Agenda Pomona College ֠ Visual pathway LCS 11: Cognitive Science 1. Neural retina 2. Lateral geniculate nucleus Vision 2 3. Primary visual cortex ֠ Construction illusory surfaces Jesse A. Harris ֠ Reading for Wednesday: Hoffman, 1998: ch 4 ֠ Group presentations May 1, 6, and 8 April 22, 2013 Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 1 Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 2 Visual pathway Eye Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 4
Neural retina ◮ Image focuses on neural retina ◮ Neural retina composed of discrete photoreceptor cells which are attuned to particular stimuli Rods Responsive to movement Cones Responsive to color and detail ◮ Early selectivity within eye itself Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 6 Rods and cones Retinal ganglion cells Retinal ganglion cells Respond to small receptive fi eld, consist of cells with ON and OFF regions. ON Excitation OFF Inhibition Some cells have ON center, others have OFF center, approx equal cells that compete with one another in terms of fi ring rates. Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 8
Retinal ganglion cells Nystagmus Fading images Many cells in retina are detectors of fading; attuned to changes in stimulus (transient cells). ◮ Constant slight tremor (nystagmus) to reactivate stimulus on photoreceptors ◮ If eye is stabilized by paralyzing eye muscles with drugs or by moving images to account for movement, world becomes a dull grey. Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 10 Troxler fading ◮ Fixate on upper cross for a while: Blurry grey lines disappear. ◮ Then fi xate on lower cross: complementary after images appear. Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 12
Troxler fading ◮ Fixate on central dot and doughnut will disappear, replaced by orange background. Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 14 ◮ Left visual fi eld inputs right V1 ◮ Right visual fi eld inputs left V1 ◮ Crossing at the optic chiasm Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 16
Lateral geniculate nucleus ◮ Cells similar to retinal ganglion cells, with small receptive fi elds. ◮ No discrimination between more complex features of image, e.g., orientation or movement. Retinotopic maps Nearby parts of image are represented in the nearby cells of the retina, LGN, and V1 areas of the brain. Physiological organization selectively mirrors organization of object perceived. Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 17 Primary visual cortex (V1) Distortion in retinotopic maps Columnal organization Cells are ordered via columns (approx 1mm strips) that are Cortical magni fi cation e ff ect sensitive to particular features. Cells most dense around fovea - far greater amount of V1 dedicated to processing information from with the fovea. Each cell has a preferred Results in a distorted map, where there is about a 2000-fold orientation which responds to magni fi cation for area within 1-2 degrees of visual angle. input from a particular receptive fi eld. But columns are not evenly distributed across V1! Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 20
Compression system High resolution in important areas, with low resolution elsewhere. ֠ If the brain were required to deal with all information from the complete visual fi eld with the same degree of acuity as the fovea, it would weight 10 tons. Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 22 Simple cells ◮ Cells in LGN don’t response to orientation ◮ Cells in V1 do - if bar covers ON center, than minimal inhibition, if it crosses inhibitory area enough, then the inhibition cancels out fi ring. Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 23 Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 24
Complex and hyper complex cells Complex cells Larger receptive fi elds, without clear division between excitatory and inhibitory areas. Hyper complex cells Responsive to orientation, as well as length. Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 25 Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 26 Rules The miracle of vision! Rule of generic view The visual system transforms a fragmented 2D image into a Construct only those visual worlds for which the image is coherent 3D construct. stable (i.e., generic) view. ◮ Physiological organization should feed into rules for constructing percepts Rule 1 ◮ We construct visual perception of everything we see, Always interpret a straight line in an image as a straight line according to basic constraints or principles. in 3D. ◮ Illustrate this best with subjective contours and surfaces, in which we create the perception of a surface which is not present in the image. Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 27 Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 28
Rules Rules Rule of generic view Rule of generic view Construct only those visual worlds for which the image is Construct only those visual worlds for which the image is stable (i.e., generic) view. stable (i.e., generic) view. Rule 2 Rule 2 If the tips of two lines coincide in an image, then always If the tips of two lines coincide in an image, then always interpret them as coinciding in 3D. interpret them as coinciding in 3D. Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 29 Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 30 Kanizsa triangle Non-accidental features (Rule 12) If two visual structures have a non-accidental relation, group them and assign them to a common origin. Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 31 Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 32
Non-accidental features Convex curves Non-accidental features (Rule 12) Convex cusps (Rule 11) If two visual structures have a non-accidental relation, group Construct subjective fi gures that occlude only if there are them and assign them to a common origin. convex cusps. Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 33 Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 34 Blivet fi gure, aka the devil’s fork Blivet fi gure, aka the devil’s fork Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 35 Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 36
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