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EDU 310
Jeffery Jutai

CHAPTER 3SPATIAL VISION FROM SPOTS TO STRIPESVISUAL ACUITY OH SAY CAN YOU SEEContrast difference in luminance between an object and the back ground or between lighter and darker parts of the same objectAcuity the smallest spatial detail that can be resolvedCycle for a grating a pair consisting of one dark bar and one bright barVisual angle the angle subtended by an object at the retinaTo calculate visual angle of your resolution acuitydivide size of the cycle by the viewing distance at which you could just barely make out the orientation of the gratings then take the arctangent of this ratioThis resolution acuity represents one of the fundamental limits of spatial vision its the finest highcontrast detail that can be resolvedThe limit is determined by the spacing of photoreceptors in the retinaSine wave grating a grating with a sinusoidal luminance profile Light intensity in such gratings varies smoothly and continuously across each cycleAliasing misperception of a grating due to undersamplingA VISIT TO THE EYE DOCTORThe method for designating visual acuity was invented in 1862 by a Dutch eye doctor Herman SnellenAlthough 2020 vision is often considered the gold standard most healthy young adults have an acuity level closer to 2015ACUITY FOR LOWCONTRAST STRIPESSpatial frequency number of times a pattern such as a sine wave grating repeats in a given unit of spaceoExample if you view your book from 120 cm away the visual angle between each pair of white stripes is about 025 degree so the spatial frequency of this grating is 10254 cycles per degreeCycles per degree the number of pair of dark and bright bars per degree of visual angleYou might think that the wider the stripes the lower the spatial frequency the easier it would be to distinguish the light stripes from dark stripes but this isnt true Schade Fergus Campbell and Dan Green demonstrated that the human contrast sensitivity function CSF is shaped like an upside down UContrast threshold smallest amount of contrast required to detect a patternFor a 1 cycledegree grating to be just distinguishable from uniform gray the dark stripes must be about 1 darker than the light stripesoExample if a light stripe reflects 1000 photons dark stripe should reflect 990 photonsoThe reciprocal of this threshold is 1001100A contrast of 100 corresponds to a sensitivity value of 1 The CSF value reaches this value at about 60 cyclesdegreeRETINAL GANGLION CELLS AND STRIPESRetinal ganglion cells respond vigorously to spots of light They also respond well to certain types of stripes or gratingsWhen the spatial frequency of the grating is too low the ganglion cell responds weakly because of the fat bright bar of the grating lands in the inhibitory surround damping the cells responseWhen the spatial frequency is too high the ganglion cell responds weakly because both dark and bright stripes fall within the receptive field center washing out the responseWhen spatial frequency is just right with bright bar filling the center and dark bar filling the surround the cell responds vigorouslyThus retinal ganglion cells are tuned to spatial frequency each cell responds best to a specific spatial frequency that matches its receptive field size and it responds less to both higher and lower spatial frequenciesChristina EnrothCugell and John Robsonfirst to record responses of retinal ganglion cells to sinusoidal gratings
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