Glossary

A B C D E F G H I J K L M N O P Q R S T U V W

A

A-beta fiber: A wide-diameter, myelinated sensory nerve fiber that transmits signals from mechanical stimulation.
A-delta fiber: An intermediate-sized, myelinated sensory nerve fiber that transmits pain and temperature signals.
abducens (VI) nerves: The sixth pair of cranial nerves, which innervate the lateral rectus muscle of the eyeballs.
absolute metrical depth cue: A depth cue that provides quantifiable information about distance in the third dimension (e.g., his nose sticks out 4 centimeters in front of his face).
absolute pitch: A rare ability whereby some people are able to very accurately name or produce notes without comparison to other notes.
absolute threshold: The minimum amount of stimulation necessary for a person to detect a stimulus 50% of the time.
absorb: To take up something—such as light, noise, or energy—and not transmit it at all.
acceleration: A change in velocity. Mathematically, acceleration is the derivative of velocity. In words, linear acceleration indicates a change in linear velocity; angular acceleration indicates a change in angular velocity.
accessory olfactory bulb (AOB): A smaller neural structure located behind the main olfactory bulb that receives input from the vomeronasal organ.
accidental viewpoint: A viewing position that produces some regularity in the visual image that is not present in the world (e.g., the sides of two independent objects lining up perfectly).
accommodation: The process by which the eye changes its focus (in which the lens gets fatter as gaze is directed toward nearer objects).
achromatopsia: An inability to perceive colors that is caused by damage to the central nervous system.
acoustic reflex: A reflex that protects the ear from intense sounds, via contraction of the stapedius and tensor tympani muscles.
acoustic startle reflex: The very rapid motor response to a sudden sound. Very few neurons are involved in the basic startle reflex, which can also be affected by emotional state.
active sensing: Sensing that includes self-generated probing of the environment. Besides our sense of equilibrium, other active human senses include vision and touch. Animal active sensing includes the use of echoes by whales and bats, the use of electrical signals by some fish, and the use of whiskers/antennae by fish, insects, and nocturnal rodents.
acuity: The smallest spatial detail that can be resolved at 100% contrast.
adaptation: A reduction in response caused by prior or continuing stimulation.
adapting stimulus: A stimulus whose removal produces a change in visual perception or sensitivity.
additive color mixture: A mixture of lights. If light A and light B are both reflected from a surface to the eye, in the perception of color the effects of those two lights add together.
aerial perspective or haze: A depth cue based on the implicit understanding that light is scattered by the atmosphere. More light is scattered when we look through more atmosphere. Thus, more distant objects are subject to more scatter and appear fainter, bluer, and less distinct.
afferent fiber: A neuron that carries sensory information to the central nervous system. Compare efferent fiber.
afferent signals: Information flowing inward to the central nervous system from sensors in the periphery. Passive sensing would rely exclusively on such sensory inflow, providing a traditional view of sensation. See also afferent fiber.
age-related macular degeneration (AMD): A disease associated with aging that affects the macula. AMD gradually destroys sharp central vision, making it difficult to read, drive, and recognize faces. There are two forms of AMD: wet and dry.
agnosia: A failure to recognize objects in spite of the ability to see them. Agnosia is typically due to brain damage.
akinetopsia: A rare neuropsychological disorder in which the affected individual has no perception of motion.
amacrine cell: A retinal cell found in the inner synaptic layer that makes synaptic contacts with bipolar cells, ganglion cells, and other amacrine cells.
ambiguous figure: A visual stimulus that gives rise to two or more interpretations of its identity or structure.
amblyopia: A developmental disorder characterized by reduced spatial vision in an otherwise healthy eye, even with proper correction for refractive error. Also known as lazy eye.
amplitude or intensity: The magnitude of displacement (increase or decrease) of a sound pressure wave. Amplitude is perceived as loudness.
amplitude: The size (increase or decrease) of a head movement (e.g., angular velocity, linear acceleration, tilt).
ampulla: An expansion of each semicircular-canal duct that includes that canal’s cupula, crista, and hair cells, where transduction occurs.
amygdala-hippocampal complex: The conjoined regions of the amygdala and hippocampus, which are key structures in the limbic system. This complex is critically involved in the unique emotional and associative properties of olfactory cognition.
analgesia: Decreasing pain sensation during conscious experience.
anamorphosis or anamorphic projection: Use of the rules of linear perspective to create a two-dimensional image so distorted that it looks correct only when viewed from a special angle or with a mirror that counters the distortion.
angular acceleration: The rate of change of angular velocity. Mathematically, the integral of angular acceleration is angular velocity, and the integral of angular velocity is angular displacement. Angular acceleration, angular velocity, and angular displacement all mathematically represent angular motion.
angular motion: Rotational motion like the rotation of a spinning top or swinging saloon doors that rotate back and forth.
anisometropia: A condition in which the two eyes have different refractive errors (e.g., one eye is farsighted and the other not).
anomia: An inability to name objects in spite of the ability to see and recognize them (as shown by usage). Anomia is typically due to brain damage.
anosmia: The total inability to smell, most often resulting from sinus illness or head trauma.
anterior cingulate cortex (ACC): A region of the brain associated with the perceived unpleasantness of a pain sensation.
aperture problem: The fact that when a moving object is viewed through an aperture (or a receptive field), the direction of motion of a local feature or part of the object may be ambiguous.
aperture: An opening that allows only a partial view of an object.
apparent motion: The illusory impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession.
aqueous humor: The watery fluid in the anterior chamber of the eye.
aromatherapy: The manipulation of odors to influence, mood, performance, and well-being, as well as the physiological correlates of emotion such as heart rate, blood pressure, and sleep.
articulation: The act or manner of producing a speech sound using the vocal tract.
astigmatism: A visual defect caused by the unequal curving of one or more of the refractive surfaces of the eye, usually the cornea.
attack: The part of a sound during which amplitude increases (onset).
attention deficit hyperactivity disorder (ADHD): A quite common childhood disorder that can continue into adulthood. Symptoms include difficulty focusing attention as well as problems controlling behavior.
attention: Any of the very large set of selective processes in the brain. To deal with the impossibility of handling all inputs at once, the nervous system has evolved mechanisms that are able to bias processing to a subset of things, places, ideas, or moments in time.
attentional blink: The tendency not to perceive or respond to the second of two different target stimuli amid a rapid stream of distracting stimuli if the observer has responded to the first target stimulus within 200–500 milliseconds before the second stimulus is presented.
audibility threshold: The lowest sound pressure level that can be reliably detected at a given frequency.
auditory nerve: A collection of neurons that convey information from hair cells in the cochlea to (afferent) and from (efferent) the brain stem.
auditory stream segregation: The perceptual organization of a complex acoustic signal into separate auditory events for which each stream is heard as a separate event.
autonomic nervous system: The part of the nervous system that innervates glands, heart, digestive system, and so on, and that is responsible for regulating many involuntary actions.
azimuth: The angle of a sound source on the horizontal plane relative to a point in the center of the head between the ears. Azimuth is measured in degrees, with 0 degrees being straight ahead. The angle increases clockwise toward the right, with 180 degrees being directly behind.

B

balance system: The sensory systems, neural processes, and muscles that contribute to postural control. Specific components include the vestibular organs, kinesthesis, vestibulo-spinal pathways, skeletal bones, and postural control muscles. Because of its crucial contributions to balance, some even informally refer to the vestibular system as the “balance system” and the vestibular organs as the “balance organs.” But the balance system is much more than just the vestibular system, and the vestibular system contributes to more than just balance.
balance: The neural processes of postural control by which weight is evenly distributed, enabling us to remain upright and stable.
basal cell: One of the three types of cells in the olfactory epithelium. Basal cells are the precursor cells to olfactory sensory neurons.
basic color terms: Color words that are monolexemic (single words “blue” not, “sky blue”), used with high frequency, and have meanings that are agreed upon by speakers of a language.
basic taste: Any of the four taste qualities that are generally agreed to describe human taste experience: sweet, salty, sour, bitter.
basilar membrane: A plate of fibers that forms the base of the cochlear partition and separates the middle and tympanic canals in the cochlea.
Bayesian approach: A way of formalizing the idea that our perception is a combination of the current stimulus and our knowledge about the conditions of the world—what is and is not likely to occur. The Bayesian approach is stated mathematically as Bayes’ theorem—P(A|O) = P(A) x P(O|A)/P(O)—which enables us to calculate the probability (P) that the world is in a particular state (A) given a particular observation (O).
belt area: A region of cortex, directly adjacent to the primary auditory cortex (A1), with inputs from A1, where neurons respond to more complex characteristics of sounds.
binaral rivalry: Competition between the two nostrils for odor perception. When a different scent is presented to each nostril simultaneously, we perceive each scent to be alternating back and forth with the other, and not a blend of the two scents.
binding problem: The challenge of tying different attributes of visual stimuli (e.g., color, orientation, motion), which are handled by different brain circuits, to the appropriate object so that we perceive a unified object (e.g., red, vertical, moving right).
binocular depth cue: A depth cue that relies on information from both eyes. Stereopsis is the primary example in humans, but convergence and the ability of two eyes to see more of an object than one eye sees are also binocular depth cues.
binocular disparity: The differences between the two retinal images of the same scene. Disparity is the basis for stereopsis, a vivid perception of the three-dimensionality of the world that is not available with monocular vision.
binocular rivalry: The competition between the two eyes for control of visual perception, which is evident when completely different stimuli are presented to the two eyes.
binocular summation: The combination (or “summation”) of signals from each eye in ways that make performance on many tasks better with both eyes than with either eye alone.
binocular: With two eyes.
biological motion: The pattern of movement of living beings (humans and animals).
bipolar cell: A retinal cell that synapses with either rods or cones (not both) and with horizontal cells, and then passes the signals on to ganglion cells.
bitter: One of the four basic tastes; the taste quality, generally considered unpleasant, produced by substances like quinine or caffeine.
blood oxygen level–dependent (BOLD) signal: The ratio of oxygenated to deoxygenated hemoglobin that permits the localization of brain neurons that are most involved in a task.
body image: The impression of our bodies in space.

C

C fiber: A narrow-diameter, unmyelinated sensory nerve fiber that transmits pain and temperature signals.
C tactile (CT) afferent: A narrow-diameter, unmyelinated sensory nerve fiber that transmits signals from pleasant touch.
cataract: An opacity of the crystalline lens.
categorical perception: For speech as well as other complex sounds and images, the phenomenon by which the discrimination of items is no better than the ability to label items.
change blindness: The failure to notice a change between two scenes. If the gist, or meaning, of the scene is not altered, quite large changes can pass unnoticed.
characteristic frequency (CF): The frequency to which a particular auditory nerve fiber is most sensitive.
chemosignal: Any of various chemicals emitted by humans that are detected by the olfactory system and that may have some effect on the mood, behavior, hormonal status, and/or sexual arousal of other humans.
chord: A combination of three or more musical notes with different pitches played simultaneously.
chorda tympani: The branch of cranial nerve VII (the facial nerve) that carries taste information from the anterior, mobile tongue (the part that can be stuck out). The chorda tympani exits the tongue with the lingual branch of the trigeminal nerve (cranial nerve V) and then passes through the middle ear on its way to the brain.
chromophore: The light-catching part of the visual pigments of the retina.
cilium: Any of the hairlike protrusions on the dendrites of olfactory sensory neurons. The receptor sites for odorant molecules are on the cilia, which are the first structures involved in olfactory signal transduction.
circadian: Referring to cells in the magnocellular layers of the lateral geniculate nucleus of the thalamus. Magno from the Greek for “large” referring to the size of the cells.
circumvallate papillae: Circular structures that form an inverted V on the rear of the tongue (three to five on each side, with the largest in the center). Circumvallate papillae are moundlike structures, each surrounded by a trench (like a moat). These papillae are much larger than fungiform papillae.
closure: In reference to perception, closure is the name of a Gestalt principle that holds that a closed contour is preferred to an open contour.
coarticulation: The phenomenon in speech whereby attributes of successive speech units overlap in articulatory or acoustic patterns.
cochlea: A spiral structure of the inner ear containing the organ of Corti.
cochlear nucleus: The first brain stem nucleus at which afferent auditory nerve fibers synapse.
cochlear partition: The combined basilar membrane, tectorial membrane, and organ of Corti, which are together responsible for the transduction of sound waves into neural signals.
cognitive habituation: The psychological process by which, after long-term exposure to an odor, one no longer has the ability to detect that odor or has very diminished detection ability.
cold fiber: A sensory nerve fiber that fires when skin temperature decreases.
color assimilation: A color perception effect in which two colors bleed into each other, each taking on some of the chromatic quality of the other.
color constancy: The tendency of a surface to appear the same color under a fairly wide range of illuminants.
color contrast: A color perception effect in which the color of one region induces the opponent color in a neighboring region.
color space: The three-dimensional space, established because color perception is based on the outputs of three cone types, that describes the set of all colors.
color-anomalous: A better term for what is usually called “color-blind.” Most “color-blind” individuals can still make discriminations based on wavelength. Those discriminations are different from the norm—that is, anomalous.
column: A vertical arrangement of neurons. Neurons within a single column tend to have similar receptive fields and similar orientation preferences.
comparator: An area of the visual system that receives one copy of the command issued by the motor system when the eyes move (the other copy goes to the eye muscles). The comparator compares the image motion signal with the eye motion signal and can compensate for the image changes caused by the eye movement.
complex cell: A cortical neuron whose receptive field does not have clearly defined excitatory and inhibitory regions.
computed tomography (CT): An imaging technology that uses X-rays to create images of slices through volumes of material (e.g., the human body).
conductive hearing loss: Hearing loss caused by problems with the bones of the middle ear.
cone: A photoreceptor specialized for daylight vision, fine visual acuity, and color.
cone monochromat: An individual with only one cone type. Cone monochromats are truly color-blind.
cone of confusion: A region of positions in space where all sounds produce the same time and level (intensity) differences (ITDs and ILDs).
cone-opponent cell: A cell type—found in the retina, lateral geniculate nucleus, and visual cortex—that, in effect, subtracts one type of cone input from another.
congenital prosopagnosia: A form of “face blindness” apparently present from birth, as opposed to “acquired prosopagnosia,” which would typically be the result of an injury to the nervous system.
conjunction search: Search for a target defined by the presence of two or more attributes (e.g., a red, vertical target among red horizontal and blue vertical distractors).
continuity constraint: In stereopsis, the observation that, except at the edges of objects, neighboring points in the world lie at similar distances from the viewer. This is one of several constraints that have been proposed as helpful in solving the correspondence problem.
contralateral: Referring to the opposite side of the body (or brain).
contralesional field: The visual field on the side opposite a brain lesion. For example, points to the left of fixation are contralesional to damage in the right hemisphere of the brain.
contrast sensitivity function (CSF): A function describing how the sensitivity to contrast (defined as the reciprocal of the contrast threshold) depends on the spatial frequency (size) of the stimulus.
contrast threshold: The smallest amount of contrast required to detect a pattern.
contrast: The difference in luminance between an object and the background, or between lighter and darker parts of the same object.
convergence: The ability of the two eyes to turn inward, often used in order to place the two images of a feature in the world on corresponding locations in the two retinal images (typically on the fovea of each eye). Convergence reduces the disparity of that feature to zero (or nearly zero).
cornea: The transparent “window” into the eyeball.
correspondence problem: 1. In binocular vision, the problem of figuring out which bit of the image in the left eye should be matched with which bit in the right eye. The problem is particularly vexing when the images consist of thousands of similar features, like dots in random dot stereograms. 2.In motion detection, the problem faced by the motion detection system of knowing which feature in frame 2 corresponds to a particular feature in frame 1.
corresponding retinal points: Two monocular images of an object in the world are said to fall on corresponding points if those points are the same distance from the fovea in both eyes. The two foveas are also corresponding points.
cortical magnification: The amount of cortical area (usually specified in millimeters) devoted to a specific region (e.g., 1 degree) in the visual field.
cranial nerves: Twelve pairs of nerves (one for each side of the body) that originate in the brain stem and reach sense organs and muscles through openings in the skull.
cribriform plate: A bony structure riddled with tiny holes, at the level of the eyebrows, that separates the nose from the brain. The axons from the olfactory sensory neurons pass through the tiny holes of the cribriform plate to enter the brain.
crista: Any of the specialized detectors of angular motion located in each semicircular canal in a swelling called the ampulla.
criterion: In signal detection theory, an internal threshold that is set by the observer. If the internal response is above criterion, the observer gives one response (e.g., “yes, I hear that”). Below criterion, the observer gives another response (e.g., “no, I hear nothing”).
critical bandwidth: The range of frequencies conveyed within a channel in the auditory system.
critical period: A phase in the life span during which abnormal early experience can alter normal neuronal development. Critical periods are proposed for the development of binocular vision and development of a first human language.
cross-adaptation: The reduction in detection of one odorant following exposure to another odorant. Cross-adaptation is presumed to occur because the components of the odors (or the odorants) in question share one or more olfactory receptors for their transduction, but the order in which odorants are presented also plays a role.
cross-modality matching: The ability to match the intensities of sensations that come from different sensory modalities. This ability allows insight into sensory differences. For example, a listener might adjust the brightness of a light until it matches the loudness of a tone.
crossed disparity: The sign of disparity created by objects in front of the plane of fixation (the horopter). The term crossed is used because images of objects located in front of the horopter appear to be displaced to the left in the right eye, and to the right in the left eye.
cue: A stimulus that might indicate where (or what) a subsequent stimulus will be. Cues can be valid (giving correct information), invalid (incorrect), or neutral (uninformative).
cultural relativism: In sensation and perception, the idea that basic perceptual experiences (e.g., color perception) may be determined in part by the cultural environment.
cycle: For a grating, a pair consisting of one dark bar and one bright bar.
cycles per degree: The number of pairs of dark and bright bars (cycles of a grating) per degree of visual angle.
Cyclopean: Referring to stimuli that are defined by binocular disparity alone. Named after the one-eyed Cyclops of Homer’s Odyssey.
cytochrome oxidase (CO): An enzyme used to reveal the regular array of “CO blobs,” which are spaced about 0.5 millimeter apart in the primary visual cortex.

D

decay: The part of a sound during which amplitude decreases (offset).
decibel (dB): A unit of measure for the physical intensity of sound. Decibels define the difference between two sounds as the ratio between two sound pressures. Each 10:1 sound pressure ratio equals 20 dB, and a 100:1 ratio equals 40 dB.
dermis: The inner of two major layers of skin, consisting of nutritive and connective tissues, within which lie the mechanoreceptors.
deuteranope: An individual who suffers from color blindness that is due to the absence of M-cones.
dichoptic: Referring to the presentation of two different stimuli, one to each eye. Different from binocular presentation, which could involve both eyes looking at a single stimulus.
diffuse bipolar cell: A bipolar retinal cell whose processes are spread out to receive input from multiple cones.
diopter (D): A unit of measurement of the optic power of a lens. It is equal to the reciprocal of the focal length, in meters. A 2-diopter lens will bring parallel rays of light into focus at ½ meter (50 cm).
diplopia: Double vision. If visible in both eyes, stimuli falling outside of Panum’s fusional area will appear diplopic.
direction: The line one moves along or faces, with reference to the point or region one is moving toward or facing.
directional transfer function (DTF): A measure that describes how the pinna, ear canal, head, and torso change the intensity of sounds with different frequencies that arrive at each ear from different locations in space (azimuth and elevation).
distractor: In visual search, any stimulus other than the target.
divergence: The ability of the two eyes to turn outward, often used in order to place the two images of a feature in the world on corresponding locations in the two retinal images (typically on the fovea of each eye). Divergence reduces the disparity of that feature to zero (or nearly zero).
dizziness: A commonly used lay term that nonspecifically indicates any form of perceived spatial disorientation, with or without instability.
doctrine of specific nerve energies: A doctrine, formulated by Johannes Müller, stating that the nature of a sensation depends on which sensory fibers are stimulated, not on how fibers are stimulated.
dorsal column–medial lemniscal (DCML) pathway: The route from the spinal cord to the brain that carries signals from skin, muscles, tendons, and joints.
dorsal horn: A region at the rear of the spinal cord that receives inputs from receptors in the skin.
double dissociation: The phenomenon in which one of two functions—such as hearing and sight, or first- and second- order motion—can be damaged without harm to the other, and vice versa.
double-opponent cell: A cell type, found in the visual cortex, in which one region is excited by one cone type, combination of cones, or color and inhibited by the opponent cones or color (e.g., R+/G–). Another adjacent region would be inhibited by the first input and excited by the second (thus, in this example, R–/G+).
dualism: The idea that the mind has an existence separate from the material world of the body.
duplex: In reference to the retina, consisting of two parts: the rods and cones, which operate under different conditions.

E

ear canal: The canal that conducts sound vibrations from the pinna to the tympanic membrane and prevents damage to the tympanic membrane.
eccentricity: The distance between the retinal image and the fovea.
efferent commands: Information flowing outward from the central nervous system to the periphery. A common example is motor commands that regulate muscle contraction. The copy of such motor commands is often called an “efferent copy.” See also efferent fiber.
efferent fiber: A neuron that carries information from the central nervous system to the periphery. Compare afferent fiber.
egocenter: The center of a reference frame used to represent locations relative to the body.
electroencephalography (EEG): A technique that, using many electrodes on the scalp, measures electrical activity from populations of many neurons in the brain.
emmetropia: The condition in which there is no refractive error, because the refractive power of the eye is perfectly matched to the length of the eyeball.
end stopping: The process by which a cell in the cortex first increases its firing rate as the bar length increases to fill up its receptive field, and then decreases its firing rate as the bar is lengthened further.
endogenous cue: In directing attention, an endogenous cue is located in (endo) or near the current location of attention
endogenous opiate: A chemical released by the body that blocks the release or uptake of neurotransmitters necessary to transmit pain sensations to the brain.
endogenous spatial attention: A form of top-down (knowledge-driven) control of spatial attention in which attention is voluntarily directed toward the site where the observer anticipates a stimulus will occur.
ensemble statistics: The average and distribution of properties like orientation or color over a set of objects or over a region in a scene.
entorhinal cortex: A phylogenetically old cortical region that provides the major sensory association input into the hippocampus. The entorhinal cortex also receives direct projections from olfactory regions.
entry-level category: For an object, the label that comes to mind most quickly when we identify it (e.g., “bird”). At the subordinate level, the object might be more specifically named (e.g., “eagle”); at the superordinate level, it might be more generally named (e.g., “animal”).
epidermis: The outer of two major layers of skin.
equal-loudness curve: A graph plotting sound pressure level (dB SPL) against the frequency for which a listener perceives constant loudness.
equilibrium: In reference to the vestibular system, our vestibular sense comprised of spatial orientation perception—encompassing our perception of linear motion, angular motion, and tilt—combined with reflexive vestibular responses like posture, vestibulo-autonomic reflexes, and vestibulo-ocular reflexes.
esotropia: Strabismus in which one eye deviates inward.
Euclidean: Referring to the geometry of the world, so named in honor of Euclid, the ancient Greek geometer of the third century BCE. In Euclidean geometry, parallel lines remain parallel as they are extended in space, objects maintain the same size and shape as they move around in space, the internal angles of a triangle always add to 180 degrees, and so forth.
event-related potential (ERP): A measure of electrical activity from a subpopulation of neurons in response to particular stimuli that requires averaging many EEG recordings.
exogenous cue: In directing attention, an exogenous cue is located out (exo) at the desired final location of attention.
exogenous spatial attention: A form of bottom-up (stimulus-driven) spatial attention in which attention is reflexively (involuntarily) directed toward the site at which a stimulus has abruptly appeared.
exotropia: Strabismus in which one eye deviates outward.
exploratory procedure: A stereotyped hand movement pattern used to touch objects in order to perceive their properties; each procedure is best for determining one (or more) object properties.
extinction: In visual attention, the inability to perceive a stimulus to one side of the point of fixation (e.g., to the right) in the presence of another stimulus, typically in a comparable position in the other visual field (e.g., on the left side).
extrastriate body area (EBA): A region of extrastriate visual cortex in humans that is specifically and reliably activated by images of the body other than the face.
extrastriate cortex: The region of cortex bordering the primary visual cortex and containing multiple areas involved in visual processing.

F

familiar size: A depth cue based on knowledge of the typical size of objects like humans or pennies.
feature integration theory: Anne Treisman’s theory of visual attention, which holds that a limited set of basic features can be processed in parallel preattentively, but that other properties, including the correct binding of features to objects, require attention.
feature search: Search for a target defined by a single attribute, such as a salient color or orientation.
Fechner’s law: A principle describing the relationship between stimulus and resulting sensation that says the magnitude of subjective sensation increases proportionally to the logarithm of the stimulus intensity.
feed-forward process: A process that carries out a computation (e.g., object recognition) one neural step after another, without need for feedback from a later stage to an earlier stage.
figure-ground assignment: The process of determining that some regions of an image belong to a foreground object (figure) and other regions are part of the background (ground).
filiform papillae: Small structures on the tongue that provide most of the bumpy appearance. Filiform papillae have no taste function.
filter: An acoustic, electrical, electronic, or optic device, instrument, computer program, or neuron that allows the passage of some range of parameters (e.g., orientations, frequencies) and blocks the passage of others.
first-order motion: The motion of an object that is defined by changes in luminance.
flavor: The combination of true taste (sweet, salty, sour, bitter) and retronasal olfaction.
focus of expansion: The point in the center of the horizon from which, when we’re in motion (e.g., driving on the highway), all points in the perspective image seem to emanate. The focus of expansion is one aspect of optic flow.
foliate papillae: Folds of tissue containing taste buds. Foliate papillae are located on the rear of the tongue lateral to the circumvallate papillae, where the tongue attaches to the mouth.
formant: A resonance of the vocal tract. Formants are specified by their center frequency and are denoted by integers that increase with relative frequency.
Fourier analysis: A mathematical procedure by which any signal can be separated into component sine waves at different frequencies. Combining these sine waves will reproduce the original signal.
fovea: A small pit, near the center of the macula, that contains the highest concentration of cones, and no rods. It is the portion of the retina that produces the highest visual acuity and serves as the point of fixation.
frame of reference: The coordinate system used to define locations in space.
free fusion: The technique of converging (crossing) or diverging the eyes in order to view a stereogram without a stereoscope.
frequency: For sound, the number of times per second that a pattern of pressure change repeats. Frequency is perceived as pitch.
functional magnetic resonance imaging (fMRI): A variant of magnetic resonance imaging that makes it possible to measure localized patterns of activity in the brain. Activated neurons provoke increased blood flow, which can be quantified by measuring changes in the response of oxygenated and deoxygenated blood to strong magnetic fields.
fundamental frequency: The lowest-frequency component of a complex periodic sound.
fundus: The back layer of the retina—what the eye doctor sees through an ophthalmoscope.
fungiform papillae: Mushroom-shaped structures (maximum diameter 1 millimeter) that are distributed most densely on the edges of the tongue, especially the tip. Taste buds (an average of six per papilla) are buried in the surface.
fusiform face area (FFA): A region of extrastriate visual cortex in humans that is specifically and reliably activated by human faces.

G

G protein–coupled receptor (GPCR): Any of the class of receptors that are present on the surface of olfactory sensory neurons. All GPCRs are characterized by a common structural feature of seven membrane-spanning α-helices.
ganglion cell: A retinal cell that receives visual information from photoreceptors via two intermediate neuron types (bipolar cells and amacrine cells) and transmits information to the brain and midbrain.
gate control theory: A description of the pain-transmitting system that incorporates modulating signals from the brain.
geon: In Biederman’s recognition-by-components model, any of the “geometric ions” out of which perceptual objects are built.
Gestalt grouping rules: A set of rules describing which elements in an image will appear to group together. The original list was assembled by members of the Gestalt school of thought.
Gestalt: In German, literally “form.” In reference to perception, a school of thought stressing that the perceptual whole could be greater than the apparent sum of the parts.
gestation: Fetal development during pregnancy.
glabrous: In reference to skin, lacking hair.
global superiority effect: The finding in various experiments that the properties of the whole object take precedence over the properties of parts of the object.
glomerulus: Any of the spherical conglomerates containing the incoming axons of the olfactory sensory neurons. Each OSN converges onto two glomeruli (one medial, one lateral).
good continuation: A Gestalt grouping rule stating that two elements will tend to group together if they seem to lie on the same contour.
graded potential: An electrical potential that can vary continuously in amplitude.
granule cells: Like mitral cells, granule cells are at the deepest level of the olfactory bulb. They comprise an extensive network of inhibitory neurons, integrate input from all the earlier projections, and are thought to be the basis of specific odorant identification.
gravity: A force that attracts a body toward the center of the Earth.
guided search: Search in which attention can be restricted to a subset of possible items on the basis of information about the target item’s basic features (e.g., its color).
gustation: The sense of taste.

H

hair cell: Any cell that has stereocilia for transducing mechanical movement in the inner ear into neural activity sent to the brain; some hair cells also receive inputs from the brain.
haptic perception: Knowledge of the world that is derived from sensory receptors in skin, muscles, tendons, and joints, usually involving active exploration.
haptic virtual environment: A synthetic world that may be experienced haptically by operation of an electromechanical device that delivers forces to the hand of the user.
harmonic spectrum: The spectrum of a complex sound in which energy is at integer multiples of the fundamental frequency.
helicotrema: The opening that connects the tympanic and vestibular canals at the apex of the cochlea.
hertz (Hz): A unit of measure for frequency. One hertz equals one cycle per second.
heterodimer: A chain of two molecules that are different from each other.
heuristic: A mental shortcut.
high-spontaneous fiber: An auditory nerve fiber that has a high rate (more than 30 spikes per second) of spontaneous firing; high-spontaneous fibers increase their firing rate in response to relatively low levels of sound.
homologous regions: Brain regions that appear to have the same function in different species.
homunculus: A maplike representation of regions of the body in the brain.
horizontal cell: A specialized retinal cell that contacts both photoreceptor and bipolar cells.
horopter: The location of objects whose images lie on corresponding points. The surface of zero disparity.
hyperalgesia: An increased or heightened response to a normally painful stimulus.
hypercolumn: A 1-millimeter block of striate cortex containing two sets of columns, each covering every possible orientation (0–180 degrees), with one set preferring input from the left eye and one set preferring input from the right eye.
hyperopia: Farsightedness, a common condition in which light entering the eye is focused behind the retina and accommodation is required in order to see near objects clearly.
hyperpolarization: An increase in membrane potential such that the inner membrane surface becomes more negative than the outer membrane surface.

I

illuminant: The light that illuminates a surface.
illusory conjunction: An erroneous combination of two features in a visual scene—for example, seeing a red X when the display contains red letters and Xs but no red Xs.
illusory contour: A contour that is perceived even though nothing changes from one side of it to the other in an image.
image: A picture or likeness.
imbalance: Lack of balance; unsteadiness; nearly falling over.
inattentional blindness: A failure to notice—or at least to report—a stimulus that would be easily reportable if it were attended.
incus: The middle of the three ossicles, connecting the malleus and the stapes.
inferior colliculus: A midbrain nucleus in the auditory pathway.
inferotemporal (IT) cortex: Part of the cerebral cortex in the lower portion of the temporal lobe, important in object recognition.
inhibition of return: The relative difficulty in getting attention (or the eyes) to move back to a recently attended (or fixated) location.
inner ear: A hollow cavity in the temporal bone of the skull, and the structures within this cavity: the cochlea and the semicircular canals of the vestibular system canals.
inner segment: The part of a photoreceptor that lies between the outer segment and the cell nucleus.
insular cortex: The primary cortical processing area for taste—the part of the cortex that first receives taste information. Also called insula or gustatory cortex.
interaural level difference (ILD): The difference in level (intensity) between a sound arriving at one ear versus the other.
interaural time difference (ITD): The difference in time between a sound arriving at one ear versus the other.
interocular transfer: The transfer of an effect (such as adaptation) from one eye to the other.
inverse-square law: A principle stating that as distance from a source increases, intensity decreases faster such that decrease in intensity is equal to the distance squared. This general law also applies to optics and other forms of energy.
ipsilateral: Referring to the same side of the body (or brain).
ipsilesional field: The visual field on the same side as a brain lesion.
iris: The colored part of the eye, consisting of a muscular diaphragm surrounding the pupil and regulating the light entering the eye by expanding and contracting the pupil.
isointensity curve: A map plotting the firing rate of an auditory nerve fiber against varying frequencies at varying intensities.

J

just noticeable difference (JND) or difference threshold: The smallest detectable difference between two stimuli, or the minimum change in a stimulus that enables it to be correctly judged as different from a reference stimulus.
juxtaglomerular neurons: The first layer of cells surrounding the glomeruli. They are a mixture of excitatory and inhibitory cells and respond to a wide range of odorants. The selectivity of neurons to specific odorants increases in a gradient from the surface of the olfactory bulb to the deeper layers.

K

kinesthesia: Perception of the position and movement of our limbs in space.
kinesthetic: Referring to perception involving sensory mechanoreceptors in muscles, tendons, and joints.
koniocellular cell: A neuron located between the magnocellular and parvocellular layers of the lateral geniculate nucleus. This layer is known as the koniocellular layer.
koniocellular: Referring to cells in the koniocellular layer of the lateral geniculate nucleus of the thalamus. Konio from the Greek for “dust” referring to the appearance of the cells.

L

L-cone: A cone that is preferentially sensitive to long wavelengths; colloquially (but not entirely accurately) known as a “red cone.”
labeled lines: A theory of sensory coding in which each nerve fiber carries a particular stimulus quality.
lateral geniculate nucleus (LGN): A structure in the thalamus, part of the midbrain, that receives input from the retinal ganglion cells and has input and output connections to the visual cortex.
lateral inhibition: Antagonistic neural interaction between adjacent regions of the retina.
lateral superior olive (LSO): A relay station in the brain stem where inputs from both ears contribute to detection of the interaural level difference.
learned taste aversion: The avoidance of a novel flavor after it has been paired with gastric illness. The smell, not the taste, of the substance is key for the learned aversion response in humans.
lens: The lens inside the eye that enables the changing of focus.
lesion: In reference to neurophysiology,1. (n) A region of damaged brain. 2. (v) To destroy a section of the brain.
limbic system: The encompassing group of neural structures that includes the olfactory cortex, the amygdala, the hippocampus, the piriform cortex, and the entorhinal cortex. The limbic system is involved in many aspects of emotion and memory. Olfaction is unique among the senses for its direct connection to the limbic system.
linear acceleration: The rate of change of linear velocity. Mathematically, the integral of linear acceleration is linear velocity, and the integral of linear velocity is linear displacement, which is also referred to as “translation.” Linear acceleration, linear velocity, and linear displacement all mathematically represent linear motion.
linear motion: Translational motion like the predominant movement of a train car or bobblehead doll.
linear perspective: A depth cue based on the fact that lines that are parallel in the three-dimensional world will appear to converge in a two-dimensional image.
lordosis: The position that females of some species (e.g., pigs and rats) need to assume in order to be impregnated. It involves the inward curving of the spinal column and exposure of the genitals.
loudness: The psychological aspect of sound related to perceived intensity (amplitude).
low-spontaneous fiber: An auditory nerve fiber that has a low rate (less than 10 spikes per second) of spontaneous firing; low-spontaneous fibers require relatively intense sound before they will fire at higher rates.
luminance-defined object: An object that is delineated by differences in reflected light.

M

M ganglion cell: A ganglion cell resembling a little umbrella that receives excitatory input from diffuse bipolar cells and feeds the magnocellular layer of the lateral geniculate nucleus.
M-cone: A cone that is preferentially sensitive to middle wavelengths; colloquially (but not entirely accurately) known as a “green cone.”
macula: In reference to vision, the central part of the retina that has a high concentration of cones. In reference to the vestibular system, any of the specialized detectors of linear acceleration and gravity found in each otolith organ.
magnetic resonance imaging (MRI): An imaging technology that uses the responses of atoms to strong magnetic fields to form images of structures like the brain. The method can be adapted to measure activity in the brain, as well (see functional magnetic resonance imaging).
magnetoencephalography (MEG): A technique, similar to electroencephalography, that measures changes in magnetic activity across populations of many neurons in the brain.
magnitude estimation: A psychophysical method in which the participant assigns values according to perceived magnitudes of the stimuli.
magnocellular layer: Either of the bottom two neuron-containing layers of the lateral geniculate nucleus, the cells of which are physically larger than those in the top four layers.
main olfactory bulb (MOB): The olfactory bulb; the blueberry-sized extension of the brain just above the nose; the first region of the brain where smells are processed. In humans we refer simply to “olfactory bulb(s)”; in animals with accessory olfactory bulbs, we distinguish between “main” and “accessory.”
malleus: One of the three ossicles. The malleus receives vibration from the tympanic membrane and is attached to the incus.
masking: Using a second sound, frequently noise, to make the detection of another sound more difficult.
materialism: The idea that the only thing that exists is matter, and that all things, including the mind and consciousness, are the results of interaction between bits of matter.
mathematical integration: Computing an integral—one of the two main operations in calculus (the other, the inverse operation, is differentiation). Velocity is the integral of acceleration. Change of position is the integral of velocity.
mechanoreceptor: A sensory receptor that responds to mechanical stimulation (pressure, vibration, or movement).
medial geniculate nucleus: The part of the thalamus that relays auditory signals to the temporal cortex and receives input from the auditory cortex.
medial superior olive (MSO): A relay station in the brain stem where inputs from both ears contribute to detection of the interaural time difference.
Meissner corpuscle: A specialized nerve ending associated with fast-adapting (FA I) fibers that have small receptive fields.
melanopsin: A photopigment, found in a class of photoreceptive retinal ganglion cells, that is sensitive to ambient light.
melody: A sequence of notes or chords perceived as a single coherent structure.
Merkel cell neurite complex: A specialized nerve ending associated with slowly adapting (SA I) fibers that have small receptive fields.
metamers: Different mixtures of wavelengths that look identical. More generally, any pair of stimuli that are perceived as identical in spite of physical differences.
method of adjustment: A method of limits in which the subject controls the change in the stimulus.
method of constant stimuli: A psychophysical method in which many stimuli, ranging from rarely to almost always perceivable (or rarely to almost always perceivably different from a reference stimulus), are presented one at a time. Participants respond to each presentation: “yes/no,” “same/different,” and so on.
method of limits: A psychophysical method in which the particular dimension of a stimulus, or the difference between two stimuli, is varied incrementally until the participant responds differently.
metrical depth cue: A depth cue that provides quantitative information about distance in the third dimension.
microsaccade: An involuntary, small, jerklike eye movement.
microvilli: Slender projections of the cell membrane on the tips of some taste bud cells that extend into the taste pore.
mid-spontaneous fiber: An auditory nerve fiber that has a medium rate (10–30 spikes per second) of spontaneous firing. The characteristics of mid-spontaneous fibers are intermediate between low- and high-spontaneous fibers.
middle (midlevel) vision: A loosely defined stage of visual processing that comes after basic features have been extracted from the image (low-level, or early, vision) and before object recognition and scene understanding (high-level vision).
middle canal: One of three fluid-filled passages in the cochlea. The middle canal is sandwiched between the tympanic and vestibular canals and contains the cochlear partition. Also called scala media.
middle ear: An air-filled chamber containing the middle bones, or ossicles. The middle ear conveys and amplifies vibration from the tympanic membrane to the oval window.
middle temporal area (MT): An area of the brain thought to be important in the perception of motion.
midget bipolar cell: A small bipolar cell in the central retina that receives input from a single cone.
mitral cell: The deepest layer of neurons in the olfactory bulb. Each mitral cell responds only to a few specific odorants.
monocular depth cue: A depth cue that is available even when the world is viewed with one eye alone.
monocular: With one eye.
monosodium glutamate (MSG): The sodium salt of glutamic acid (an amino acid).
motion aftereffect (MAE): The illusion of motion of a stationary object that occurs after prolonged exposure to a moving object.
motion parallax: An important depth cue that is based on head movement. The geometric information obtained from an eye in two different positions at two different times is similar to the information from two eyes in different positions in the head at the same time.
muscle spindle: A sensory receptor located in a muscle that senses its tension.
myopia: Nearsightedness, a common condition in which light entering the eye is focused in front of the retina and distant objects cannot be seen sharply.

N

naïve template theory: The proposal that the visual system recognizes objects by matching the neural representation of the image with a stored representation of the same “shape” in the brain.
nasal dominance: The asymmetry characterizing the intake of air by the two nostrils, which corresponds to differing sensitivity to odorants between the two nostrils. Nasal dominance alternates nostrils throughout the day, but there is no predictability about when the nostrils alternate.
Necker cube: An outline that is perceptually bi-stable. Unlike the situation with most stimuli, two interpretations continually battle for perceptual dominance.
negative afterimage: An afterimage whose polarity is the opposite of the original stimulus. Light stimuli produce dark negative afterimages. Colors are complementary; for example, red produces green, and yellow produces blue.
neglect: As a neurological symptom, in visual attention: (1) The inability to attend or respond to stimuli in the contralesional visual field (typically, the left field after right parietal damage). (2) Ignoring half of the body or half of an object.
neural plasticity: The ability of neural circuits to undergo changes in function or organization as a result of previous activity.
neuroimaging: A set of methods that generate images of the structure and/or function of the brain. In many cases, these methods allow us to examine the brain in living, behaving humans.
neurotransmitter: A chemical substance used in neuronal communication at synapses.
neutral point: The point at which an opponent color mechanism is generating no signal. If red-green and blue-yellow mechanisms are at their neutral points, a stimulus will appear achromatic. (The black-white process has no neutral point.)
nociceptor: A sensory receptor that responds to painful input, such as extreme heat or pressure.
nonaccidental feature: A feature of an object that is not dependent on the exact (or accidental) viewing position of the observer.
nonmetrical depth cue: A depth cue that provides information about the depth order (relative depth) but not depth magnitude (e.g., his nose is in front of his face).
nontaster (of PTC/PROP): An individual born with two recessive alleles for the TAS2R38 gene and unable to taste the compounds phenylthiocarbamide and propylthiouracil.

O

occlusion: A cue to relative depth order in which, for example, one object obstructs the view of part of another object.
octave: The interval between two sound frequencies having a ratio of 2:1.
ocular dominance: The property of the receptive fields of striate cortex neurons by which they demonstrate a preference, responding somewhat more rapidly when a stimulus is presented in one eye than when it is presented in the other.
oculomotor (III) nerves: The third pair of cranial nerves, which innervate all the extrinsic muscles of the eye except the lateral rectus and the superior oblique muscles, and which innervate the elevator muscle of the upper eyelid, the ciliary muscle, and the sphincter muscle of the pupil.
odor hedonics: The liking dimension of odor perception, typically measured by ratings of an odors’s perceived pleasantness, familiarity, and intensity.
odor: The translation of a chemical stimulus into a smell sensation. For example, “The cake has a chocolate odor.”
odorant receptor (OR): The region on the cilia of olfactory sensory neurons where odorant molecules bind.
odorant: A molecule that is defined by its physicochemical characteristics, which is capable of being translated by the nervous system into the perception of a smell. For example, “You were given the odorant methyl salicylate to smell, which has the odor of wintergreen mint.”
OFF bipolar cell: A bipolar cell that responds to a decrease in light captured by the cones.
OFF-center cell: A cell that depolarizes in response to a decrease in light intensity in its receptive-field center.
olfaction: The sense of smell.
olfactory (I) nerves: The first pair of cranial nerves. The axons of the olfactory sensory neurons bundle together after passing through the cribriform plate to form the olfactory nerve, which conducts impulses from the olfactory epithelia in the nose to the olfactory bulb.
olfactory bulb: A blueberry-sized extension of the brain just above the nose, where olfactory information is first processed. There are two olfactory bulbs, one in each brain hemisphere, corresponding to the right and left nostrils.
olfactory cleft: A narrow space at the back of the nose into which air flows and where the olfactory epithelium is located.
olfactory epithelium: A secretory mucous membrane in the human nose whose primary function is to detect odorants in inhaled air. Located on both sides of the upper portion of the nasal cavity and the olfactory clefts, the olfactory epithelium contains three types of cells: olfactory sensory neurons, basal cells, and supporting cells.
olfactory sensory neuron (OSN): One of three cell types—the main one—in the olfactory epithelium. OSNs are small neurons located beneath a mucous layer in the epithelium. The cilia on the OSN dendrites contain the receptor sites for odorant molecules.
olfactory tract: The bundle of axons of the mitral and tufted cells within the olfactory bulb that sends odor information to the primary olfactory cortex.
olfactory white: The olfactory equivalent of “white noise” or the color white. When at least 30 odorants of equal intensity that span olfactory physiochemical and psychological (perceptual) space are mixed, they produce a resultant odor percept that is the same as every other mixture of 30 odorants meeting the span and equivalent intensity criteria, even though the various mixtures do not share any common odorants.
ON bipolar cell: A bipolar cell that responds to an increase in light captured by the cones.
ON-center cell: A cell that depolarizes in response to an increase in light intensity in its receptive-field center.
opponent color theory: The theory that perception of color is based on the output of three mechanisms, each of them resulting from an opponency between two colors: red-green, blue-yellow, and black-white.
optic (II) nerves: The second pair of cranial nerves, which arise from the retina and carry visual information to the thalamus and other parts of the brain.
optic array: The collection of light rays that interact with objects in the world that are in front of a viewer. Term coined by J. J. Gibson.
optic flow: The pattern of apparent motion of objects in a visual scene produced by the relative motion between the observer and the scene.
optokinetic nystagmus (OKN): A reflexive eye movement in which the eyes will involuntarily track a continually moving object.
orbitofrontal cortex (OFC): The part of the frontal lobe of the cortex that lies behind the bone (orbit) containing the eyes. The OFC is responsible for the conscious experience of olfaction, as well as the integration of pleasure and displeasure from food; and it has been referred to as the secondary olfactory cortex and the secondary taste cortex. The OFC is also involved in many other functions, and it is critical for assigning affective value to stimuli—in other words, determining hedonic meaning.
organ of Corti: A structure on the basilar membrane of the cochlea that is composed of hair cells and dendrites of auditory nerve fibers.
orientation tuning: The tendency of neurons in striate cortex to respond optimally to certain orientations and less to others.
orthonasal olfaction: Sniffing in and perceiving odors through our nostrils, which occurs when we are smelling something that is outside of us.
oscillatory: Referring to back-and-forth movement that has a constant rhythm.
ossicle: Any of three tiny bones of the middle ear: malleus, incus, and stapes.
otitis media: Inflammation of the middle ear, commonly in children as a result of infection.
otoconia: Tiny calcium carbonate stones in the ear that provide inertial mass for the otolith organs, enabling them to sense gravity and linear acceleration.
otolith organ: Either of two mechanical structures (utricle and saccule) in the vestibular system that sense both linear acceleration and gravity.
otosclerosis: Abnormal growth of the middle-ear bones that causes hearing loss.
ototoxic: Producing adverse effects on cochlear or vestibular organs or nerves.
outer ear: The external sound-gathering portion of the ear, consisting of the pinna and the ear canal.
outer segment: The part of a photoreceptor that contains photopigment molecules.
oval window: The flexible opening to the cochlea through which the stapes transmits vibration to the fluid inside.

P

P ganglion cell: A small ganglion cell that receives excitatory input from single midget bipolar cells in the central retina and feeds the parvocellular layer of the lateral geniculate nucleus.
Pacinian corpuscle: A specialized nerve ending associated with fast-adapting (FA II) fibers that have large receptive fields.
panpsychism: The idea that the mind exists as a property of all matter—that is, that all matter has consciousness.
Panum’s fusional area: The region of space, in front of and behind the horopter, within which binocular single vision is possible.
papilla: Any of multiple structures that give the tongue its bumpy appearance. From smallest to largest, the papilla types that contain taste buds are fungiform, foliate, and circumvallate; filiform papillae, which do not contain taste buds, are the smallest and most numerous.
parabelt area: A region of cortex, lateral and adjacent to the belt area, where neurons respond to more complex characteristics of sounds, as well as to input from other senses.
parahippocampal place area (PPA): A region of extrastriate visual cortex in humans that is specifically and reliably activated more by images of places than by other stimuli.
parallel search: A search in which multiple stimuli are processed at the same time.
parallelism: A rule for figure-ground assignment stating that parallel contours are likely to belong to the same figure.
parietal lobe: In each cerebral hemisphere, a lobe that lies toward the top of the brain between the frontal and occipital lobes.
parvocellular layer: Any of the top four neuron-containing layers of the lateral geniculate nucleus, the cells of which are physically smaller than those in the bottom two layers.
parvocellular: Referring to cells in the parvocellular layers of the lateral geniculate nucleus of the thalamus. Parvo from the Greek for “small” referring to the size of the cells.
perception: The act of giving meaning to a detected sensation.
period: For hearing, the time required for a full wavelength of an acoustic sine wave to pass by a point in space.
phantom limb: Sensation perceived from a physically amputated limb of the body.
phase locking: Firing of a single neuron at one distinct point in the period (cycle) of a sound wave at a given frequency. (The neuron need not fire on every cycle, but each firing will occur at the same point in the cycle.)
phase: A fraction of the cycle of the sine wave described in degrees (0° to 360°) or radians (0π to 2π). In hearing, phase can be used to describe fractions of a period that relate to time.
phase: The relative position of a grating.
pheromone: A chemical emitted by one member of a species that triggers a physiological or behavioral response in another member of the same species. Pheromones are signals for chemical communication and do not need to have any smell.
phonation: The process through which vocal folds are made to vibrate when air pushes out of the lungs.
photoactivation: Activation by light.
photon: A quantum of visible light or other form of electromagnetic radiation demonstrating both particle and wave properties.
photopic: Referring to light intensities that are bright enough to stimulate the cone receptors and bright enough to “saturate” the rod receptors (that is, drive them to their maximum responses).
photoreceptor: A light-sensitive receptor in the retina.
pictorial depth cue: A cue to distance or depth used by artists to depict three-dimensional depth in two-dimensional pictures.
pinna: The outer, funnel-like part of the ear.
pitch: The psychological aspect of sound related mainly on the frequency of vibration.
place code: Tuning of different parts of the cochlea to different frequencies, in which information about the particular frequency of an incoming sound wave is coded by the place along the cochlear partition that has the greatest mechanical displacement.
placebo effect: Decreasing pain sensation when people think they’re taking an analgesic drug but actually are not.
polysensory: Referring to blending multiple sensory systems.
positivism: A philosophical position arguing that all we really have to go on is the evidence of the senses, so the world might be nothing more than an elaborate hallucination.
positron emission tomography (PET): An imaging technology that enables us to define locations in the brain where neurons are especially active by measuring the metabolism of brain cells using safe radioactive isotopes.
preattentive stage: The processing of a stimulus that occurs before selective attention is deployed to that stimulus.
prefrontal cortex: A region of the brain concerned with cognition and executive control.
presbyopia: Literally “old sight.” The age-related loss of accommodation, which makes it difficult to focus on near objects.
primary auditory cortex (A1): The first area within the temporal lobes of the brain responsible for processing acoustic information.
primary olfactory cortex or piriform cortex: The neural area where olfactory information is first processed. It comprises the amygdala, parahippocampal gyrus, and interconnected areas; and it interacts closely with the entorhinal cortex.
primary visual cortex (V1), area 17, or striate cortex: The area of the cerebral cortex of the brain that receives direct inputs from the lateral geniculate nucleus, as well as feedback from other brain areas.
primer pheromone: A pheromone that triggers a physiological (often hormonal) change among conspecifics. This effect usually involves prolonged pheromone exposure.
principle of univariance: The fact that an infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor. One photoreceptor type cannot make color discriminations based on wavelength.
probability summation: The increased detection probability based on the statistical advantage of having two (or more) detectors rather than one.
projective geometry: For purposes of studying perception of the three-dimensional world, the geometry that describes the transformations that occur when the three-dimensional world is projected onto a two-dimensional surface. For example, parallel lines do not converge in the real world, but they do in the two-dimensional projection of that world.
proprioception: Perception mediated by kinesthetic and internal receptors.
prosopagnosia: An inability to recognize faces.
protanope: An individual who suffers from color blindness that is due to the absence of L-cones.
proximity: A Gestalt grouping rule stating that the tendency of two features to group together will increase as the distance between them decreases.
psychoacoustics: The study of the psychological correlates of the physical dimensions of acoustics; a branch of psychophysics.
psychophysics: The science of defining quantitative relationships between physical and psychological (subjective) events.
pupil: The dark, circular opening at the center of the iris in the eye, where light enters the eye.

Q

qualia (sing. quale): In philosophy, private conscious experiences of sensation or perception.

R

random dot stereogram (RDS): A stereogram made of a large number (often in the thousands) of randomly placed dots. Random dot stereograms contain no monocular cues to depth. Stimuli visible stereoscopically in random dot stereograms are Cyclopean stimuli.
rapid serial visual presentation (RSVP): An experimental procedure in which stimuli appear in a stream at one location (typically the point of fixation) at a rapid rate (typically about eight per second).
rate saturation: The point at which a nerve fiber is firing as rapidly as possible and further stimulation is incapable of increasing the firing rate.
rate-intensity function: A graph plotting the firing rate of an auditory nerve fiber in response to a sound of constant frequency at increasing intensities.
reaction time (RT): A measure of the time from the onset of a stimulus to a response.
realism: A philosophical position arguing that there is a real world to sense.
receiver operating characteristic (ROC) curve: In studies of signal detection, the graphical plot of the hit rate as a function of the false-alarm rate. If these are the same, points fall on the diagonal, indicating that the observer cannot tell the difference between the presence and absence of the signal. As the observer’s sensitivity increases, the curve bows upward toward the upper left corner. That point represents a perfect ability to distinguish signal from noise (100% hits, 0% false alarms).
receptive field: The region on the retina in which visual stimuli influence a neuron’s firing rate.
receptor adaptation: The biochemical phenomenon, occurring after continual exposure to an odorant, whereby receptors stop responding to an odorant and detection ceases.
receptor potential: A change in voltage across the membrane of a sensory receptor cell (in the vestibular system, a hair cell) in response to stimulation.
recognition-by-components model: Biederman’s model of object recognition, which holds that objects are recognized by the identities and relationships of their component parts.
reflect: To redirect something that strikes a surface—especially light, sound, or heat—usually back toward its point of origin.
reflectance: The percentage of light hitting a surface that is reflected and not absorbed into the surface. Typically reflectance is given as a function of wavelength.
reflexive eye movement: A movement of the eye that is automatic and involuntary.
refract: 1. To alter the course of a wave of energy that passes into something from another medium, as water does to light entering it from the air. 2. To measure the degree of refraction in a lens or eye.
Reissner’s membrane: A thin sheath of tissue separating the vestibular and middle canals in the cochlea.
relatability: The degree to which two line segments appear to be part of the same contour.
related color: A color, such as brown or gray, that is seen only in relation to other colors. For example, a “gray” patch in complete darkness appears white.
relative height: As a depth cue, the observation that objects at different distances from the viewer on the ground plane will form images at different heights in the retinal image. Objects farther away will be seen as higher in the image.
relative metrical depth cue: A depth cue that could specify, for example, that object A is twice as far away as object B without providing information about the absolute distance to either A or B.
relative size: A comparison of size between items without knowing the absolute size of either one.
releaser pheromone: A pheromone that triggers an immediate behavioral response among conspecifics.
response enhancement: An effect of attention on the response of a neuron in which the neuron responding to an attended stimulus gives a bigger response.
retina: A light-sensitive membrane in the back of the eye that contains rods and cones, which receive an image from the lens and send it to the brain through the optic nerve.
retinitis pigmentosa (RP): A progressive degeneration of the retina that affects night vision and peripheral vision. RP commonly runs in families and can be caused by defects in a number of different genes that have recently been identified.
retronasal olfaction: Perceiving odors through your mouth while breathing and chewing. This occurs when we are smelling something that is inside our mouth and is what gives us the experience of flavor.
retronasal olfactory sensation: The sensation of an odor that is perceived when chewing and swallowing force an odorant in the mouth up behind the palate into the nose. Such odor sensations are perceived as originating from the mouth, even though the actual contact of odorant and receptor occurs at the olfactory mucosa.
rhodopsin: The visual pigment found in rods.
rod monochromat: An individual with no cones of any type. In addition to being truly color-blind, rod monochromats are badly visually impaired in bright light.
rod: A photoreceptor specialized for night vision.
round window: A soft area of tissue at the base of the tympanic canal that releases excess pressure remaining from extremely intense sounds.
Ruffini ending: A specialized nerve ending associated with slowly adapting (SA II) fibers that have large receptive fields.

S

S-cone: A cone that is preferentially sensitive to short wavelengths; colloquially (but not entirely accurately) known as a “blue cone.”
saccade: A type of eye movement, made both voluntarily and involuntarily, in which the eyes rapidly change fixation from one object or location to another.
saccadic suppression: The reduction of visual sensitivity that occurs when we make saccadic eye movements. Saccadic suppression eliminates the smear from retinal image motion during an eye movement.
saccule: One of the two otolith organs. A saclike structure that contains the saccular macula. Also called sacculus.
salience: The vividness of a stimulus relative to its neighbors.
salty: One of the four basic tastes; the taste quality produced by the cations of salts (e.g., the sodium in sodium chloride produces the salty taste). Some cations also produce other taste qualities (e.g., potassium tastes bitter as well as salty). The purest salty taste is produced by sodium chloride (NaCl), common table salt.
scatter: To disperse something—such as light—in an irregular fashion.
scene-based guidance: Information in our understanding of scenes that helps us find specific objects in scenes (e.g., objects do not float in air, faucets are near sinks).
scotoma: The blind spot in the middle of the visual field.
scotopic: Referring to light intensities that are bright enough to stimulate the rod receptors but too dim to stimulate the cone receptors.
second-order motion: The motion of an object that is defined by changes in contrast or texture, but not by luminance.
selective attention: The form of attention involved when processing is restricted to a subset of the possible stimuli.
semicircular canal: Any of three toroidal tubes in the vestibular system that sense angular motion.
sensation: The ability to detect a stimulus and, perhaps, to turn that detection into a private experience.
sense of angular motion: The perceptual modality that senses rotation.
sense of linear motion: The perceptual modality that senses translation.
sense of tilt: The perceptual modality that senses head inclination with respect to gravity.
sensitivity: 1. The ability to respond to transmitted signals. 2. In signal detection theory, a value that defines the ease with which an observer can tell the difference between the presence and absence of a stimulus or the difference between stimulus 1 and stimulus 2.
sensorineural hearing loss: Hearing loss due to defects in the cochlea or auditory nerve.
sensory exafference: Change in afference caused by external stimuli. For the vestibular system, vestibular afference evoked by passive head motion would yield sensory exafference. Compare sensory reafference.
sensory integration: The process of combining different sensory signals. Typically, combining several signals yields more accurate and/or more precise information than can be obtained from individual sensory signals. This is not the mathematical process of integration learned in calculus (e.g., the integral of acceleration is velocity).
sensory reafference: Change in afference caused by self-generated activity. For the vestibular system, vestibular afference evoked by an active self-generated head motion would yield sensory reafference. Compare sensory exafference.
serial self-terminating search: A search from item to item, ending when a target is found.
set size: The number of items in a visual display.
shape-pattern theory: The current dominant biochemical theory for how chemicals come to be perceived as specific odors. Shape-pattern theory contends that different scents—as a function of the fit between odorant shape to OR shape—activate different arrays of olfactory receptors in the olfactory epithelia. These various arrays produce specific firing patterns of neurons in the olfactory bulb, which then determine the particular scent we perceive.
sharper tuning: An effect of attention on the response of a neuron in which the neuron responding to an attended stimulus responds more precisely. For example, a neuron that responds to lines with orientations from –20 degrees to +20 degrees might come to respond to ±10-degree lines.
signal detection theory: A psychophysical theory that quantifies the response of an observer to the presentation of a signal in the presence of noise. Measures obtained from a series of presentations are sensitivity (d′) and criterion of the observer.
similarity: A Gestalt grouping rule stating that the tendency of two features to group together will increase as the similarity between them increases.
simple cell: A cortical neuron whose receptive field has clearly defined excitatory and inhibitory regions.
simultagnosia: An inability to perceive more than one object at a time. Simultagnosia is a consequence of bilateral damage to the parietal lobes (Balint syndrome).
sine wave: A simple, smoothly changing oscillation that repeats across space. Higher frequency sine waves have more oscillations and lower frequencies have fewer oscillations over a given distance. 1. In hearing, a waveform for which variation as a function of time is a sine function. Also called pure tone. 2. In vision, a pattern for which variation in a property like brightness or color as a function of space is a sine function.
sine wave or pure tone: The waveform for which variation as a function of time is a sine function.
sine wave grating: A grating with a sinusoidal luminance profile as shown in Figure 3.4b.
single-opponent cell: Another way to refer to cone-opponent cells, in order to differentiate them from double-opponent cells.
sinusoidal: Referring to any oscillation, such as a sound wave or rotational motion, whose waveform is that of a sine curve. The period of a sinusoidal oscillation is the time that it takes for one full back-and-forth cycle of the motion to occur. The frequency of a sinusoidal oscillation is defined as the numeral 1 divided by the period.
smooth pursuit: A type of voluntary eye movement in which the eyes move smoothly to follow a moving object.
somatosensation: Collectively, sensory signals from the skin, muscles, tendons, joints, and internal receptors.
somatosensory area 1 (S1): The primary receiving area for touch in the cortex.
somatosensory area 2 (S2): The secondary receiving area for touch in the cortex.
somatotopic: Spatially mapped in the somatosensory cortex in correspondence to spatial events on the skin.
somatotypical: Having normal somatosensation.
sour: One of the four basic tastes; the taste quality produced by the hydrogen ion in acids.
source segregation or auditory scene analysis: Processing an auditory scene consisting of multiple sound sources into separate sound images.
spatial disorientation: Any impairment of spatial orientation. More specifically, any impairment of our sense of linear motion, angular motion, or tilt.
spatial frequency: The number of cycles of a grating (e.g., dark and bright bars) per unit of visual angle (usually specified in cycles per degree).
spatial layout: The description of the structure of a scene (e.g., enclosed, open, rough, smooth) without reference to the identity of specific objects in the scene.
spatial orientation: A sense consisting of three interacting modalities: perception of linear motion, angular motion, and tilt.
spatial-frequency channel: A pattern analyzer, implemented by an ensemble of cortical neurons, in which each set of neurons is tuned to a limited range of spatial frequencies.
specific anosmia: The inability to smell one specific compound amid otherwise normal smell perception.
specific hungers theory: The idea that deficiency of a given nutrient produces craving (a specific hunger) for that nutrient. Curt Richter first proposed this theory and demonstrated that cravings for salty or for sweet are associated with deficiencies in those substances. However, the idea proved wrong for other nutrients (e.g., vitamins).
spectral power distribution: The physical energy in a light as a function of wavelength.
spectral reflectance function: The percentage of a particular wavelength that is reflected from a surface.
spectral sensitivity: Refering to the sensitivity of a cell or a device to different wavelengths on the electromagnetic spectrum.
spectrogram: In sound analysis, a three-dimensional display that plots time on the horizontal axis, frequency on the vertical axis, and amplitude (intensity) on a color or gray scale.
spectrum: A representation of the relative energy (intensity) present at each frequency.
spinothalamic pathway: The route from the spinal cord to the brain that carries most of the information about skin temperature and pain.
staircase method: A psychophysical method for determining the concentration of a stimulus required for detection at the threshold level. The staircase method is an example of a method of limits. A stimulus (e.g., odorant) is presented in an ascending concentration sequence until detection is indicated, and then the concentration is shifted to a descending sequence until the response changes to “no detection.” This ascending and descending sequence is typically repeated several times, and the concentrations at which reversals occur are averaged to determine the threshold detection level of that odorant for a given individual. Also called reverse staircase method.
stapedius: The muscle attached to the stapes; tensing the stapedius decreases vibration.
stapes: One of the three ossicles. Connected to the incus on one end, the stapes presses against the oval window of the cochlea on the other end.
stereoacuity: A measure of the smallest binocular disparity that can generate a sensation of depth.
stereoblindness: An inability to make use of binocular disparity as a depth cue. This term is typically used to describe individuals with vision in both eyes. Someone who has lost one (or both) eyes is not typically described as “stereoblind.”
stereocilium: Any of the hairlike extensions on the tips of hair cells in the cochlea that, when flexed, initiate the release of neurotransmitters.
stereoisomers: Isomers (molecules that can exist in different structural forms) in which the spatial arrangements of the atoms are mirror-image rotations of one another, like a right and left hand.
stereopsis: The ability to use binocular disparity as a cue to depth.
stereoscope: A device for simultaneously presenting one image to one eye and another image to the other eye. Stereoscopes can be used to present dichoptic stimuli for stereopsis and binocular rivalry.
Stevens’ power law: A principle describing the relationship between stimulus and resulting sensation that says the magnitude of subjective sensation is proportional to the stimulus magnitude raised to an exponent.
stimulus onset asynchrony (SOA): The time between the onset of one stimulus and the onset of another.
strabismus: A misalignment of the two eyes such that a single object in space is imaged on the fovea of one eye and on a nonfoveal area of the other (turned) eye.
structural description: A description of an object in terms of the nature of its constituent parts and the relationships between those parts.
structuralism: A school of thought believing that complex objects or perceptions could be understood by analysis of the components.
substantia gelatinosa: A jellylike region of interconnecting neurons in the dorsal horn of the spinal cord.
subtraction method: In function magnetic imaging, brain activity is measured in two conditions: one with and one without the involvement of the mental process of interest. Subtracting the two conditions shows regions of brain specifically activated by that process.
subtractive color mixture: A mixture of pigments. If pigments A and B mix, some of the light shining on the surface will be subtracted by A, and some by B. Only the remainder contributes to the perception of color.
superior colliculus: A structure in the midbrain that is important in initiating and guiding eye movements.
superior olive: An early brain stem region in the auditory pathway where inputs from both ears converge.
supertaster: Supertasters are those individuals who experience the most intense taste sensations; for some stimuli, they are dramatically more intense than for medium tasters or nontasters. Supertasters also tend to experience more intense oral burn and oral touch sensations. A variety of factors may contribute to this heightened perception, among the most important is the density of fungiform papillae.
supporting cell: One of the three types of cells in the olfactory epithelium. Supporting cells provide metabolic and physical support for the olfactory sensory neurons.
suppression: In vision, the inhibition of an unwanted image. Suppression occurs frequently in people with strabismus.
surroundedness: A rule for figure-ground assignment stating that if one region is entirely surrounded by another, it is likely that the surrounded region is the figure.
sweet: One of the four basic tastes; the taste quality produced by some sugars, such as glucose, fructose, and sucrose. These three sugars are particularly biologically useful to us, and our sweet receptors are tuned to them. Some other compounds (e.g., saccharin, aspartame) are also sweet.
symmetry: A rule for figure-ground assignment stating that symmetrical regions are more likely to be seen as figure.
synapse: The junction between neurons that permits information transfer.
synaptic terminal: The location where axons terminate at the synapse for transmission of information by the release of a chemical transmitter.
syncopation: Any deviation from a regular rhythm.

T

tactile agnosia: The inability to identify objects by touch.
target: The goal of a visual search.
tastant: Any stimulus that can be tasted.
taste: Sensations evoked by solutions in the mouth that contact receptors on the tongue and the roof of the mouth that then connect to axons in cranial nerves VII, IX, and X.
taste bud: A globular cluster of cells that has the function of creating neural signals conveyed to the brain by the taste nerves. Some of the cells in a taste bud have specialized sites on their apical projections that interact with taste stimuli. Some of the cells form synapses with taste nerve fibers.
taste receptor cell: A cell within the taste bud that contains sites on its apical projection that can interact with taste stimuli. These sites fall into two major categories: those interacting with charged particles (e.g., sodium and hydrogen ions), and those interacting with specific chemical structures.
taster (of PTC/PROP): An individual born with one or both dominant alleles for the TAS2R38 gene and able to taste the compounds phenylthiocarbamide and propylthiouracil. PTC/PROP tasters who also have a high density of fungiform papillae are PROP supertasters.
tau (τ): Information in the optic flow that could signal time to collision (TTC) without the necessity of estimating either absolute distances or rates. The ratio of the retinal image size at any moment to the rate at which the image is expanding is tau, and TTC is proportional to tau.
tectorial membrane: A gelatinous structure, attached on one end, that extends into the middle canal of the ear, floating above inner hair cells and touching outer hair cells.
tempo: The perceived speed of the presentation of sounds.
temporal code: Tuning of different parts of the cochlea to different frequencies, in which information about the particular frequency of an incoming sound wave is coded by the timing of neural firing as it relates to the period of the sound.
temporal integration: The process by which a sound at a constant level is perceived as being louder when it is of greater duration. The term also applies to perceived brightness, which depends on the duration of light.
tensor tympani: The muscle attached to the malleus; tensing the tensor tympani decreases vibration.
texture gradient: A depth cue based on the geometric fact that items of the same size form smaller images when they are farther away. An array of items that change in size smoothly across the image will appear to form a surface tilted in depth.
texture segmentation: Carving an image into regions of common texture properties.
texture-defined object or contrast-defined object: An object that is defined by differences in contrast, or texture, but not by luminance.
thermoreceptor: A sensory receptor that signals information about changes in skin temperature.
threshold tuning curve: A graph plotting the thresholds of a neuron or fiber in response to sine waves with varying frequencies at the lowest intensity that will give rise to a response.
tilt aftereffect: The perceptual illusion of tilt, produced by adaptation to a pattern of a given orientation.
tilt: To attain a sloped position like that of the Leaning Tower of Pisa.
timbre: The psychological sensation by which a listener can judge that two sounds with the same loudness and pitch are dissimilar. Timbre quality is conveyed by harmonics and other high frequencies.
time to collision (TTC): The time required for a moving object (such as a cricket ball) to hit a stationary object (such as a batsman’s head). TTC = distance/rate.
tip link: A tiny filament that stretches from the tip of a stereocilium to the side of its neighbor.
tip-of-the-nose phenomenon: The inability to name an odor, even though it is very familiar. Contrary to the tip-of-the-tongue phenomenon, one has no lexical access to the name of the odor, such as first letter, rhyme, number of syllables, and so on, when in the tip-of-the-nose state. This is an example of how language and olfactory perception are deeply disconnected.
tone chroma: A sound quality shared by tones that have the same octave interval.
tone height: A sound quality corresponding to the level of pitch. Tone height is monotonically related to frequency.
tonotopic organization: An arrangement in which neurons that respond to different frequencies are organized anatomically in order of frequency.
topographical mapping: The orderly mapping of the world in the lateral geniculate nucleus and the visual cortex.
transduce: To convert from one form of energy to another (e.g., from light to neural electrical energy, or from mechanical movement to neural electrical energy).
transmit: To convey something (e.g., light) from one place or thing to another.
transparent: Allowing light to pass through with no interruption, so that objects on the other side can be clearly seen.
triangle test: A test in which a participant is given three odorants to smell, of which two are the same and one is different. The participant is required to state which is the odd odor out. Typically, the order in which the three odorants are given (e.g., same, same, different; different, same, same; same, different, same) is manipulated and the test is re-peated several times for greater accuracy.
trichromatic theory of color vision or trichromacy: The theory that the color of any light is defined in our visual system by the relationships of three numbers—the outputs of three receptor types now known to be the three cones. Also called the Young-Helmholtz theory.
trigeminal (V) nerve: The fifth cranial nerve, which transmits information about the “feel” of an odorant (e.g., mint feels cool, cinnamon feels warm), as well as pain and irritation sensations (e.g., ammonia feels burning).
tritanope: An individual who suffers from color blindness that is due to the absence of S-cones.
trochlear (IV) nerves: The fourth pair of cranial nerves, which innervate the superior oblique muscles of the eyeballs.
tufted cell: The next layer of cells after the juxtaglomerular neurons. They repond to fewer odorants than the juxtaglomerular cells, but more than neurons at the deepest layer of cells.
two-point touch threshold: The minimum distance at which two stimuli (e.g., two simultaneous touches) are just perceptible as separate.
two-tone suppression: A decrease in the firing rate of one auditory nerve fiber due to one tone, when a second tone is presented at the same time.
tympanic canal: One of three fluid-filled passages in the cochlea. The tympanic canal extends from the round window at the base of the cochlea to the helicotrema at the apex. Also called scala tympani.
tympanic membrane: The eardrum; a thin sheet of skin at the end of the outer ear canal. The tympanic membrane vibrates in response to sound.

U

umami: The taste sensation produced by monosodium glutamate.
uncrossed disparity: The sign of disparity created by objects behind the plane of fixation (the horopter). The term uncrossed is used because images of objects located behind the horopter will appear to be displaced to the right in the right eye, and to the left in the left eye.
unique hue: Any of four colors that can be described with only a single color term: red, yellow, green, blue. Other colors (e.g., purple or orange) can be described as compounds (reddish blue, reddish yellow).
uniqueness constraint: In stereopsis, the observation that a feature in the world is represented exactly once in each retinal image. This constraint simplifies the correspondence problem.
unrelated color: A color that can be experienced in isolation.
utricle: One of the two otolith organs. A saclike structure that contains the utricular macula. Also called utriculus.

V

vanishing point: The apparent point at which parallel lines receding in depth converge.
vection: An illusory sense of self-motion caused by moving visual cues when one is not, in fact, actually moving.
velocity: The speed and direction in which something moves. Mathematically, velocity is the integral of acceleration. In words, linear velocity is distance divided by time to traverse that distance; angular velocity is rotation angle divided by time to traverse that angle.
vergence: A type of eye movement in which the two eyes move in opposite directions; for example, both eyes turn toward the nose (convergence) or away from the nose (divergence).
vertigo: A sensation of rotation or spinning. The term is often used more generally to mean any form of dizziness.
vestibular canal: One of three fluid-filled passages in the cochlea. The vestibular canal extends from the oval window at the base of the cochlea to the helicotrema at the apex. Also called scala vestibuli.
vestibular organs: The set of five sense organs—three semicircular canals and two otolith organs—located in each inner ear that sense head motion and head orientation with respect to gravity. See also vestibular system.
vestibular system: The set of five sense organs—three semicircular canals and two otolith organs—located in each inner ear that sense head motion and head orientation with respect to gravity and the neural pathways directly associated with these sense organs. See also vestibular organs.
vestibulo-ocular reflex (VOR): A short-latency reflex that helps stabilize vision by counterrotating the eyes when the vestibular system senses head movement.
vestibulocochlear (VIII) nerves: The eighth pair of cranial nerves, which connect the inner ear with the brain, transmitting impulses concerned with hearing and spatial orientation. The vestibulocochlear nerve is composed of the cochlear nerve branch and the vestibular nerve branch.
vibration theory: An alternative to shape-pattern theory for describing how olfaction works. Vibration theory proposes that every odorant has a different vibrational frequency, and that molecules that produce the same vibrational frequencies will smell the same.
Vieth-Müller circle: The location of objects whose images fall on geometrically corresponding points in the two retinas. If life were simple, this circle would be the horopter, but life is not simple.
viewpoint invariance: 1. A property of an object that does not change when observer viewpoint changes. 2. A class of theories of object recognition that proposes representations of objects that do not change when viewpoint changes.
visual acuity: A measure of the finest detail that can be resolved by the eyes.
visual angle: The angle subtended by an object at the retina.
visual crowding: The deleterious effect of clutter on peripheral object recognition.
visual search: Search for a target in a display containing distracting elements.
visual-field defect: A portion of the visual field with no vision or with abnormal vision, typically resulting from damage to the visual nervous system.
vitalism: The idea that there is a force in life that is distinct from physical entities.
vitreous humor: The transparent fluid that fills the vitreous chamber in the posterior part of the eye.
vocal tract: The airway above the larynx used for the production of speech. The vocal tract includes the oral tract and nasal tract.
volley principle: The idea that multiple neurons can provide a temporal code for frequency if each neuron fires at a distinct point in the period of a sound wave but does not fire on every period.
vomeronasal organ (VNO): A chemical sensing organ at the base of the nasal cavity with a curved tubular shape. The VNO evolved to detect chemicals that cannot be processed by ORs, such as large and/or aqueous molecules, the types of molecules that constitute pheromones. Also called Jacobson’s organ.

W

warmth fiber: A sensory nerve fiber that fires when skin temperature increases.
wave: An oscillation that travels through a medium by transferring energy from one particle or point to another without causing any permanent displacement of the medium.
wavelength: The distance required for one full cycle of oscillation for a sine wave.
Weber fraction: The constant of proportionality in Weber’s law.
Weber’s law: The principle describing the relationship between stimulus and resulting sensation that says the just noticeable difference (JND) is a constant fraction of the comparison stimulus.
white noise: Noise consisting of all audible frequencies in equal amounts. White noise in hearing is analogous to white light in vision, for which all wavelengths are present.