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Higher Processing of Visual Information: Lecture III --- April 9, 2007 by Mu-ming Poo 1. Columnar Organization a. orientation columns b. ocular dominance columns 2. Anatomy of higher visual areas 3. Two processing pathways - “Where” pathway for motion and depth - “What” pathway for form and color 4. The binding problem Columnar Organization --Cells in the same column have similar properties (RF position, orientiation preference, ocular dominance) Orientation columns Olique penetration in V1 --preferred orientation gradually shifts Vertical penetration in V1 --same preferred oritentation Ocular dominance columns Olique penetration in V1 --eye dominance shift in alternating manner Vertical penetration in V1 --same eye dominance A complete set of orientation columns is about 1 mm wide. Ocular dominance columns Cell number Monocular labeling show zebra stripes in layer IV (0.5mm wide) Ocular dominance The pinwheel-like orientation maps revealed by optical imaging (Blasdel & Grinvald, 1980s). Optical imaging visualizes the changes of intrinsic optical properties of neural tissues due to neuronal activity. Iso-orientation maps of cat V1 Orientation & direction maps of monkey V1 Orientation preference map Two anatomical pathways 1. Ventral Pathway (for form and color), “What” Pathway: Retinal P cells → Parvo LGN → V1 (4Cb) → V2 → V4 → IT (Inferior Temporal Cortex) 2. Dorsal Pathway (for motion), “Where” Pathway: Retinal M cells → Magno LGN → V1 (4Ca) → V2 → V3 → MT (Medial Temporal Cortex) → Posterior Parietal cortex Ventral Pathway – Two parallel channels for form and color 1. Parvocellular – interblob system process “form” information: (V1) L4 (4Cb) → (V1) L2/3 interblob → (V2) pale interstripe → V4 → IT 2. Parvocellular – blob system process “color” information: (V1) L4 (4Cb) → (V1) L2/3 blob → (V2) thin stripe → V4 → IT 4B (Magno) - Thick stripe Blobs – thin stripe Interblobs - interstripe Illusory contours can trip firing of V2 cells, while only real contours fire V1 cells. … V3 --- Receives input from thick stripe and interstripe areas of V2 --- No thin stripe (Blob) input, generally color insensitive --- Edges of a particular orientation --- Some motion perception --- Depth perception V4 --- Inputs mainly from foveal regions of V1 and V2 (blobs/thin stripes) --- Perceived color of surfaces (not wavelengths entering the eye) --- Lesions here lead to loss of color vision (Cerebral achromatopsia). V5 (MT: Medial temporal cortex) --- Input from thick stripes of V2 (i.e. Magnocellular) --- Specialized for detection of speed and overall motion of the entire object. --- Lesions lead to inability to perceive objects in motion, perception is frozen (Cerebral akinetopsia) --- Columnar organization of direction selectivity --- Some MT cells (20%) are “pattern direction-selective” Direction-selective cells: cells responding to stimuli moving in one direction but not in the opposite direction. Aperture Problem Due to small aperture of the receptive field, motion in three directions is perceived as in one direction. Solution: Several lowerorder V1 cells project to higher order MT(V5) cells to integrate the local movements. V1 cells are “component direction-selective” MT cells are “pattern direction-selective”, responding not to the direction of components of an object, but to the vectorially summed direction of many component. Inferotemporal Cortex --- Cells respond to a single complex stimulus (e.g. an apple) --- Lesions here leads to inability to identify an object (visual agnosia), picking it up is no problem Superior Temporal Cortex --- Lesions lead to inability to recognize faces (prospagnosia) Complex Cell Responses in Inferior Temporal Cortex 1. Primary cells – respond to simple stimuli 2. Elaborate cells – shapes with color or texture (complex stimuli) 3. Size neurons Invariant neurons: respond to object regardless of size (near or far) Variant neurons: respond to object of a specific size 4. Location neurons – respond to object only in a specific location in the visual field Agnosias (Sigmund Freud) Specific defects in vision due to cortical lesion (stroke or tumor) Movement agnosia: Selective loss of movement perception without loss of other perceptual functions, due to bilateral damage in MT or MST Achromatopsia (color agnosia) - loss of color vision due to lesion of temporal cortex (V4) Prosopagnosia – loss of form recognition, due to lesion of inferior temporal cortex Stereopsis -- Depth Perception for near objects (<100 ft). binocular disparity The difference between the images of an object on the two retinas due to the slightly different location of the two eyes relative to the viewed object (Look at one figure with alternative closing of the left and right eye). Cues for depth are provided by points just proximal or distal to the fixation point. <> Activation pattern (revealed by optical imaging) of area TE of inferior temporal cortex by manipulating visual stimuli. The color circles (left panel) are used to indicate activation areas in response to the corresponding stimuli (right panel) (Adapted from Tsunoda, Yamane, Nishizaki, and Tanifuji Nature Neuroscience 2001). The binding problem: ----How the varied aspects of sensory information processed in different cortical areas are integrated to yield the coherent percepts and representations that we experience as the external world. --- Existence of “Grandmother cell?” Hypothesis: 1. Synchronous oscillation -- temporal synchrony of neuronal firing may underlie binding. 2. Cell assembly (Donald Hebb) -- The perception is represented by synchronous firing a specific group of cells. Each cell participates in many different cell assemblies.