Transcript Higher Visual Areas

Higher Visual Areas
1. Anatomy of higher visual areas
2. Two processing pathways
- “Where” pathway for motion and depth
- “What” pathway for form and color
3.
The binding problem
Two anatomical pathways
1. Ventral Pathway:
Retinal P cells → Parvo LGN → V1 (4Cb) → V2 → V4 → IT
(Inferior Temporal Cortex)
2. Dorsal Pathway:
Retinal M cells → Magno LGN → V1 (4Ca) → V2 → MT
(Medial Temporal Cortex) → Posterior Parietal cortex
Ventral Pathway – Two parallel channels for form and color
1. Parvocellular – interblob system (form):
(V1) L4 (4Cb) → (V1) L2/3 interblob → (V2) pale interstripe →
V4 → IT
2. Parvocellular – blob system (color)x:
(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.
…
Temporal Cortex
• Inferotemporal Cortex
–--- Cells respond to a single complex stimulus such as an apple
–--- Lesions here leads to inability to identify an object
(visual agnosia), picking it up is no problem
Superior Temporal Cortex
–--- Lesions here lead to inability to recognize faces (prospagnosia)
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 actual wavelengths entering 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 entire objects.
--- Lesions lead to inability to perceive objects in
motion, perception is frozen (Cerebral akinetopsia)
l
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
•
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
Dorsal Pathway - motion and depth Processing
.
Direction-selective Cells – Cells responding to moving bar in one
direction, but not in the opposite direction.
V1: Many cells with simple and complex RF are direction-selective
MT: 1. Direction-selective cells for moving bar or moving dots
2. Columnar organization of direction selectivity
3. RF larger than those of V1 cells
4. Some MT cells (20%) are “pattern direction-selective”
5. Lesion of MT cells impairs motion perception
Aperture Problem
Due to small aperture of the receptive field, motion in three
directions is perceived as in one direction.
Solution: Several lower-order cells project to higher order cells
to integrate the local movements.
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. Temporal synchrony of neuronal firing may
underlie binding.
2. Cell assembly (Donald Hebb) - The first step of
perception is represented by the synchronous
firing a specific group of cells. Each cell
participate in many different cell assemblies.
Stereopsis
-- Perception of solidity or depth
for near objects (<100ft) .
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.
Cues for depth are provided by
points just proximal or distal to
the fixation point.