Gabor Function

One Dimensional:

 

Two Dimensional:

 

Where the constant  controls the standard deviation (fall-off) of the function and the constant  controls the spatial frequency.

Ganglion Cell

Output neuron of the retina. Axons form the optic nerve. Divided into several different classifications by morphological and physiological features for cats and primates.

(See also On-Center Cells and Off-Center Cells).

 

Morphological Classifications

Cat

Cell

 

Correspond physiologically to Y Cells.

 

Cell

 

Correspond physiologically to X Cells.

 

Cell

 

Correspond physiologically to W Cells.

 

Primate

Parasol Ganglion Cell

 

Large cell bodies, large dendritic arborization. Correspond physiologically to Magno Ganglion Cells.

Midget Ganglion Cell

 

Small cell bodies and small dendritic arborizations. Correspond physiologically to Parvo Ganglion Cells

 

Bistrated Ganglion Cell

 

Carry signal of the short wavelength cones. Dendritic trees stratified into two tiers near the inner and outer borders of the inner plexiform layer. Send output to parvocellular layer of LGN.

 

Physiological Classifications

Cat

X Cell

 

Cells signals combine linearly, sensitive to local contrast of image, respond poorly to uniform illumination of the receptive field, spot moving in any direction gives rise to same response, a dim spot in the receptive field will give rise to the same response as a smaller brighter one. Primarily responsible for the cats contrast sensitivity at high spacial frequencies. Separated into two classes, on-center and off-center cells. Correspond mophologically to  cells.

 

Y Cell

 

Concentrically organized receptive fields in both on- and off-center from. Receptive field 3x as large as X Cells. Show a pronounced non-linearity in the summation of signals of from photoreceptors. Rectified on/off response so cell discharge a burst of impulses at both the presentation and removal of a contrast pattern. Mediate grating detection at low spacial frequencies. Correspond morphologically to  cells.

 

W Cells

 

Correspond morphologically to  cells.

 

Primate

M Cell, Magno Cell, or P Cell

Receptive field 2-3 larger than P Cells. Show some non-linearities like Y-cell in cat, but bulk are like X-cells. More sensitive to achromatic contrast than P-cells. Transient response to sustained illumination. Appear to work on gross features and movement. Send output to magnocellular layer of LGN. Associated morphologically with Parasol Ganglion Cells.

 

P Cell, Parvo Cell, or P Cells

Similar to x-cells in cat but with color opponent organization. Make contact with only a single bipolar cell. Wave-length selective. Two main types, red-green chromatic opponency and yellow-blue chromatic opponency. Mediate color and fine-detail. Send output to parvocellular layer of LGN. Associated morphological with Midget Ganglion Cells.

 

Ganglion Cell Layer

(See Retinal Layers).

 

Ganzfeld Phenomenon

A uniformly illuminated surface of a uniform field (a ganzfeld) quickly disappears from visual perception and one experiences a gray fog.

 

Gaussian Filter

Algorithm smoothing spatial or temporal variation in an image by averaging neighboring values of light intensity, the contribution of values to the average being weighted according to a Gaussian function.

 

Gaussian Function

One Dimension:

 

Two Dimensions:

 

Where the constant c controls the height of the gaussian and the constant  controls its standard deviation.

Gelb-Experiment

A spinning black disc is illuminated by a projector in a darkly illuminated room. The disk appears to be white until a small white piece of paper is placed in front of the projected light at which time the true color of the disk is perceived (i.e. black).

 

Generalized Cone

(Marr & Nishihara 1978)

 

A surface representation created by moving a cross-section of a constant shape but variable size along an axis.

 

Generic View

A view of an object that is not special. In other words, the view won't change drastically with small perturbations in lighting or viewing direction. (As opposed to Accidental View).

 

Geniculostriate Pathway

Pathway from the retina via the LGN to the striate cortex

 

Geons

(Biederman 1987)

 Geometric ions. A set of basic shape primitive (wedges, cylinders) from which complex objects can be described.

 

Gestalt Laws of Perceptual Organization

(Koffka 1935, Kohler 1947)

 

Proximity

Things which are close together are grouped together

 

Similarity

Things which look similar are grouped together

 

Common Fate

Things which appear to move together are grouped together

 

Good Continuation

Perceptual organization will tend to preserve smooth continuity rather than yielding abrupt changes

 

Closure

Of several geometrically possible perceptual organizations the one producing a closed rather than an open figure is preferred

 

Relative Size

Other things being equal, the smaller of two areas will be seen as figure against a larger background

 

Surroundedness

Surrounded areas tend to be seen as figures as opposed to ground

 

Orientation

There is a preference for horizontal or vertically oriented regions to be seen as figure

 

Symmetry

Symmetrical area tend to be perceived as figures against asymmetrical backgrounds

 

Gestalt Psychologists

(Wertheimer, Kohler, Koffka)

 The whole is greater than the parts

 

Law of Pragnanz

Of Several geometrically possible organizations the one that will actually occur which posses the best, simplest and most stable shape

 

Doctrine of Isomorphism

Underlying every sensory experience is a brain event which is structurally similar to that experience

 

(See also Gestalt Laws of Perceptual Organization).

 

Glass Pattern

(Glass 1969)

If one takes a random set of dots and a slightly rotated copy of the same set of dots, local groupings create a globally coherent pattern.

Glaucoma

A symptomatic condition characterized by raised intra-ocular pressure, visual field loss, enlargement of the blind spot and changes in the appearance of the optic nerve head. If left untreated ganglion cell axon death may be caused by prolonged exposure of the eye to elevated introcular pressure. (More Information)

 

Grouping, Perceptual