Basic Processes in Visual Perception

1. Basic Processes in Visual Perception

2. What is perception good for? • We often receive incomplete information through our senses. Information can be highly ambiguous • Perceptual system must resolve ambiguities by drawing inferences from a large set of perceptual cues and conceptual knowledge of the world

3. Mapping of Visual Fields Left visual field  right visual cortex Right visual field  left visual cortex

4. The Retina-geniculate-striate System • The parvocellular (or P) pathway • Sensitive to color and to fine detail • Most of its input comes from cones • The magnocellular (or M) pathway • Most sensitive to information about movement • Most of its input comes from rods

5. A very simplified illustration of the pathways and brain areas involved in vision. There is much more interconnectivity within the brain (VI onwards) than is shown, and there are additional (not shown) brain areas involved in vision.

6. Are there behavioral consequences for individual differences in brain anatomy?

7. Primary and Secondary Visual Cortex (V1 and V2) • Retinotopic maps • Receptive fields: • On-off cells; Off-on cells • Simple cells • Lateral inhibition

8. Retinotopic maps in V1 • Retinotopic mapping: locations on retina are mapped to cortex in orderly fashion. Note: more of visual cortex is dedicated to foveal vision Response in monkey primary visual cortex (V1) measured by radio-active tracers Stimulus pattern Tootell, R. B., M. S. Silverman, et al. Science (1982)

9. Stimulus Cortical Mapping: Left Hemisphere Cortical Mapping: Right Hemisphere

10. Revealing retinotopic maps with fMRI From: Geoff Boynton, SALK institute

11. Revealing retinotopic maps with fMRI From: Geoff Boynton, SALK institute

12. Measuring Neural Activity

13. Receptive Fields • The receptive field (RF) of a neuron is the area of retina cells that trigger activity of that neuron • On-off cells and off-on cells:

14. On-off cell Video STIMULUS RESPONSE APPROX. FIRING RATE 4 25 5 0 LGN On cell: responses as shown on left LGN Off cell opposite response pattern

15. Simple Cells (bar detectors) Video:

16. A wiring diagram for building simple cells out of on-off cells Hierarchical organization of the brain: by aggregating responses over several on-off cells, the brain can detect more complicated features (e.g. bars and edges)

17. Hierarchical Organization

18. - - - - - - - - - - - - - - - ++++ ++++ ++++ ++++ ++++ - - - - - - - - - - - - - - - - - - - - - - - - - Lateral Inhibition • Lateral inhibition sets up competition between neurons so that if one neuron becomes adept at responding to a pattern, it inhibits other neurons from doing so. Light: On-Off Cells with lateral inhibition: Response  Edge detection DEMO APPLETS: 1) http://serendip.brynmawr.edu/%7Ebbutoi/latinh.html 2) http://www.psychology.mcmaster.ca/4i03/demos/lateral-demo.html

19. Functional Specialization Theory (Zeki) • Spatially different areas are functionally specialized for processing visual attributes such as shape, color, orientation, and direction of motion • Examples: • V1 and V2 • Early stage of visual perception • V3 and V3A • Form, especially the shapes of objects in motion • V4 • Responsive to colour • V5 • Visual motion

20. Evidence for Functional Specialization • Single-cell recording • Patient data: • Achromatopsia (damage to V4) • Akinetopsia (damage to V5 or MT)

21. Specialization for form processing in IT (Inferotemporal-Cortex) Kobatake & Tanaka, 1994

22. There is some evidence for specialization to face processing Bruce, Desimone & Gross (1981)

23. The percentage of cells in six different visual cortical areas responding selectively to orientation, direction of motion, disparity, and colour.

24. Sensory Binding Problem • If spatially different areas are functionally specialized for processing visual attributes such as shape, color, orientation, and direction of motion…. • then how does the brain then “bind” together the sensory attributes of an object to construct a unified perception of the object? Binding Problem

25. Binding Problem

26. Alternative View: Hierarchical Model Lennie (1998): • Visual processing is hierarchical • Areas serve multiple functions (except for MT)

27. Hierarchical Organization

28. “What and Where” or “What and How” Systems • Mishkin and Ungerleider (1982) • Object perception (whatis it?) • Ventral pathway running from the primary visual area in the cortex to the inferior temporal cortex • Spatial perception (whereis it?) • There is a dorsal pathway running from the primary visual area in the cortex to the posterior parietal cortex

30. Perception–Action Model • Milner and Goodale (1995, 1998) • Vision for perception • Based on the ventral pathway • Long-lasting, viewpoint-independent representations • Vision for action • Based on the dorsal pathway • Short lasting, viewpoint-dependent representations

31. Evidence • Double dissociation: some patients would show reasonably intact vision for perception but severely impaired vision for action, and others would show the opposite pattern • Optic ataxia • Visual agnosia

32. Differential Sensitivity to Visual Illusions Performance on a 3-D version of the Müller-Lyer illusion as a function of task (grasping vs. matching) and type of stimulus (ingoing fins vs. outgoing fins). Haart et al. (1999).

33. Appropriate grasping requires theretrieval of object knowledge from long-term memory Mean percentages of objects grasped appropriately in the control (grasping only), spatial imagery, and paired associate learning conditions. Creem and Proffitt (2001b).