Absorption Spectrum

The relationship between the wavelength of light striking a pigment and how strongly the light is absorbed.

 

Accessory Oculomotor Nucleus

Receives input from the pretectal area, innervates the ciliary ganglion. Mediates pupillary light reflexes.

Accessory Optic System (AOS)

Region of the vertebrate midbrain to which some optic nerve fibers project. Cells respond to large slowly moving textured patterns and are selective for both direction and speed of motion suggesting they are involved in the computation of global motion. Possibly used for the detection of retinal slip and used in image stabilization. Consists of two sets of retina ganglion cell fibers and three target nuclei in the anterior portion of the midbrain, the dorsal, medial and lateral nuclei.

 

Accidental View

A special view of an object in which small perturbations in lighting or viewing direction drastically change its appearance. For example, imagine looking at a pencil straight on from the tip. (As opposed to Generic View).

 

Accommodation

Adjustment of the optics of an eye to keep an object in focus on the retina as its distance from the eye varies. Process of adjusting the focal length of a lens.

Acuity

The highest spatial frequency rsolvable with a grating of unity contrast (i.e. the maximum possibly contrast). (Vernier vs. Stereo vs. ?)

Achromatopsia

A loss of color vision unaccompanied by attendant loss of other perceptual functions.

 

Action Potential

Action Spectra

A photoreceptor's relative spectral sensitivity.

 

Adaptation

A change in the sensitivity to light of a photoreceptor or the visual system as a whole to match the average light intensity present.

Afferent

Heading towards. A system's afferent signals are those entering the system from elsewhere. (As opposed to Efferent)

 

Affordance

(Gibson)

A possibility for action afforded to a perceiver by an object. The affordances of an object depend upon the perceiver as well as upon the characteristics of the object. For example, a stream affords such actions as jumping and paddling to a person, but to an frog it affords swimming.

 

Aftereffect

Afterimage

A long lasting change in perceived brightness that occurs after prolonged viewing of a given luminance that remains after the luminance has been changed. Presumably due to adaptation in the exposed region.

Aggregate field

The area within which the receptive fields of cells in a single hypercolumn of the visual cortex fall.

 

Aliasing

Results when density of sampling is less than density of signal. One can only measure the portion of the input falling over the sampled positions. Sampled values for two different signals can be the same when the in-between values are different. (See also Nyquist Limit).

 

Allelotropia

A depth percept can cause a shift in perceived form, namely change in the perceived distance between patterns in a configuration. For example that a pattern AB C is viewed in one eye and the pattern A BC is viewed through the other, the letter B is seen in depth at a position halfway between A and C.

 

Amacrine Cell

A type of neuron seen in the retina.

Ambient Vision

(Ingle, Schneider, Tevarthen, Held 67-68)

 

The role of vision involved in orienting an animal in space and guiding its larger movements. Sensitive to motion and dependent on peripheral vision. (As opposed to Focal Vision).

 

Amblyopia

An abnormal development of spatial vision associated with anisometropia or strabismus in early life. Visual acuity is reduced in the defocused or deviated eye as a result. (More Information)

 

Amodal

Perceptually present, but not having a real phenomenological presence. (As opposed to Modal).

 

Amodal Completion

(Michotte, Tines and Grabbe 1964)

 The creation of a subjective contour that is without local sensory attributes e.g. contrast difference. It is neither a cognitive inference nor a projection but a direct perception. Seen most often in displays that have partially occluded objects. (See also Modal Completion).

Anaglyph

Device for creating 3-D images using red and blue filters for the two eyes.

 

Anchoring

In any scene, the object which is the lightest is perceived as being white, or is 'anchored' to white. (Ref. Alan Gilchrist)

Angle of Incidence

Angle between an incident ray of light and the perpendicular to a surface.

Angle of Reflection

Angle between a reflected ray of light and the perpendicular reflecting surface.

(See Angle of Incidence).

 

Angle of Refraction

Angle between a refracted ray of light and the perpendicular to the refracting surface.

(See Angle of Incidence).

 

Anisometropia

A disorder involving unequal refractive errors in the two eyes.

 

Anisotropic

Any process or filter which is directionally selective or biased. (As opposed to Isotropic).

 

APB

(Slaughter & Miller 1981)

2-amino-4-phosoponobutyrate. When selectively applied to the mud puppy retina the On-bipolars, On-amicrines and On-ganglion cells become unresponsive to light, whereas the Off-cells retain their response properties. Allows for the study of the Off-cell pathway in isolation. Major finding is that the center-surround antagonism of Off-center retinal ganglion cells is not significantly altered suggesting that it is not a product of the interaction between On and Off cells.

 

Aperture Problem

Any motion detecting device with a field of view which is small relative to an edge moving through it can only detect the component of velocity at right angles to the edge, while the component parallel to the edge is invisible.

Apparent Motion

A observer presented with a display in which two items (lines, dots) in different locations are alternately exposed at certain interstimulus intervals will report seeing the item move from the first position to the second position. Illusion depends on the spatial separation, luminance and duration of the items. (See also Transformational Apparent Motion)

 

Area 17

One of Broadman's areas. (See Appendix I: Cortical Areas, V1)

 

Area 7A

Monkey parietal cortex

 

Area Centralis

Area in a vertebrate retina rich in cones, with little pooling of receptor outputs. In the human eye, the area centralis corresponds to the fovea, but this in not so in all species.

 

Area Contrast

A contrast effect which is not limited to the borders of an enclosed region. The change of brightness produced by contrast at the edges of the region spreads uniformly over the entire enclosed region. (As opposed to Border Contrast).

 

Assimilation

A neutral pattern of intermediate luminance appears brighter if a superimposed pattern is made of high luminance lines, yet darker if the pattern consists of low luminance lines. This effect is in the opposite direction as is seen in simultaneous contrast.

Automatic Gain Control

A fast gain control that aims to a constant output by changing gain in proportion to signal strength

 

Auto-Stereogram

Unlike classical Random Dot Stereograms (RDS Ref) which require two separate images presented dichoptically, an autostereogram consists of just one images viewed by two eyes simultaneously. A depth percept is attained from this image.