Glossary of Vision Terms

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


Please email me any terms you would like to have included.


Accommodation
The changes in the shape of the lens made to accommodate changes in viewing distance. Click here for an animated illustration.

Agnosia
A neuropsycholigcal condition that usually results from brain damage. Visual agnosics have damage to their visual association cortex and are unable to identify objects from vision alone. Agnosics are able to "see" visual images in the sense of being able to identify features of objects, but they cannot "unify" or interpret their images to obtain visual "gestalts". The term literally means "not knowing".

Amacrine Cells
Large retinal cells with long processes that spread laterally to interconnect bipolar cells, ganglion cells, and other amacrine cells.

Aperture Colors
Colors viewed through a reduction screen that seem to float in space rather than belonging to any surface. These colors are seen in isolation against a neutral background. The motivation of presenting such colors is to view "pure" colors without simultaneous constrast effects. They are contrasted with surface colors.

Bezold-Brucke Hue Shift
Color samples will look more reddish or greenish at lower light levels, and more blueish or yellowish at higher light levels. This is explained by assuming that the R-G system is more active at low light levels, and the Y-B system proportionately more active at higher light levels.

Bipolar Cells
Found in the intermediate cell layer of the retina. These cells relay information from the rods and cones to the ganglion cells. They bridge the two plexiform layers of the retina.

Blind Spot
Also known as the optic disk. The area of the retina where the optic nerve leaves the eye. There are no photoreceptors in this area therefore it is "blind". The brain fills in the area on the basis of information available from the other eye's view.

Cones
Photoreceptors located in the retina. In the human eye there are three different types of cones which are maximally sensitive to light of 440(blue-violet), 550(green), and 570nm(orange-red). The fovea contains only cones.

Constancy
The ability to perceive a property of an object as constant despite variations in the retinal stimulation it produces. The term is applied to size, depth, color and brightness.

Deuteranope
A dichromat missing the second cone type that contains green or medium wavelength absorbing pigment. A deuteranope has difficulty distinguishing between green and red mixed with a little blue (this condition is sometimes referred to as a red/green deficiency). This is the most common form of dichromacy.

Dichromat
A person who has only two out of the three cone types normally found in the human retina. Dichromats can match any colour with a mixture of two other wavelengths of light, in contrast to trichromats who require lights of three distinct wavelengths to match all other colours.

Emmert's Law
This describes perceived size-distance relationships and was originally formulated in terms of afterimages: the perceived size of an afterimage is proportional to its apparent distance.

Fovea
The area of the retina devoted to central vision. This is a small region densely packed with cone photoreceptors. This area is responsible for high acuity vision.

Ganglion Cells
The ouput neurons of the retina; their axons form the optic nerve which leaves the eye at the optic disk or blind spot. They are first order neurons of the visual system: the first retinal cells that fire action potentials. Ganglion cell receptive fields are circular with a center and an antagonistic surround.

Gestalt Theory
An experimental and theoretical movement that originated in Germany in 1911. The leading figures were Max Wertheimer, Kurt Koffka and Wolfgang Kohler. They were concerned with the perception of forms (Gestalts) and theorized that we always actively organize our perception of forms in the most parsimonious and meaningful way (Prägnanz). Various principles contribute to the attainment of Prägnanz and are known as the Gestalt Laws.

Hypermetropia
An inability to focus objects at near distances caused by a mismatch between the lens-retina distance and the focal length of the eye's dioptric apparatus. This could be caused by an eyeball that is not long enough. Commonly known as farsightedness. Corrected with convex or plus lenses.

The Inner Nuclear Layer of the Retina
Contains the cells bodies of bipolar, horizontal, and amacrine cells.

The Inner Plexiform Layer of the Retina
Contains the processes of bipolar, amacrine and ganglion cells.

Infero-temporal Cortex
Areas of the temporal lobe involved in higher level visual processing.

Magno Pathway



Medial Temporal Cortex
Part of the temporal cortex. Sometimes referred to as V5. This area is is considered primary in the processing of motion.

Medial Superior Temporal Cortex
Part of the medial temporal cortex and, by extension, the temporal cortex.

Metamers Two color stimuli that differ in terms of spectral composition but produce the same color under given viewing conditions.

Myopia
An inability to focus objects at far distances caused by a mismatch between the lens-retina distance and the focal length of the eye's dioptric apparatus. This could be caused by an eyeball that has grown too long. Commonly known as nearsightedness. Corrected with concave or minus lenses.

Occipital Lobe
The occipital lobe is the lobe of the cerebral cortex near the back of the head. Much of the preliminary processing of visual information occurs within this region.

The Outer Nuclear Layer of the Retina
Contains rod and cone phororeceptors.

The Outer Plexiform Layer of the Retina
Contains the processes of receptor, bipolar and horizontal cells.

Parietal Lobe
The parietal lobe lies between the temporal lobe and the occipital lobe.

Parvo Pathways



The Pigment Epithelium
A layer of cells packed with the black pigment melanin that lines the back of the eye. The pigment epithelium absorbs any stray light that has passed through the retina so that it does not bounce back and cause degredation of the image.

Prägnanz
The tendency to move towards simplicity, symmetry and wholeness. Whatever input our sensory systems are faced with they attempt to organize it into a coherent, meaningful percept. For example, simple repetitive patterns are organized into particular groupings, and we see figures in the fire or the clouds.

Protanope
A dichromat missing the first cone type that contains the red or long wavelength absorbing pigment. A protanope is highly insensitive to deep red light.

Purkinje Effect
Under conditions of dim illumination blue-green objects appear brighter than objects of other colours.This effect is attributed to rod sensitivity which peaks at 480nm.

Receptive Field
The area of the retina where stimulation of photoreceptors with light causes either an increase or decrease in firing. In the fovea receptive fields are small with centers of only a few minutes of arc, in the periphery the centers are 3-5 degrees.

The Retina
Derived from the Greek for "net". The retina lines the eye cavity and contains the eye's receptor sheet.

Rods
Photoreceptors located in the retina. They are sensitive to dim illumination and thus responsible for night vision. They are not involved in colour vision.

Stereopsis
Derived from the Greek for "solid sight". Stereopsis (often abbreviated as stereo) is the source of three-dimensional shape and distance information provided by analysis of the minute horizontal positional differences between the two eyes' views of a scene or object.

Temporal lobe
Located approximately over the ears, this lobe of the cerebral cortex is involved in some higher level processing in the infero-temporal cortex of visual information. See also medial temporal cortex.

Tritanope
A dichromat missing the third cone type that contains the "blue" or short wavelength absorbing pigment. A tritanope is highly insensitive to deep blue light. Tritanopia is an extremely rare condition.

20/20 Vision
Defined by Hermann Snellen as normal or emmetropic vision. From a viewing distance of 20ft the observer can resolve a letter of 5 min arc. If the denominator is less than 20 the observer is described as farsighted, whereas if it is greater than 20 the observer is said to be nearsighted.

Ventral Pathway The ventral pathway includes areas V1, V2, V4, and inferotemporal cortex. It is thought to be responsible for perception: the phenomenological awareness of visual object representations.

V1
The primary receiving area of visual cortex. Also known as area 17 (Brodmann's), and striate cortex (due to its striped appearance). Receives input from the lateral geniculate nucleus of the thalamus. The cells in the entry layer, IVca have circularly symmetric, centre/surround antagonistic receptive fields, like those of the LGN. In other layers the cells are orientation selective, direction selective and binocular (Hubel & Wiesel, 1979). Staining with cytochromoxidase (a marker of metabolic activity) yields blob and interblob areas in layers 2,3,5, and 6.

V2
The area adjacent to V1. When stained with cytochromoxidase a characteristic striping pattern is revealed, consisting of thick, thin and inter stripes. The thick stripes are considered part of the magno pathway. The thin stripes are part of the parvo pathway.
V4
Semir Zeki has designated this area as the colour processing area of visual cortex. It is part of the parvo-blob pathway.