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Transcript
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.
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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.