s and p exam #2 textbook terms

S cone

Short-wavelength sensitive cone; peaks at ~420 nm (blue); least numerous of the three cone types; contributes to blue-yellow opponent processing.

M cone

Medium-wavelength sensitive cone; peaks at ~535 nm (green); one of the two longer-wavelength cones; contributes to red-green opponent processing.

L cone

Long-wavelength sensitive cone; peaks at ~565 nm (red/yellow-green); most numerous cone type; contributes to red-green opponent processing and luminance.

Photopic

Daylight vision; cone-dominated; high acuity and full color vision; operates at high light levels.

Scotopic

Dim-light vision; rod-dominated; no color and low acuity; operates at low light levels; relies on rod sensitivity.

Problem of univariance

A single photoreceptor cannot distinguish between a change in wavelength and a change in intensity; it only signals the total number of photons absorbed, so one cone type alone cannot encode color and at least two cone types must be compared.

Trichromatic theory of color vision

Color is encoded by the relative activity of three cone types (S, M, L); explains color matching and metamers but cannot by itself explain opponent phenomena such as negative afterimages.

Metamers

Physically different light spectra that appear the same color because they produce identical responses across the three cone types.

Additive color mixture

Mixing lights so that wavelengths add together (e.g., red + green = yellow); primary colors are red, green, and blue; applies to lights and screens.

Subtractive color mixture

Mixing pigments so that wavelengths are absorbed/subtracted (e.g., blue + yellow = green); primary colors are cyan, magenta, and yellow; applies to paints and inks.

Lateral geniculate nucleus

Thalamic relay for visual information; color is processed mainly in the parvocellular layers and some koniocellular layers before reaching visual cortex.

Cone-opponent cell

Neuron whose response is excited by one cone type and inhibited by another (e.g., L+ M−); the basic neural building block of opponent color coding.

Color space

A three-dimensional representation of color (e.g., RGB, HSV, or cone excitation space) in which each color is a point defined by its coordinates.

Achromatic

Without color; shades of gray varying only in luminance.

Hue

The perceptual attribute of color (red, green, blue, etc.); related to the dominant wavelength of light.

Saturation

Colorfulness or purity of a color; how much white is mixed in; a fully saturated hue has no white added.

Brightness

Perceived intensity or luminance of light; the achromatic dimension of color.

Opponent color theory

Color is encoded as opposing pairs: red–green, blue–yellow, and black–white; explains negative afterimages, unique hues, and why we never see reddish-green or bluish-yellow.

Unique hue

A hue that contains no other hue (pure red, green, blue, or yellow); supports opponent theory because each unique hue corresponds to one pole of an opponent channel.

Double-opponent cell

Cell with a center excited by one color pair and a surround by the opposite (e.g., L+M− center, M+L− surround); important for color contrast and constancy.

Single-opponent cell

Cell with a spatially uniform cone-opponent response; signals the overall color of a patch rather than spatial contrast.

Achromatopsia

Loss of color vision from cortical damage; the world appears gray despite intact cones and retinae.

Afterimage

Visual experience that persists after the stimulus is removed; can be positive (same color) or negative (complementary color).

Adapting stimulus

The stimulus stared at to induce adaptation and produce an aftereffect.

Negative afterimage

Afterimage in the complementary color (e.g., stare at red then see green); evidence for opponent processing.

Neutral point

Wavelength at which an observer cannot detect a hue (e.g., in dichromats); the point where the two remaining opponent channels balance.

Deuteranope

Dichromat missing M cones (green-weak); the most common form of dichromacy.

Protanope

Dichromat missing L cones (red-weak).

Tritanope

Dichromat missing S cones (blue-weak); rare.

Color-anomalous

Anomalous trichromat; has all three cone types but one is shifted in sensitivity, causing reduced color discrimination.

Cone monochromat

Has only one functioning cone type; severe color deficiency; can match any color with a single wavelength.

Rod monochromat

No functioning cones; sees only with rods; no color vision, poor acuity, and photophobia.

Agnosia

Inability to recognize objects despite intact sensation (e.g., color agnosia: cannot recognize colors despite seeing them).

Anomia

Inability to name colors despite perceiving them correctly.

Cultural relativism

Idea that color categories and color naming vary by culture and language (e.g., Whorfian hypothesis).

Color contrast

A color appears more different from its surround (e.g., gray looks darker on white than on black).

Color assimilation

A color appears more similar to its surround; opposite of color contrast.

Unrelated color / related color

Unrelated color is seen in isolation (aperture mode, no context); related color is seen as a surface color in context and is subject to constancy.

Color constancy

Perceiving stable object color despite changes in illumination; the brain discounts the illuminant.

Illuminant

The light source illuminating a scene; its spectral power distribution affects the light reaching the eye.

Spectral reflectance function

The proportion of light a surface reflects at each wavelength; the physical basis of an object's color.

Spectral power distribution

The amount of light energy a source emits at each wavelength.

Reflectance

The proportion of incident light a surface reflects; combined with the illuminant it determines the light reaching the eye.

Realism

The view that perception reflects the real world accurately; perception is a faithful copy of reality.

Positivism

The view that only observable/measurable phenomena are valid; perception is actively constructed rather than a passive copy.

Euclidean

Geometry of flat space in which parallel lines never meet; not always true for visual space, which can be non-Euclidean.

Binocular

Using both eyes; enables binocular summation and stereopsis.

Binocular summation

The combined benefit of two eyes over one (better detection, acuity, and sensitivity).

Binocular disparity

The difference between the two retinal images of an object; the basis of stereopsis and a powerful binocular depth cue.

Monocular

Using one eye only.

Stereopsis

Depth perception based on binocular disparity; the perception of depth from the differences between the two eyes' images.

Depth cues

Sources of information about the distance and three-dimensional layout of objects.

Monocular depth cue

A depth cue available with one eye (e.g., occlusion, relative size, perspective, motion parallax).

Binocular depth cue

A depth cue requiring both eyes (e.g., binocular disparity, convergence).

Occlusion

Near objects block far objects; the strongest and most reliable depth cue; a nonmetrical cue.

Nonmetrical depth cue

A depth cue that gives only ordinal depth information (which object is closer), not distance.

Metrical depth cue

A depth cue that gives quantitative distance information (e.g., how many meters away).

Projective geometry

The geometry of how 3D scenes project onto 2D surfaces such as the retina; describes perspective and explains why the retinal image is ambiguous.

Relative size

A cue in which smaller objects appear farther away, assuming objects have the same real size.

Texture gradient

A cue in which texture becomes denser and finer with increasing distance.

Relative height

A cue in which objects higher in the visual field (below the horizon) appear farther away.

Familiar size

Using knowledge of an object's known real size to judge its distance.

Relative metrical depth cue

A depth cue that gives relative distances (e.g., A is twice as far as B) but not absolute distance.

Absolute metrical depth cue

A depth cue that gives absolute distance (e.g., in meters).

Aerial perspective (or haze)

A cue in which distant objects appear hazier and bluer due to atmospheric scattering.

Linear perspective

A cue in which parallel lines converge with distance; the basis of vanishing points.

Vanishing point

The point on the horizon where parallel lines in a picture converge.

Pictorial depth cue

A depth cue used in 2D pictures (e.g., perspective, shading, occlusion, relative size).

Anamorphosis (or anamorphic projection)

A distorted image that looks correct only from one specific viewpoint.

Motion parallax

A cue in which near objects move faster than far objects when the observer moves; provides strong depth information.

Accommodation

The lens changes shape to focus on objects at different distances; a weak depth cue.

Convergence

The eyes turn inward for near objects; a weak binocular depth cue.

Divergence

The eyes turn outward for far objects.

Corresponding retinal points

Points on each retina that project to the same location in space; stimulation of these points produces single vision.

Vieth-Müller circle

The theoretical circle through the fixation point on which corresponding retinal points lie; an approximation of the horopter.

Horopter

The locus of points in space with zero disparity; objects on the horopter are seen as single.

Diplopia

Double vision; occurs when an object's disparity falls outside Panum's fusional area.

Panum's fusional area

The small region around the horopter where disparity is fused into single vision without diplopia.

Crossed disparity

Disparity for an object closer than the fixation point; the images cross the midline; signals "near."

Uncrossed disparity

Disparity for an object farther than the fixation point; the images do not cross; signals "far."

Stereoscope

A device that presents separate images to each eye (e.g., the Wheatstone stereoscope) to produce stereopsis.

Free fusion

Seeing stereograms without a device by crossing or diverging the eyes.

Stereo blindness

The inability to perceive stereopsis; can result from strabismus or lack of binocular vision early in life.

Random dot stereogram (RDS)

A stereogram made of random dots in which only disparity reveals a shape; proves stereopsis is independent of monocular cues.

Cyclopean

A stimulus defined by disparity alone and not visible monocularly; named after the mythical one-eyed giant.

Correspondence problem

The problem of how the brain matches points in the left image with points in the right image; solved by constraints such as uniqueness and continuity.

Uniqueness constraint

Each point in one eye matches at most one point in the other eye.

Continuity constraint

Matching points tend to lie on continuous surfaces; neighboring points match neighboring points.

Bayesian approach

Perception as probabilistic inference that combines prior knowledge with sensory likelihood to estimate the world.

Ideal observer

A theoretical observer that optimally uses all available information; a benchmark for comparing human performance.

Stereoacuity

The threshold for detecting binocular disparity; a measure of fine depth discrimination.

Dichoptic

Presenting different stimuli to each eye.

Critical period

A developmental window (early childhood) during which normal binocular vision must be established; deprivation can cause permanent deficits.

Strabismus

Misalignment of the eyes (crossed or wall-eyed); can lead to suppression and loss of stereopsis.

Esotropia

An inward turn of the eye (cross-eyed).

Exotropia

An outward turn of the eye (wall-eyed).

Tilt aftereffect

Adaptation to tilted lines shifts perceived orientation of subsequently viewed lines; evidence for orientation-selective neurons.

Suppression

The brain ignores input from one eye (common in strabismus) to avoid diplopia.

Attention

The selective processing of some information over others; can be spatial, feature-based, or object-based.

Selective attention

Focusing processing on relevant stimuli while ignoring irrelevant ones.