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Chapter 3

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Department
Psychology
Course
PSYB51H3
Professor
Matthias Niemeier
Semester
Winter

Description
Chapter 3 – Spatial Vision: From Stars To Stripes Visual Acuity  Contrast: difference in luminance between object and background, or between lighter and darker parts of same object  Acuity: smallest spatial detail that can be resolved  Cycle: for a grating, pair consisting of one dark bar and one bright bar  Visual angle: angle subtended by an object at retina o To calculate visual angle of resolution acuity, divide size of cycle by viewing distance at which you could just barely make out orientation of gratings, then take arctangent of ratio  Resolution acuity represents one of fundamental limits of spatial vision – finest high contrast detail that can be resolved  Sine wave gratings: grating with sinusoidal luminance profile  Visual system “samples” the grating discretely, through array of receptors at back of retina  If receptors are spaced such that whitest and blackest parts of grating fall on separate cones, should be able to make out grating o But if cycle falls on single cone, see nothing but gray field  Aliasing: misperception of grating due to undersampling  Cones in fovea have center-to-center separation of 0.5 minute of arc (0.008 degree)  Rods and cones in periphery packed together less tightly  As result, visual acuity is poorer in periphery than in fovea  Visit To The Eye Doctor o Snellen constructed set of block letters for which letter as a whole was five times as large as strokes that formed letter  Defined visual acuity as o Distance at which person can just identify letters / distance at which person with “normal” vision can just identify the letters o Normal vision is 20/20 and a 20/20 letter is designed to subtend an angle of 5 arc minutes (0.083 degree) at eye o Most healthy young adults have acuity level of 20/15  Acuity of Low Contrast Stripes o Schade showed people sine wave gratings with different spatial frequencies and had them adjust contrast of gratings until they could be detected o Spatial frequency: number of cycles of a grating per unit of visual angle (usually in degrees). o Cycles per degree: number of dark and bright bars per degree of visual angle o Contrast sensitivity function (CSF): function describing how sensitive to contrast (defined as reciprocal of contrast threshold)depends on spatial frequency (size) of stimulus o Campbell and Green demonstrated that human contrast sensitivity function is shaped like an upside down U  Obtain units for Y axis by taking reciprocal of contrast threshold  Contrast threshold: smallest about of contrast required to detect a pattern  Contrast of 100% corresponds to sensitivity value of 1 and CSF reaches value at about 60 cycles/degree = 1 minute of arc  Why Sine Wave Gratings? o Although “pure” sine wave gratings may be rare in real world, patterns of stripes with more or less fuzzy boundaries are quite common o On larger scale, visual system appears to break down real world images into vast number of components Retinal Ganglion Cells and Stripes  Each ganglion cell also responds well to certain types of stripes or gratings  When spatial frequency of grating is too low, ganglion cell responds weakly because part of fat, bright bar of grating lands in inhibitory surround, damping cell’s response  When spatial frequency is to high, ganglion cell responds weakly because both dark and bright stripes fall within receptive field center, washing out response  When spatial frequency just right, with bright bar filling center and dark bars in surround, cell responds vigorously  Retinal ganglion cells are “tuned” to spatial frequency – each cell responds best to specific spatial frequency that matches its receptive field size and responds less to both higher and lower spatial frequencies  Enroth-Cugell & Robson showed that cells respond vigorously to gratings of right size and discovered that responses depend on phase of grating – its position within receptive field  Phase: relative position of grating  If grating phase shifted by 90 degrees, half of receptive field center will be filled by light bar and half by dark and similarly for surround – no net difference between light intensity in receptive field’s center and its surround  Second 90 degree shift puts dark bar in center and light bars in surround, producing negative response  Third phase shift returns to situation after first shift, with overall intensities in center and surround equivalent The Lateral Geniculate Nucleus  Lateral geniculate nucleus (LGNs): structure in thalamus, part of mid brain that receives input from retinal ganglion cells and has input and output connections to visual cortex  Neurons in bottom 2 layers of primate LGN are physically larger than those in top 4 layers  Magnocellular layers: neurons in bottom 2 layers of LGN, which are physically larger than those in top 4 layers  Parvocellular layers: neurons in top 4 layers of LGN, which are smaller than bottom 2 layers  2 types of layers differ in important way – magnocellular layers receive input from M ganglion cells in retina and parvocellular layers receive input from P ganglion cells  Left LGN receives projections from left side of retinas in both eyes, and right LGN receives projectsions from right sides of retinas  Second, each layer of LGN receives input from one or other eye  From bottom to top, layers 1, 4 and 6 or right LGN listen to left (contralateral) eye, while layers 2, 3, and 5 receive input from right (ipsilateral) eye o Contralateral: referring to opposite side of body (or brain) o Ipsilatera: referring to same side of body (or brain)  Each LGN layer contains highly organized map of complete half of visual field  Topographical mapping: orderly mapping of world in lateral geniculate nucleus and visual cortex  LGN neurons have concentric receptive fields that are similar to those of retinal ganglion cells – respond well to spots and gratings  LGN appears to serve as staging area where feedback from other parts of brain modulates input from eyes Striate Cortex  Receiving area for LGN inputs in cerebral cortex lies below inion  Primary visual cortex (V1)/area 17/striate cortex: area of cerebral cortex of brain that receives direct inputs from lateral geniculate nucleus, as well as feedback from other brain areas, and is responsible for processing visual info o Striate cortex consists of 6 major layers, some sublayers o Fibers from LGN project mainly to layer 4, with magnocellurlar axons coming into subway layer 4Ca and parvocellular axons projecting to sublayer 4CB  Like LGN, striate cortex has systematic topographical mapping of visual fiend  Major and complex transformation of visual info takes place in striate cortex – contains on order of 200 million cells – more than 100 times as many as LGN  Cortical Topography and Cortical Magnification o Objects imaged on or near fovea are processed by neurons in large part of striate cortex, but objects imaged in far right or left periphery are allocated only a tiny portion of striate cortex o Cortical magnification: amount of cortical area (usually specified in millimeters) devoted to specific region in visual field o Consequence of cortical magnification is that visual acuity declines in orderly fashion with eccentricity (distance from fovea) o Fovea representation in cortex is highly magnified because visual system must make trade off o High resolution requires great number of resources – dense array of photoreceptors, one-to-one lines from photoreceptors to retinal ganglion cells, large chunk of striate cortex Receptive Fields in Striate Cortex  Hubel and Wiesel found that cat’s cortical cells hardly responded at all to same spots that made ganglion cells fire like crazy  Inserted glass slide with black spot into slot in special ophthalmoscope and when it fired strong burst, realized that response had nothing to do with spot itself; cell had been responding to shadow cast by edge of glass slide as it swept across ophthalmoscope’s light path  Most fundamental discovery was that neurons in striate cortex responds to stripes, not stars – receptive fields of striate cortex neurons aren’t circular o Rather, they’re elongated, responding much more vigorously to bars, lines, edges and gratings than to round circles of light  Orientation Selectivity o Hubel and Wiesel uncovered number of other properties of receptive fields of neurons in striate cortex o First, individual neuron won’t respond equivalently to just any old stripe in its receptive field – responds best when line or edge is at just right orientation and hardly at all when line is tilted more than 30 degrees away from optimal orientation o Orientation turning: tendency of neuron in striate cortex to respond optimally to certain orientation, and less to others o Cells in striate cortex are selective for 0, 45, 20, 62 degrees and so on, so that population of neurons as whole detects all possible orientations  Other Receptive Field Properties o Cortical cells respond not just to bars, lines and edges – like retinal ganglion cells, also respond well to gratings  And like ganglion cells, respond best to gratings that have just right spatial frequency to fill receptive field center o Cortical cells much more narrowly tuned than retinal ganglion cells o Narrow tuning functions mean that each striate cortex neuron functions as filter for portion of image that excites cell o Filter: acoustic, electrical, electronic or optical device, instrument, computer program, or neuron that allows passage of some frequencies or digital elements and blocks passage of others o Another discover was that many cortical cells respond especially well to moving lines, bars, edges and gratings o Many neurons respond strongly when line moves in one direction o Info from 2 e
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