Sensation and Perception Exam 4 Vision Spring 2015 Vision III 1 Striate cortex How many layers What is cortical magnification What is visual crowding Where is the fovea represented Striate cortex V1 Primary Visual Cortex Area 17 6 layers Cortical magnification more neurons in cortex process info from fovea compared with info from peripheral retina Visual acuity decreases with eccentricity Convergence in rod pathway and cortical magnification aren t only reasons we don t see well in periphery Perceptual problem of visual crowding in periphery Makes it hard to identify objects Periphery anterior fovea gets lots of representation posteriorly 2 How is striate cortex organized answer topographically aka retinotopically The striate cortex is organized topographically retinotopically 3 What types of stimuli do cells in striate cortex respond to They respond to lines bars edges and gratings For each cell the line it responds to has to be a particular angle and the cell responds best at that angle orientation tuning Each line must also be a certain width some respond best when line is moving in a particular direction 4 Compare and contrast simple cells and complex cells Simple Cells have receptive fields respond best to object of given shape and orientation Best stimuli are bars lines rectangles with definite edges Also respond to gratings tuned to spatial frequency Complex cells Orientation specific Larger receptive field than simple cells Stimulus can occur in wider range of visual field Best response to movement if stimulus is properly oriented and movement is in particular direction Simple Cells Orientation Specific Sometimes Length Specific Smaller visual field Complex Cells Orientation specific and movement specific Sometimes length specific Larger visual field 5 What do hypercomplex cells respond to Hypercomplex cells Like simple Orientation specific Or Good for detecting edges corners and borders Like complex Orientation specific movement specific Plus length specific End stopping 6 What are columns in cortex and how do the response properties of neurons in a given column compare to each other What are ocular dominance columns What is a hypercolumn Columns in cortex are composed of the very picky receptive cells All cell types in a certain column respond to the same type of stimulus Neighboring columns respond to similar but not identical stimuli Ocular Dominance columns cortical cells show preference to stimulus from eye or the other so they are the L R L R columns A hypercolumn includes 18 20 adjacent orientation columns enough for all orientations of a stimulus 7 How does early experience play a role in the development of columns and of visual experience later in life There is a critical period for development of columns in V1 If one eye doesn t receive appropriate stimulation the neurons destined to respond to that eye don t become properly connected EX when only exposed to vertical stripes during critical period of development cats can t ever see horizontal stripes Cataracts can reduce blur image into 1 eye Strabismus 1 eye is turned out leads to 2 different images on the 2 foveas Amblyopia reduced visual acuity in 1 eye due to abnormal visual experience Can lead to decreased binocular vision depth perception Can t be corrected later in life 8 Understand the tilt aftereffect Vision IV 1 What is middle vision After basic features have been extracted from the image early vision up to and including striate cortex Before object recognition and scene understanding high level vision Goal is to organizes elements of a visual scene into groups that we can then recognize as objects Middle vision is like a collection of specialists each with an area of expertise individual opinions about what the input might mean Bring together that which should be brought together Similarity proximity parallelism symmetry etc Split asunder that which should be split asunder Edges that define regions figure ground relationships Use what you know 2 D edges tell you about 3 D corners or occlusion borders Avoid accidents Accidental viewpoints Seek consensus and avoid ambiguity Use committees to eliminate all but 1 possibility Remember that middle vision occurred after striate cortex and before the temporal lobe specifically before inferotemporal gyrus which is where we do object recognition 2 Illusory contour a contour that is perceived even though nothing changes from one side of the contour to the other in the image 3 Why do you think the Gestalt school of thought was instrumental in identifying how we perceive images To answer this you need to know what Gestalt means Gestalt grouping rules rules describing which elements in an image will appear to group together Gestalt The whole is greater than the sum of its parts The perception is greater than the sum of the sensations 4 Know the gestalt grouping principles good continuation occlusion texture segmentation similarity proximity connectedness common region how do those latter 2 relate to proximity what trumps what parallelism symmetry Also know dynamic occlusion Good continuation same contour 2 lines will tend to group together if they seem to lie on the Occlusion if an edge seems to stop the visual system guesses that it is stopping because something else is in the way occluding it Dynamic occlusion when visual regions are discontinuous not only in space but over time occlusion across time Persistence One frame persists in visual memory after it is no longer physically present and is integrated with the next frame When a visible region of a moving object Position Updating goes behind an occluding surface its shape information persists in visual memory and we update its position over time segmentation carving an image into regions of common texture Texture properties Similarity chunks that are similar to each other are grouped together Proximity items near each other are grouped together Here the similarity is based on form Look like lines as opposed to columns because proximity is important Common Region things enclosed in a common space must go together Connectedness things that are connected must go together Parallelism looks the same Symmetry the opposite or backwards 5 How does color fit in to the gestalt grouping principles Color trumps good continuation as an organizing principle 6 What is an ambiguous figure Ambiguous figure when two committees in middle vision shout equally loudly This is the exception that proves the rule the rule being
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