creates a relative vacuum

2. Pitch (high or low) is determined by the frequency (wavelength) of the sound wave, measured in

cycles per second, or hertz (Hz) a. Changes in frequency over time allow for the identification of the origin of the sound

3. Loudness (sound intensity) is determined by the amplitude (height) of the sound wave, measured

in decibels (dBs) a. Excess of 85 dB can cause hearing damage

4. Timbre (sound quality or resonance) is determined by the complexity of the sound waves or their

mix of frequencies C. The Human Ear 1. Outer Ear collects sound waves and funnels them toward the middle ear a. Pinna – visible part of the ear outside of the head b. Auditory Canal

c. Eardrum – an airtight flap of skin that vibrates in response to sound waves 2. Middle Ear transmits vibrations to the inner ear

a. Contains three tiny bones, or ossicles (hammer, anvil, and stirrup) that form a lever for

transmitting and intensifying vibrations from the eardrum 3. Inner Ear is embedded in the skull, transduces vibrations into neural impulses a. Cochlea – a fluid-filled tube that is the organ of auditory transduction

i. Basilar Membrane – a structure in the inner ear that undulates when vibrations from the

ossicles reach the cochlear fluid

(a) Hair Cells – specialized auditory receptor neurons embedded in the basilar membrane,

causing the release of neurotransmitter, initiating a neural signal in the auditory nerve that goes to the brain

D. Perceiving Pitch

1. Signals from the auditory nerve travel to the thalamus, then to the contralateral (or opposite)

hemisphere of the auditory cortex 2. Area A1 – a portion of the temporal lobe that contains the primary auditory cortex

a. Typically the left hemisphere processes sounds related to language, and the right hemisphere

processes rhythmic sounds and music

b. Topographically organized, area A1 responds to sounds between 20 and 20,000 Hz in a young

adult, with the most sensitive frequencies around 1,000 to 3,000 Hz

3. Place Code – the cochlea encodes different frequencies at different locations along the basilar

membrane

a. Best at high frequencies, stimulating the base of the membrane versus the less accurate

stimulation of the tip (apex)

4. Temporal Code – the cochlea registers low frequencies via the firing rate of action potentials

entering the auditory nerve

E. Localizing Sound Sources 1. The placement of our ears on the opposite sides of the head gives us stereophonic hearing a. Sound arriving at the ear closer to the sound is louder than sound arriving at the far ear VI. The Body Senses: More Than Skin Deep

A. Somatosenses, or body senses, are for detecting things close to our bodies

B. Touch

1. Haptic Perception – the active exploration of the environment by touching and grasping objects

with our hands

2. Four receptors underneath the skin provide a rich tactile experience by sensing pressure,

texture, pattern, or vibrations

a. Touch receptors have receptive fields much like visual receptive fields, with excitatory centers

surrounded by inhibitory zones 3. A set of thermoreceptors add the dimension of skin temperature 4. Neural representation of the body’s surface a. There is contralateral organization

b. More of the tactile brain is devoted to parts of the skin surface that have greater spatial

resolution (e.g., fingertips and lips) C. Pain

1. Pain indicates damage or potential damage to the body

a. Congenital Insensitivity to Pain – a rare inherited disorder that specifically impairs pain

perception; tends to increase childhood mortality b. Tissue damage is transduced by pain receptors

i. A-delta fibers – transmit the initial sharp pain from sudden injury ii. C-fibers – transmit the longer-lasting, duller pain that persists after the initial injury c. Two pain pathways

i. Somatosensory cortex identifies where the pain is occurring and what sort of pain it is

(sharp, burning, dull)

ii. Motivational and emotional areas (amygdala, hypothalamus, frontal cortex) to motivate the

escape from the source of the pain

d. Referred Pain – feeling of pain when sensory information from internal and external areas

converge on the same nerve cells in the spinal cord

i. Example: Heart attack victims feel pain radiating in the left arm rather than inside of the

chest

e. Pain type and pain intensity show a less-than-perfect correlation

i. Gate-Control Theory – a theory of pain perception based on the idea that signals arriving

from pain receptors in the body can be stopped, or gated, by interneurons in the spinal cord via feedback from two directions

(a) Feedback from the periacqueductal gray (PAG) can send inhibitory signals to the spinal cord to stop pain perception in the brain when stress activates the PAG via endorphin release or opiate drugs

(b) Feedback can increase the perception of pain when you are ill to discourage strenuous activity and encourage healing

(c) Bottom-up Control – senses feed information to the brain (d) Top-down Control – brain exerts control over what we sense D. Body Position, Movement, and Balance

1. Sensations related to position, movement, and balance depend on stimulation produced within

our bodies

a. Receptors in the muscles, tendons, and joints signal the position of the body in space,

whereas information about balance and head movement originate in the inner ear

2. Vestibular System – the three fluid-filled semicircular canals and adjacent organs located next to

the cochlea in each inner ear

a. Canals are in perpendicular orientations and have hair cells to detect movement and

acceleration of the head 3. Vision also helps to maintain balance

VII. The Chemical Senses: Adding Flavor

A. The chemical senses of olfaction (smell) and gustation (taste) respond to the molecular structure of substances floating in the nasal cavity as you inhale or dissolving in saliva as you ingest

1. Smell and taste combine to produce the perceptual experience we call flavor B. Smell

1. Olfactory signals are sent directly to the frontal lobe, amygdala, and other parts of the forebrain without stopping at the thalamus first

2. Odorant molecules released from substances travel through the air into our noses and interact with olfactory receptor neurons (ORNs) in the olfactory epithelium, sending action potentials down the olfactory nerve

a. Olfactory Receptor Neurons – receptor cells that initiate the sense of smell i. Humans possess about 350 types that permit about 10,000 odor discriminations

b. Olfactory Bulb – a brain structure located above the nasal cavity and beneath the frontal lobes

i. Glomerulus – a site where ORNs converge in the olfactory bulb to begin perception of odorants

3. Adaptation to smell occurs after initial detection, much like what occurs in vision and hearing

4. Pheromones – biochemical odorants emitted by other members of a species that can affect an animal’s behavior or physiology

a. Parents can identify their children

b.

A testosterone-based odor activated the hypothalamus in heterosexual women but not heterosexual men, and the reverse was true for an estrogen-based odor; but homosexual men responded to the testosterone-based odor like heterosexual women did and lesbian women responded to the estrogen-based odor like heterosexual men did

c. Women living in close proximity tend to synchronize their menstrual periods C. Taste 1. A primary function of taste is to identify things that are poisonous and lethal a. Aversion to bitterness is partly genetic and partly learned 2. Tongue is covered in papillae, or small bumps, with each one containing hundreds of taste buds a. Taste Bud – the organ of taste transduction i. Approximately 5,000 to 10,000 cover the tongue, roof of the mouth, and upper throat ii. Each bud contains 50 to 100 taste receptor cells (a)You lose about 50% of taste receptors by age 20

(b)Five main types of taste receptors: salt (NaCl), sour (acids), bitter, sweet, and umami (savory; responding to high protein and MSG)

(c)Tips of receptors, called microvilli, react with tastant molecules in food 3. Taste and smell collaborate to produce the complex perception of flavor

a. Odorants from substances outside the mouth enter the nasal cavity via the nostrils, and odorants in the mouth enter through the back of the throat