04 | Sensation and Perception

selective attention

focusing conscious awareness on a particular stimulus.

inattentional blindness

failing to see visible objects when our attention is directed elsewhere.

change blindness

failing to notice changes in the environment; a form of inattentional blindness.

perceptual set

a mental predisposition to perceive one thing and not another.

gestalt

an organized whole. Gestalt psychologists emphasized our tendency to integrate pieces of information into meaningful wholes.

figure-ground

the organization of the visual field into objects (the figures) that stand out from their surroundings (the ground).

grouping

the perceptual tendency to organize stimuli into coherent groups.

depth perception

the ability to see objects in three dimensions, although the images that strike the retina are two-dimensional; allows us to judge distance.

visual cliff

a laboratory device for testing depth perception in infants and young animals.

binocular cue

a depth cue, such as retinal disparity, that depends on the use of two eyes.

convergence

a cue to nearby objects' distance, enabled by the brain combining retinal images.

retinal disparity

a binocular cue for perceiving depth. By comparing retinal images from the two eyes, the brain computes distance — the greater the disparity (difference) between the two images, the closer the object.

monocular cue

a depth cue, such as interposition or linear perspective, available to either eye alone.

stroboscopic movement

an illusion of continuous movement (as in a motion picture) experienced when viewing a rapid series of slightly varying still images.

phi phenomenon

an illusion of movement created when two or more adjacent lights blink on and off in quick succession.

autokinetic effect

the illusory movement of a still spot of light in a dark room.

perceptual constancy

perceiving objects as unchanging (having consistent color, brightness, shape, and size) even as illumination and retinal images change.

color constancy

perceiving familiar objects as having consistent color, even if changing illumination alters the wavelengths reflected by the object.

perceptual adaptation

the ability to adjust to changed sensory input, including an artificially displaced or even inverted visual field.

sensation

the process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment.

sensory receptors

sensory nerve endings that respond to stimuli.

perception

the process by which our brain organizes and interprets sensory information, enabling us to recognize objects and events as meaningful.

bottom-up processing

information processing that begins with the sensory receptors and works up to the brain's integration of sensory information.

top-down processing

information processing guided by higher-level mental processes, as when we construct perceptions drawing on our experience and expectations.

transduction

conversion of one form of energy into another. In sensation, the transforming of physical energy, such as sights, sounds, and smells, into neural impulses the brain can interpret.

psychophysics

the study of relationships between the physical characteristics of stimuli, such as their intensity, and our psychological experience of them.

absolute threshold

the minimum stimulus energy needed to detect a particular stimulus 50 percent of the time.

signal detection theory

a theory predicting how and when we detect the presence of a faint stimulus (signal) amid background stimulation (noise); assumes there is no single absolute threshold and that detection depends partly on a person's experience, expectations, motivation, and alertness.

subliminal

below one's absolute threshold for conscious awareness.

priming

the activation, often unconsciously, of certain associations, thus predisposing one's perception, memory, or response.

difference threshold

the minimum difference between two stimuli required for detection 50 percent of the time. We experience the difference threshold as a just noticeable difference (jnd).

Weber's law

the principle that, to be perceived as different, two stimuli must differ by a constant minimum percentage (rather than a constant amount).

sensory adaptation

diminished sensitivity as a consequence of constant stimulation.

wavelength

the distance from the peak of one light wave or sound wave to the peak of the next. Electromagnetic wavelengths vary from the short gamma waves to the long pulses of radio transmission.

hue

the dimension of color that is determined by the wavelength of light; what we know as the color names blue, green, and so forth.

intensity

the amount of energy in a light wave or sound wave, which influences what we perceive as brightness or loudness. Intensity is determined by the wave's amplitude (height).

cornea

the eye's clear, protective outer layer, covering the pupil and iris.

pupil

the adjustable opening in the center of the eye through which light enters.

iris

a ring of muscle tissue that forms the colored portion of the eye around the pupil and controls the size of the pupil opening.

lens

the transparent structure behind the pupil that changes shape to help focus images on the retina.

retina

the light-sensitive back inner surface of the eye, containing the receptor rods and cones plus layers of neurons that begin the processing of visual information.

accommodation

the process by which the eye's lens changes shape to focus images of near or far objects on the retina.

rods

retinal receptors that detect black, white, and gray, and are sensitive to movement. Rods are necessary for peripheral and twilight vision, when cones don't respond.

cones

retinal receptors that are concentrated near the center of the retina and that function in daylight or in well-lit conditions. Cones detect fine detail and give rise to color sensations.

optic nerve

the nerve that carries neural impulses from the eye to the brain.

blind spot

the point at which the optic nerve leaves the eye, creating a "blind" spot because no receptor cells are located there.

fovea

the central focal point in the retina, around which the eye's cones cluster.

three-color

theory

the theory that the retina contains three different types of color receptors — one most sensitive to red, one to green, one to blue — which, when stimulated in combination, can produce the perception of any color.

opponent-process theory

the theory that opposing retinal processes (red-green, blue-yellow, white-black) enable color vision. For example, some cells are stimulated by green and inhibited by red; others are stimulated by red and inhibited by green.

feature detectors

nerve cells in the brain's visual cortex that respond to specific features of the stimulus, such as shape, angle, or movement.

audition

the sense or act of hearing.

frequency

the number of complete wavelengths that pass a point in a given time (for example, per second).

pitch

a tone's experienced highness or lowness; depends on frequency.

middle ear

the chamber between the eardrum and the cochlea containing three tiny bones that concentrate the vibrations of the eardrum on the cochlea's oval window.

cochlea

a coiled, bony, fluid-filled tube in the inner ear; sound waves traveling through the cochlear fluid trigger nerve impulses.

inner ear

the innermost part of the ear, containing the cochlea, semicircular canals, and vestibular sacs.

sensorineural hearing loss

the most common form of hearing loss, caused by damage to the cochlea's receptor cells or to the auditory nerve; also called nerve deafness.

conduction hearing loss

a less common form of hearing loss, caused by damage to the mechanical system that conducts sound waves to the cochlea.

cochlear implant

a device for converting sounds into electrical signals and stimulating the auditory nerve through electrodes threaded into the cochlea.

place theory

in hearing, the theory that links the pitch we hear with the place where the cochlea's membrane is stimulated. (Also called place coding.)

frequency theory

in hearing, the theory that the rate of nerve impulses traveling up the auditory nerve matches the frequency of a tone, thus enabling us to sense its pitch. (Also called temporal coding.)

gate-control theory

the theory that the spinal cord contains a neurological "gate" that blocks pain signals or allows them to pass on to the brain. The "gate" is opened by the activity of pain signals traveling up small nerve fibers, and is closed by activity in larger fibers or by information coming from the brain.

gustation

our sense of taste.

olfaction

our sense of smell.

kinesthesis

our movement sense; our system for sensing the position and movement of individual body parts.

vestibular sense

our balance sense; our sense of body movement and position that enables our sense of balance.

sensory interaction

the principle that one sense can influence another, as when the smell of food influences its taste.

embodied cognition

the influence of bodily sensations, gestures, and other states on cognitive preferences and judgments.

shape constancy

we perceive the form of familiar objects, such as a door,, as constant even while our retinas receive changing images of them.

brightness constancy

similarly depends on context. We perceive an object as having a constant brightness even as its illumination varies.depends on relative luminance — the amount of light an object reflects relative to its surroundings

size constancy,

we perceive an object as having an unchanging size, even while our distance from it varies. We assume a bus is large enough to carry people, even when we see its tiny image from two blocks away

relative clarity

Because more light passes through objects that are farther away, we perceive these objects as hazy, blurry, or unclear. Nearby objects, by contrast, appear sharp and clear.

relative size

if we assume two objects are similar in size, most people perceive the one that casts the smaller retinal image as farther away

texture gradient

moving toward or away from an object changes our perception of its smoothness or texture. When we view a wall from a distance, we will perceive it as smooth, but the same wall up close will reveal greater texture and detail

linear perspective

parallel lines appear to converge with distance

interposition

if one object partially blocks our view of another, we perceive it as closer