Transcript Document

Anatomy/Physiology of
Binocular Vision
• Goals
– Follow the M and P pathway out of primary
visual cortex
– Answer where binocularly and disparity
driven cells appear
– Learn a bit about stereopsis
– Answer (partially) how an oculocentric
neuronal organization gives rise to an
egocentric visual perception
Parallel Pathways:
Magnocellular (M) and
Parvocellular (P)
• Each pathway is sensitive to specific
visual stimuli
• Each pathway has its own timing
characteristics
• Each pathway is NOT strictly parallel!
– More of a “Bob ‘N Weave” pathway
arrangement
Magnocellular (M-pathway)
The Table Setter
•
•
•
•
•
Coarse visual form
Moving (or modulating)target
Processing time: rapid
Peripheral fusion
Coarse stereopsis
Parvocellular (P-pathway)
The Details
•
•
•
•
•
Spatial detail
Chromatic detail
Stationary (or moving slowly) target
Processing time: slow
Fine stereopsis
Parallel Pathways On the Move
• Lateral geniculate nucleus
• Segregation of P and M pathways into layers
(1-2 Magno.; 3-6 Parvo.)
• LGN serves as a relay station to primary visual
cortex (18)
– Where vision will become a conscious event
– Where stereopsis and fusion takes place
– Where visual and cognitive processing take place
Primary Visual Cortex (V1)
• Located along calcarine sulcus
• M and P pathways continue in different
paths as they reach layer 4 of V1
– M pathway to layer 4 Ca
– P pathway to layer 4Cb and layer 4A
• Organized into ocular dominance zones
– Monocular cells in layer 4C
– Binocular driven cells outside of layer 4C
Parallel Pathways in V1
• M pathway:
– From 4Ca to layer 4B in same vertical
column (1 mm wide)
– From 4B to layers 2/3 in same vertical
column (1 mm wide)and neighboring
columns
Parallel Pathways in V1
• P pathway:
– From 4Cb to layers 4A and 3 in same
vertical column (1 mm wide)
– In layer 3, cytochrome oxidase, a metabolic
marker, has dense staining in layer 2/3;
absent in layer 4
– Called “blobs”
– Although considered “P-cells only”, a
significant M-pathway input exists
Parallel Pathways in V1
• Blob and interblob regions:a split in the
parvocellular pathway
• Blob regions are situated in the center of
ocular dominance columns
– Blob regions: color opponency, low contrast
and spatial frequency, not orientation
selective
– Interblob regions: little color opponency,
high contrast and spatial frequency, very
orientation selective
M and P Pathways In V2
• V2 has areas of high
cytochrome oxidase
activity in form of
thick and thin stripes
• M pathways project
to thick stripes
• P pathway
– Blob cells: thin
stripes
– Interblob cells: inter
stripes
Other Visual Areas
• V2: in area 18,
flanking V1
– Thin/inter stripe
regions (P pathway)
projects to V4
– Thick stripe (M
pathway) projects to
V3 and MT
– Some overlap in
response
characteristics in V2
due to “cross-talk”
between M and P at
blob region
Other Visual Areas
• V3: in area 18
flanking V2
– Receives M pathway
input
– Output to middle
temporal area (MT)
– Also output to V4!?!
• V4
– Receives P-pathway
input from thin/inter
stripe regions of V2
– Receives strong Minput
Vision Association Areas
• Area MT
– In parietal lobe
– M-pathway input
– Output to parietal
areas and V4
– Sensitive to motion
– Some areas have
receptive fields in
head-centric
coordinates, NOT
oculocentric
Vision Association Areas
• Inferotemporal
cortex
• Posterior parietal
cortex
• P-input (V4): fine
stereopsis, color
vision, fine pattern
vision
• M-input
(MT/V4): coarse
stereopsis, low
spatial freq., fast
flicker and motion
• Complex object
recognition: faces
• Spatial position
and object motion
Final Words About M/P
Pathways
• Significant cross-talk in V1, V4 and
beyond
• Ultimately, these two independent, yet
overlapping streams must converge to
form unitary perceptions of objects
• We do not process the world like a poorly
printed photograph, with the colors offset
Ocular Dominance Columns
• Vertical columns that
respond most strongly
to one eye
• Extends through the
full thickness of V1
• Absent in areas outside
V1
• Binocular cells outside
layer 4C respond
predominantly to one
eye over the other
Orientation Columns
• If ocular dominance
columns are loaves of
bread, orientation
selective columns are
slices (parallel to pia)
• Orientation
selectivity is
interrupted by blobs
Binocular Cells and Stereopsis
• Binocular cells in V1
receptive fields for
each eye share most
characteristics
– Corresponding
retinal loci
– Latency
– Size/shape of
receptive field
Binocular Cells and Stereopsis
• Binocular cells in V1
receptive fields for
each eye share most
characteristics
– Corresponding
retinal loci
– Latency
– Size/shape of
receptive field
• If perfect overlap of
receptive fields exist,
it argues for a
creation of an
EGOCENTRIC
PERCEPTION early
in visual processing
• It cannot explain,
however, why we are
sensitive to binocular
disparity (stereopsis)
Binocular Disparity
• Results from
different perspective
of each eye to a
particular visual
target
• Neurons tuned to
disparity have been
found in V1
• Receptive fields for
each eye do not
PERFECTLY
overlap
• More prevalent in V2
(75% cells tuned to
disparity)
• 4 main classifications
of disparity tuned
cells
– Near cells/ Far cells
– Excitatory cells tuned
to zero disparity
– Tuned excitatory
– Tuned inhibitory
Profiles of Disparity Tuned
Cells
• Near cells: resp. to
targets closer than
fixation distance
• Far cells: resp. to targets
farther than fixation
distance
• Excitatory cells tuned to
zero disparity: narrow
peak responses around
zero disparity
Profiles of Disparity Tuned
Cells
• Tuned excitatory:
stim. by stimuli near
zero disparity BUT
ON EITHER SIDE/
suppressed by
uncorrelated images
• Tuned inhibitory:
suppressed by stimuli
near zero disparity
BUT ON EITHER
SIDE / stim. by
uncorrelated images
QuickTime™ and a
Video decompressor
are needed to see this picture.