P Cell
(See Ganglion Cell).
Pale Stripes
(See Appendix I: Cortical Areas, V2).
Palinopsia
(Literally 'seeing again') is a form of visual perseveration. It refers to reappearance of an image after some time when the original external stimulus is no longer available. Neural basis of palinopsia is unknown but many palinopsia patients have some right-hemisphere disturbances.
Pandemonium
(Selfridge, Neisser, Lindsay
& Norman)
A feature based object recognition scheme. Feature demons signal the presence of features in an image. Cognitive demons look for feature combinations. Decision demon picks most active cognitive demons.
Panum's Fusional Area
Region of space within which perception of a single object is possible. (See also Stereopsis).
Panum's Limiting Case
The minimum conditions for the perception of stereopsis consisting of three lines, one for one eye and two for the other.
Parallel Pathways
Anatomical and physiological evidence exists for parallel pathways in the visual system e.g. Ventral and Dorsal streams for spatial and object processing. Parvo and Magno pathways from LGN to V2, V2 and V4. Hubel and Livingstone (1987) supported a few for strong segregation of the M and P pathways. Recent evidence (see Merrigan and Maunsell, 1994 for a review) strongly disputes this claim and the general viewpoint today is that much inter-stream communication occurs and that strict segregation of paths is unlikely.
Parasol Ganglion Cell
(See Ganglion Cell).
Parvocellular Pathway
Pathway that begins with the midget (parvo) ganglion cells in the retina and terminates within the parvocellular layer of the LGN. Conduction slower than that of Magnocellular Pathway. (See also Magnocellular Pathway and Ganglion Cell).
Pattern Adaptation
The perceived width and orientation of a grating pattern can be altered by first adapting to grating pattern of different frequencies and orientations. After adapting the contrast sensitivity to stimuli near the frequency or orientation of the adapting pattern is reduced. Suggests the existence of neurons tuned for frequency and orientation at are fatigued by the adapting pattern.
Pedestaled Motion Stimulus
A compound stimulus consisting of a linear superposition of a drifting sine wave (motion) and a stationary sine wave (pedestal). Defeats feature tracking, meaning that if observers attempt to track peaks to discover motion they would not be able to perceive coherent motion as the peaks merely oscillate back and forth. (i.e. computing motion from peaks, valleys or zero-crossing fails). Subjects, however, are able to perceive the correct motion of the moving sine wave as long as the amplitude of the pedestal is not large compared to the amplitude of the motion.
Perceptive Field
Area of the retina or visual space that when stimulated by a visual stimulus produces a change in behavioral response (for example in a monkey).
(See also Receptive Field).
Perceptual Constancy
The phenomenon that the perception of an object remains constant despite changes in the its size, lighting conditions and orientation.
Perceptually Lossless Compression
An image processing technique with loses some information about contents of the image, but the distortions produced by the loss aren't visible to the human image processing system. (See also Lossy Compression and Lossless Compression).
Peri-Stimulus Time Histogram
Plot of the average number of action potentials at each moment in time following a brief test flash.
Photometric Units
Photopic
Bright light conditions where only the cones are functional. (See also Mesopic and Scotopic).
Photoreceptor
Cells specialized for the transduction of light. (See also Cone and Rod).
Photoreceptor Mosaic
The spacial arrangement of photoreceptors in the retina.
Photon Noise
The fact that a stream of photons is not steady but fluctuates in intensity around an average value
Phototransduction
The transformation of incoming light to a neural impulse
1) Light activates visual pigment in photoreceptors
2) Activated molecules cause stimulation of cGMP phosphodiesterase, an enzyme that reduces the cytoplasmic concentration of cGMP
3) Reduction of cGMP concentration close the cGMP-gated channels hyperpolarizing the photoreceptor
Pigment Epithelium
Cells lining the back of the eye
containing melanin which resynthesize photopigment for photoreceptors.
(See also Melanin).
Poggendorf Illusion
Perception of colinearity can be distorted by an intervening figure. (More on Optical Illusions)
Pointspread
A measure of optical blur produced by natural imperfections of the lens of the eye. The response to a point of light produces a certain amount of blur, the amount of this blurring is known as the pointspread.
Poles (of optic flow field)
The two points in the optic flow field surrounding an observer moving through the environment at which there is no flow of optic texture. One is the point toward which the observer is moving and the other is the point away from which the observer is moving.
Ponzo Illusion
When two horizontal rods of equal length are drawn over an upside down V the upper rod appears longer than the lower rod. (See also Corridor Illusion). (More on Optical Illusions)
Pool Hypothesis
(Richard 1970)
Hypothesis for the mechanism for stereopsis. Suggests the visual system contains three pools, one for crossed disparities, one for uncrossed disparities and one for non-zero disparities. A particular depth is signaled by the relative activity of crossed and uncrossed pools rather than by a sharply tuned neuron coding a particular depth.
Pop-Out
Target items in a visual search that have features which allow them to be detected quickly regarless of the nummber of distractors presents are said to "pop-out".
Post Stimulus Time Histogram
A method for examining cell response to a particular stimulus. The time after stimulus onset is divided into a number of bins. The stimulus is then presented repeatedly and the count in each bin increased each time the cell fires during that time bin.
Prelunate Cortex
Area V4 in rhesus monkey brain.
(See also Appendix I: Cortical Areas, V4).
Presbyopia
The condition whereby the amplitude of accommodation, or ability to focus on objects at near distances, decreases with increasing age. It is corrected by a different prescription for reading, which is additive to the normal spectacle correction used for distance vision. Some recent research indicates that presbyopia may be caused by structural changes in the tendons and elastic fibres of the posterior ciliary body. The age related increase in fibrillar material could cause decreased compliance of the posterior insertion of the ciliary muscle.
Pretectal Area
Midbrain area just rostral to the
superior colliculus. Receives input from the optic tract. Projects bilaterally
to the preganglionic parasympathetic neurons of the accessory oculomotor
nucleus. Mediates pupillary light reflexes.
(See Figure).
Primal Sketch
(See Appendix III: Marr).
Principle of Univariance
The absorbtion of a long wavelength (low frequency, low energy) quantum has the same effect on a receptor as the absorbtion of a short wavelength (high frequency, high energy) quantum. It is the probability of absorption that changes photoreceptor sensitivity.
Prosopagnosia
A loss of the ability to identify members of complex categories such as faces.
PSTH
(See Post Stimulus Time Histogram).
Psychophysical Function
A plot showing the variation in threshold (e.g. the dimmest light required for response) with some stimulus dimension (e.g. wavelength).