AnatomyPhysiology of Binocular Vision Goals Follow the M

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 (V 1) • Located along calcarine sulcus • M and P pathways continue in different paths as they reach layer 4 of V 1 – M pathway to layer 4 Ca – P pathway to layer 4 Cb and layer 4 A • Organized into ocular dominance zones – Monocular cells in layer 4 C – Binocular driven cells outside of layer 4 C

Parallel Pathways in V 1 • M pathway: – From 4 Ca to layer 4 B in same vertical column (1 mm wide) – From 4 B to layers 2/3 in same vertical column (1 mm wide)and neighboring columns

Parallel Pathways in V 1 • P pathway: – From 4 Cb to layers 4 A 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 V 1 • 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 V 2 • V 2 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 • V 2: in area 18, flanking V 1 – Thin/inter stripe regions (P pathway) projects to V 4 – Thick stripe (M pathway) projects to V 3 and MT – Some overlap in response characteristics in V 2 due to “cross-talk” between M and P at blob region

Other Visual Areas • V 3: in area 18 flanking V 2 – Receives M pathway input – Output to middle temporal area (MT) – Also output to V 4!? ! • V 4 – Receives P-pathway input from thin/inter stripe regions of V 2 – Receives strong Minput

Vision Association Areas • Area MT – In parietal lobe – M-pathway input – Output to parietal areas and V 4 – Sensitive to motion – Some areas have receptive fields in head-centric coordinates, NOT oculocentric

Vision Association Areas • Inferotemporal cortex • Posterior parietal cortex • P-input (V 4): fine stereopsis, color vision, fine pattern vision • M-input (MT/V 4): 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 V 1, V 4 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 V 1 • Absent in areas outside V 1 • Binocular cells outside layer 4 C 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 V 1 receptive fields for each eye share most characteristics – Corresponding retinal loci – Latency – Size/shape of receptive field

Binocular Cells and Stereopsis • Binocular cells in V 1 receptive fields for each eye share most characteristics • If perfect overlap of receptive fields exist, it argues for a creation of an EGOCENTRIC – Corresponding retinal loci PERCEPTION early – Latency in visual processing – Size/shape of • It cannot explain, receptive field 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 V 1 • Receptive fields for each eye do not PERFECTLY overlap • More prevalent in V 2 (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