Visual projection systems

1. Visual projection systems • The primary visual pathway is the retina-lateral geniculate-striate cortex pathway. • It is responsible for detection of form, movement, and color

2. Other visual pathways • To the SCN of hypothalamus • To accessory optic nuclei of brainstem and to cerebellum: Synchronize eye and head movements • To pretectum to control pupil diameter • To superior colliculi, for control of visual attention • To ventral LGN, as a relay

3. Feature detection • Edge enhancement: Mach bands • Contrast enhancement: Lateral inhibition • Receptive fields

4. Mach bands

5. Moving mach bands

6. Lateral inhibition • Edges as contrast elements between lighter and darker areas may be enhanced by lateral inhibition. • When one receptor is activated, it inhibits its neighbors.

7. Lateral inhibition... • Cells receiving more light on one side of an edge are less inhibited by their neighbors receiving less light on the other side of the edge. • Thus, cells on the brighter side of an edge are less inhibited and those on the darker side are more inhibited, physiologically enhancing the contrast across the edge.

8. Lateral inhibition... Receptors across an edge show the greatest difference in response. The receptor on the brighter side of an edge is inhibited by fewer neighbors, while the receptor on the darker side is inhibited by more.

9. Another view of lateral inhibition

10. Receptive fields • The piece in the retinal mosaic to which a given cell responds. • Receptive fields for rods and cones are simple and round. • Receptive fields for ganglion cells, LGN cells, and cortical cells are complex.

11. Receptive fields... • Complex receptive fields are often doughnut shaped, with stimuli in the center producing a response opposite to that in the surround. • Some are center off - surround on. • Others are center on - surround off. • Cells with complex on - off fields enhance contrast from edges and from movement.

12. on on on off on on off off off on off off Examples of receptive fields Center-surround Cells: -Ganglion cells, LGN cells (both M and P), and layer IV of striate cortex Center off-surround on Center on-surround off

13. on off Examples... Simple cortical cells: Line border responds best to -contrasting bars -single straight edges -at a particular angle off on

14. Complex cortical cells • Merge inputs from simple cells to detect • Stimuli over a larger area of the visual field • An edge at a particular angle anywhere in the field (not “on-off”) • Movement, often directionally • About half are binocular • Half of the binocular cells show ocular dominance • Some are retinal disparity detectors

15. Complex cell fields Note: - the larger receptive field - no subdivision on-off -orientation responsiveness -directional sensitivity

16. Cortical columns • Organization of cells in cortex probed perpendicular to the surface of the cortex • All cells in a column respond to the same • portion of the visual field • orientation of stimuli • ocular dominance, if present • These make up the aggregate field of that column

17. Cortical columns, illustrated

18. Cortical columns...

19. Left Right

20. Cortical layers • Moving horizontally through a layer, or laterally from column to column, we find sequential shifts in • portion of the visual field • orientation of stimuli • ocular dominance

21. The DeValois expansion • Spatial frequency theory • Not edges, but frequency gratings • Sine wave gratings and Fourier analysis

22. Color vision • Thomas Young, Hermann von Helmholtz, and the trichromatic theory: Cone pigments • Ewald Hering and the opponent process theory: Ganglion and LGN cells • Yellow-blue • Green-red • Black-white

23. More on color • Color constancy: The phenomenon • Edwin Land (1977) and the retinex theory • Mondrians • Reflectance vs. wavelength • Dual-opponent color cells • Cytochrome oxidase rich • Blobs: Peg-like columns of these cells

24. Land’s Retinex theory • The phenomenon of color constancy shows that color perception is not merely the assessment of reflected wavelengths. • When a composite of Mondrians is bathed in colored light, the perceived colors remain unchanged. • Two differently-colored Mondrians can be bathed in colored light so that they reflect the exact same wavelengths (white).

25. Retinex theory... • If the Mondrians reflecting the composite white light are seen alone, that is, with no background, they appear white. • However, if they are seen in the context of the larger Mondrian, they retain their original colors.

26. Mondrian composites When a composite of Mondrians is bathed in colored light, the perceived colors remain unchanged.

27. Lighting individual Mondrians Two differently-colored Mondrians can be bathed in colored light so that they reflect the exact same wavelengths (white).

28. Equal reflectances viewed alone If the Mondrians reflecting the composite white light are seen alone, that is, with no background, they appear white.

29. Equal reflectances in context However, if they are seen in the context of the larger Mondrian composite, they retain their original colors.

30. Visual association cortex • The striate cortex is the primary (first-stage) visual cortex. • Striate cortex cells send axons to the surrounding areas of the extrastriate cortex • Each area of extrastriate cortex contains a specialized map of the visual field: orientation, movement, spatial frequency, retinal disparity, or color. • The areas are identified with V (for visual area) and a number, counting up from V1, the striate cortex. • The arrangement is hierarchical.

31. Visual streams • In V2, the visual information divides into • a dorsal stream, heading to the parietal cortex and identifying where an object is, and • a ventral stream, heading to the inferior temporal cortex and identifying what an object is. • V2 surrounds V1, but subsequent areas are either dorsal or ventral stream. • V4, in the ventral stream, responds to color, but in a range rather than a specific frequency. Damage to V4 eliminates color constancy (Walsh, et al., 1993)

32. More vision disorders • Achromatopsia, absence of color vision while retaining visual acuity, is due to damage in V8 (TEO in the rhesus monkey), at the back of the inferior temporal lobe. • Achromatopsia may be unilateral. • People with achromatopsia cannot remember or imagine colors. • Conversely, Zeki et al. (1999) report a patient who could see color but not form: extensive damage to extrastriate cortex.

33. The visual agnosias • Ultimate object recognition occurs in the inferior temporal cortex. • Cells here respond well to 3-D objects or photos, but poorly to simple features. • They are arranged in columns, corresponding to changing points of view. • Some respond especially well to faces: when damaged, prosopagnosia results.

34. Apperceptive or associative? • Apperceptive visual agnosia: inability to recognize objects by sight • Prosopagnosia and the fusiform facial area • Facial recognition without object recognition • Expert abilities of visual discrimination rely on the fusiform facial area • Associative visual agnosia can perceive, but not verbalize the perception; nor can she draw from a word cue. • However, she can copy the object.

35. Movement and location • V5 receives magnocellular input from several earlier areas of cortex and from superior colliculus • V5a or MST takes V5 information and responds to complex movement patterns, including optic flow • Akinetopsia • Bilateral damage to lateral occipital cortex and V5 • Balint’s syndrome • Bilateral damage to parieto-occipital cortex • Optic ataxia, ocular apraxia, simultanagnosia • Suggests that dorsal stream is not where but how, that is, by directing and adjusting movement.