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Visual Cognition I basic processes

Visual Cognition I basic processes. What is perception good for?. We often receive incomplete information through our senses. Information can be highly ambiguous

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Visual Cognition I basic processes

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  1. Visual Cognition Ibasic processes

  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. Perceptual system is always looking for meaningful patterns Slide from Rob Goldstone

  4. Object recognition system must be flexible to recognize variations in object shapes Letter “A” “Dog”

  5. Basic processes in the Visual System

  6. Hierarchical Organization Visual Input Oriented bars/edges, Motion, Texture, Depth Object/Face/Scenerecognition Low Level Vision High Level Vision Knowledge

  7. Functional Specialization • Spatially different areas are functionally specialized for processing visual attributes such as shape, color, orientation, and direction of motion • Achromatopsia (damage to V4) • cortical color blindness all color vision is lost and the world appears in shades of gray. And in achromatopsia, unlike as in blindness caused by damage to the eyes or optic nerve, even memory of color is gone • Akinetopsia (damage to V5 or MT) • or motion blindness—the loss of the ability to see objects move. Those affected report that they perceive a collection of still images.

  8. Primary and Secondary Visual Cortex (V1 and V2) • Retinotopic maps • Receptive fields: • On-off cells; Off-on cells • Orientation sensitive cells (“simple” cells) • Lateral inhibition

  9. 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)

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

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

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

  13. Single Cell Recording(usually in animal studies) Measure neural activity with probes. E.g., research byHubel and Wiesel:

  14. Hubel and Wiesel (1962) • Studied LGN and visual cortex in the cat. Found cells with different receptive fields – different ways of responding to light in certain areas • What are cells 1, 2, and 3 doing ? • detecting edges • detecting oriented bars • detecting movement in particular direction • detecting cat faces • What are likely locations for cells 1, 2, and 3? • LGN • V1 (primary visual cortex) • V5 Cell 1 Cell 2 Cell 3

  15. 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: Simple cells (bar detectors)

  16. A wiring diagram for building orientation-sensitive 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. What is this cell coding for?a) any faceb) monkey facec) human faced) eyese) hands “spike train” : each individual line represents a neuron firing. The axis represents time Bruce, Desimone & Gross (1981)

  19. Mach Bands and Lateral Inhibition

  20. - - - - - - - - - - - - - - - ++++ ++++ ++++ ++++ ++++ - - - - - - - - - - - - - - - - - - - - - - - - - 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: http://www.psychology.mcmaster.ca/4i03/demos/lateral-demo.html http://serendip.brynmawr.edu/bb/latinhib_app.html

  21. Lateral Inhibition enhances edges

  22. Craik-Cornsweet-O’Brien Illusion Left part of the picture seems to be darker than the right one. In fact they have the same brightness. The same image as above, but the edge in the middle is hidden. Left and right part of the image look to be equally dark How is this different from mach bands?

  23. Another demo of the same effect

  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

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