Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySpatial frequency tuningPSY 310Greg FrancisLecture 09Waves in the brain.Purdue UniversityWhat letter do you see? D, E, orF?Purdue UniversityVisual angle Sizes of objects are in terms of visual angle Visual angle changes with distancePurdue UniversityVisual angle Sizes of objects are in terms of visual angle Visual angle changes with distancePurdue UniversityVisual angle Sizes of objects are in terms of visual angle Visual angle changes with distancePurdue UniversityVisual angle Sizes of objects are in terms of visual angle Visual angle changes with distanceProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityVisual angle So how does the visual system respond Consider the response of ganglion cells Similar for orientation-sensitive cells If you sit close…Purdue UniversityVisual angle So how does the visual system respond Consider the response of ganglion cells Similar for orientation-sensitive cells If you sit further…Purdue UniversityVisual angle So how does the visual system respond Consider the response of ganglion cells Similar for orientation-sensitive cells If you sit at the back…Purdue UniversitySpatial frequency We can think of how the visual system interpretsinformation in terms of spatial frequencyPurdue UniversitySpatial frequency This image contains lots of different frequencies ofsine wavesLow frequenciesHigh frequenciesPurdue UniversitySpatial frequency Vertically, it changes low contrastHigh contrastLow contrastProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySpatial frequency The question is how sensitive are we to contrast atdifferent spatial frequencies?Purdue UniversitySpatial frequency Most people report the lines fade into the background along acurve something like Contrast Sensitivity FunctionPurdue UniversitySpatial frequency There is a similar effect for this odd grating The receptive field responds better to somefrequencies than othersPurdue UniversitySpatial frequency There is a similar effect for this odd grating A larger receptive field responds better to lowerfrequenciesPurdue UniversitySpatial frequency There is a similar effect for this odd grating A larger receptive field responds better to lowerfrequencies You can make the same kind of observations aboutorientation-sensitive cellsPurdue UniversityVisual cortex Measurecontrastsensitivity fororientation-sensitive cells Different cellsprefer sinewaves ofdifferentfrequencies Fourieranalysis?Prof. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityVisual perception Does this have anything to do with visualperception? Easy to show with an adaptation experiment Show a stimulus, lots of cells respondImagecortexPurdue UniversityVisual perception Does this have anything to do with visualperception? Easy to show with an adaptation experiment Show a stimulus, lots of cells respondImagecortexPurdue UniversityVisual perception The pattern across the cells indicates informationabout the grating For example, it’s spatial frequencyImagecortexPurdue UniversityVisual perception Change the spatial frequency of the image And the cell pattern responses changeImagecortexPurdue UniversityVisual perception Change the spatial frequency of the image And the cell pattern responses changeImagecortexPurdue UniversityVisual perception Suppose we start with the high frequency (smallbars) grating We get this patternImagecortexProf. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityVisual perception Over time, all of these cells weaken theirresponse More for the ones that respond strongestImagecortexPurdue UniversityVisual perception After adaptation Look at a the grating of intermediate spatialfrequency The relative pattern may be more like the lowfrequency gratingImagecortexPurdue UniversitySpatial frequency Try it!Purdue UniversitySpatial frequency Try it!Purdue UniversityConclusions Spatial frequency in visual cortex Different receptive fields are sensitive to differentranges of spatial frequencies It’s actually more complicated than this Provides a language to explain a variety ofpercepts F-E-D image Spatial frequency adaptationPurdue UniversityNext time Higher level visual processing Dorsal and ventral streams of the
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