The largest displacement between successive presentations for which observers still obtain a coherent sense of motion. For larger steps the displaced object seems to disappear and then reappear at a different location. Increases with retinal eccentricity and is a function of the spatial frequencies in the image. Note that apparent motion can occur at distance much larger than Dmax.

Dark Current

Dark Adaptation

Dark Light

Rods spontaneously produce a signal that is indistinguishable from a photon elicited event even in complete darkness about once every 160 msec.

Degree of Convergence

T he angle of inclination of the two eyes used in depth perception

Delayed Match-to-Sample

Subject is shown a stimulus and then after a delay is required to choose the same stimulus from a number of stimuli to obtain reward

Depth Capture

Objects in an image with no apparent depth information can be captured by surrounding surfaces which do contain depth information.

Depth Cues

(See also Monocular Depth Cues).


Double vision.

Dichoptic Stimulation

When different images are presented to each eye. (See also Binoptic Stimulation and Monoptic Stimulation).

Dichromatic Color Vision

Color vision based on two cone photoreceptor pigments in the retina. (See also Trichromatic Color Vision and Tetrachromatic Color Vision).

Difference of Gaussians

One Dimensional:

Two Dimensional:

Where the constants c1 and c2 control the height of the individual gaussians and the constants 1 and 2 control their standard deviations. Generally the inhibitory gaussian (the subtracted term) is chosen with a smaller magnitude and larger standard deviation than the excitatory gaussian (the additive term).


The scattering of rays of light by collision with particles of matter as they pass through a medium such as air or water, or pass by a edge or narrow aperture. Plays a role when the pupil is small.


A diopter is a reciprocal meter. It is a measure of the power of a lens or optical system in bringing rays to a focus. The dioptric power of a thin lens is the reciprocal of the distance between the center of the lens and its focal point as measured in meters. (See also Optical Power).

Direction Selectivity

A difference in the response of a cell to a pattern of light moving through it receptive field according to the direction of movement. Cells respond well when a stimulus moves in one direction and not in the opposite direction.

Discounting the Illuminant

Illumination of the visual world tens to be non-uniform, and fluctuates. in order for the visual system to create a consistent perception of color and lightness it must somehow discount fluctuation and gradient in the lighting source.

Discrete Cosine Transform

A multiresolution image coding technique used in JPEG. DCT features are products of harmonic functions where j and k refer to postion along the hrizontal and vertical directions. Generates a series of transfrom coefficient corresonding to the DCT features in the image. (See also JPEG).

Disparity Gradient

The ratio of the difference between two feature's disparities and their cyclopean separation. Steeply slanted surfaces have large disparity gradients. Human binocular fusion will only tolerate a disparity gradient of about 1.

Doctrine of Isomorphism

(See Gestalt Psychologists).

Dorsal Pathway

One of two theorized systems of visual information processing. Information thought to progress toward the parietal cortex V1-> V2 -> MT -> MST -> STP. Functions in comprehension of spatial arrangement. (See also Ventral Pathway).