Anatomy/Physiology of Binocular Vision

1. 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

2. 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

3. Magnocellular (M-pathway) The Table Setter • Coarse visual form • Moving (or modulating)target • Processing time: rapid • Peripheral fusion • Coarse stereopsis

4. Parvocellular (P-pathway) The Details • Spatial detail • Chromatic detail • Stationary (or moving slowly) target • Processing time: slow • Fine stereopsis

5. 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

6. 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

8. 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

9. 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

10. 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

11. 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

14. 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

15. 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

16. 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

17. Vision Association Areas • Posterior parietal cortex • M-input (MT/V4): coarse stereopsis, low spatial freq., fast flicker and motion • Spatial position and object motion • Inferotemporal cortex • P-input (V4): fine stereopsis, color vision, fine pattern vision • Complex object recognition: faces

18. 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

19. 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

20. Orientation Columns • If ocular dominance columns are loaves of bread, orientation selective columns are slices (parallel to pia) • Orientation selectivity is interrupted by blobs

21. 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

22. 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 and Stereopsis • Binocular cells in V1 receptive fields for each eye share most characteristics • Corresponding retinal loci • Latency • Size/shape of receptive field

23. 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 Binocular Disparity

24. 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

25. 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