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 C– P pathway to layer 4C 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 4C 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 4C 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 M-input

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

• P-input (V4): fine stereopsis, color vision, fine pattern vision

• Complex object recognition: faces

• Posterior parietal cortex

• M-input (MT/V4): coarse stereopsis, low spatial freq., fast flicker and motion

• 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

• 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 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 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

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