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PSYB51 ch.3

Contrast difference in luminance b/w lighter & darker parts of the same object
Acuity smallest spatial detail that can be resolved; acuity specified using terms 20/20 (eye doctors, scientists talk abt smallest visual angle of a cycle of the grating that can be perceived
Cycle for a grating, a pair consisting of one dark bar and one bright bar
Visual angle the angle subtended by an object at the retina; angle that would be formed by lines going from top to bottom of a cycle on the page through the center of the lens & on to the retina
Sine wave grating a grating w/ a sinusoidal luminance profile
Aliasing misperception of a grating due to undersampling; perceiving the gratings to be longer than they actually are
Problem w/ resolution acuity to spatial vision finest high-contrast detail that can be resolved; limit determined primarily by spacing of photoreceptors in the retina (p.53)
Herman Snellen constructed a set of block letters for which the letter as a whole was 5 times as large as the strokes that formed the letter
Snellen defined visual acuity as the distance at which a person can just identify the letters over the distance at which a person w/ “normal” vision can just identify the letters (20/20 vision)
Spatial frequency number of cycles of a grating per unit of visual angle (usually specified in degrees
Cycles per degree number of dark & bright bars per degree of visual angle
Contrast sensitivity function function describing how the sensitivity to contrast (defined as the reciprocal of constant threshold) depends on the spatial frequency (size) of the stimulus shaped like an upside down U
Otto Schade showed ppl sine wave gratings w/different spatial frequencies & had them adjust the contrast of the gratings until they could be detected
Contrast Threshold smallest amount of contrast required to detect a pattern; eg. For a 1-cycle/degree grating to be distinguishable from uniform gray, the dark stripes must be 1% darker than the light stripes
Retinal ganglion cells tuned to spatial frequency: each cell responds best to a specific spatial frequency that matches its receptive field size & it responds less to both higher & lower spatial frequencies
Response of a ganglion cell depends on the phase of the grating
Lateral geniculate nucleus (LGN) a structure in the thalamus, part of the midbrain, that receives input from the retinal ganglion cells & has input & output connections to the visual cortex
magnocellular layers neurons in the bottom 2 layers of the LGN which are physically larger than those in the top 4 layers; receives input from the M ganglion cells
parvocellular layers neurons in the top 4 layers of the LGN, which are physically smaller than those in the bottom 2 layers; receives input from the P ganglion cells
magnocellular pathway responds to large fast-moving objects
parvocellular pathway is responsible for processing details of stationary targets
contralateral refers to the opposite side of the body and/or brain
LGN layers 1,4,6 of right LGN listen to the left eye (contralateral)
LGN layers 2,3,5 of the right LGN receive input from the right eye (ipsilateral)
ipsilateral refers to the same side of the body and/or brain
topographical mapping orderly mapping of the world in the LGN & the visual cortex
primary visual cortex area of the cerebral cortex of the brain that receives direct inputs from the LGN, as well as feedback from other brain areas, & is responsible for processing visual info. (also known as area 17 or striate cortex) has 6 major layers
Corticol Magnification amount of corticol area (usually specified in milimeters) devoted to specific region (eg. 1 degree in the visual field)
Corticol Magnification 2 objects imaged on or near the fovea are processed by neurons in large part of striate cortex, but objects imaged in the far right or left periphery are allocated only a tiny portion of the striate cortex
corticol representation of the fovea is greatly magnified compared to the corticol representation of peripheral vision
consequence of corticol magnification is that visual acuity declines in an orderly fashion w/eccentricity (distance from the fovea)
orientation tuning tendency of neurons in striate cortex to respond optimally to certain orientations, and less to others
filter acoustic, electrical, electronic, or optical device, instrument, computer program, or neuron that allows the passage of some frequencies or digital elements & blocks the passage of others
ocular dominance property of the receptive fields of striate cortex neurons by which they demonstrate a preference responding somewhat more rapidly when a stimulus is presented in one eye than when it is presented in the other
simple cells: phase-sensitive a corticol neuron w/ CLEARLY DEFINED excitatory & inhibitory regions;
complex cell: phase insensitive neuron whose receptive field characteristics cannot be easily predicted by mapping w/spots of light; attuned to particular orientation & spatial frequency & shows ocular preference
end stopping: play an imp. role in human ability to detect luminance boundaries & discontinuities process by which a cell in a cortex 1st INCREASES its firing rate as the bar length increases to fill up its receptive field, & then DECREASES its firing rate as the bar is lengthened further- subclass of simple & complex cells
column vertical arrangement of neurons in the striate cortex, arranged by having similar orientation preferences
hypercolumn: eg. hypercolumn in part of cortex that represents fovea may "see" portion of visual field that is 0.05 degrees of visual angle across 1-mm block of striate cortex containing two sets of columns, each covering every possible orientation (0-180 degrees), w/1 set preferring input frm the left eye & 1 set preferring input from the right eye
cytochrome oxidase (CO) enzyme used to reveal the regular array of “CO blobs,” which are spaced abt 0.5mm apart in the primary visual cortex
adaptation reduction in response caused by prior or continuing stimulation (eg. Looking @ 20 degree lines then look @ 0 degree lines, they'll appear to be -20 degree b/c the neurons most sensitive to the adapting stimulus is fatigued
tilt aftereffect perceptual illusion of tilt, produced by adaptation to a pattern of a given orientation; strongly supports idea that the visual system contains individual neurons selective for different orientations
spatial frequency channel pattern analyzer, implemented by an ensemble of corticol neurons, in which each set of neurons is tuned to a limited range of spatial frequencies (Campbell & Robson 1968)
strabismus misalignment of the 2 eyes such that a single object in space is imaged on the fovea of 1 eye, & on a nonfoveal area of the other (turned) eye
amblyopia developmental disorder, characterized by reduced spatial vision in an otherwise healthy eye even w/ proper correction for refractive error. Often referred to as “lazy eye”
Hubel & Wiesel neurons in the striate cortex respond to stripes not stars. Receptive fields of the striate cortex neurons not circular (i.e in retina or LGN); they are elongated & respond more vigorously to bars, lines, edges, & gratings than to round circles of light
more cells in the striate cortex are responsive to horizontal & vertical orientations than to obliques
Neurons that share the same eye preference also have a columnar arrangement; eye preference switches every 0.5mm or so
cytochrome oxidase (CO) "blobs" spaced @ 0.5 mm apart, CO blobs have been implicated in processing colour w/interblob regions & processing motion & spatial structure
info. frm 2 eyes kept completely separate in retinas and the 2 LGNs; no single neuron receives input frm both eyes until the striate cortex transfer of adaptation effects from 1 eye to another implies that selective adaptation occurs in corticol neurons
multiple spatial frequency model of vision implies that spatial frequencies that stimulate different pattern analyzers will be detected independently, even if different frequencies are combined in the same image
figure 3.30 p. 74 images show that low frequencies emphasize the broad outlines of the face & high frequencies carry info. abt fine details
Robert Fantz research on vision in infants figure 3.32 (a) p.75 forced-choice preferential-looking stimuli & the experimental setup
(b) visual evoked potential (VEP)setup
(c) results of a sweep VEP experiment in which the spatial frequency of the stimulus is swept (continuously varied frm low to high spatial frequency), illustrating the extrapolated acuity
Created by: Ugly.Beauty