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Chapter 3

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Matthias Niemeier

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 commono 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 cells 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 fields 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 n
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