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Object Vision 2

Object Vision 2. PSY 295 – Sensation & Perception Christopher DiMattina , PhD. Object recognition. A world of identifiable objects. Inferotemporal cortex. Neurons selective for very complex stimuli like faces. How do we get such fancy neurons?. Visual processing hierarchy

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Object Vision 2

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  1. Object Vision 2 PSY 295 – Sensation & Perception Christopher DiMattina, PhD

  2. Object recognition PSY 295 - Grinnell College - Fall 2012

  3. A world of identifiable objects PSY 295 - Grinnell College - Fall 2012

  4. Inferotemporal cortex • Neurons selective for very complex stimuli like faces PSY 295 - Grinnell College - Fall 2012

  5. How do we get such fancy neurons? • Visual processing hierarchy • Mid-level vision asks how we go from simple edge detectors to neurons sensitive to complex objects • Problem of region labeling and grouping PSY 295 - Grinnell College - Fall 2012

  6. Pandemonium model PSY 295 - Grinnell College - Fall 2012

  7. Web activity: Pandemonium • http://sites.sinauer.com/wolfe3e/chap4/pandemoniumF.htm PSY 295 - Grinnell College - Fall 2012

  8. Hierarchical processing • Hubel and Wiesel’s investigations of primary visual cortex suggest hierarchical processing models • Center surround cells combine to form simple cells • Simple cells combine to form complex cells • Increasing feature selectivity, less position-dependence PSY 295 - Grinnell College - Fall 2012

  9. Ventral stream model PSY 295 - Grinnell College - Fall 2012

  10. Middle vision and object recognition PSY 295 - Grinnell College - Fall 2012

  11. Perceptual committees • Mid-level vision is optimized to detect specific features – committee of experts • Committee outputs are combined according to rules to arrive at decision PSY 295 - Grinnell College - Fall 2012

  12. Committee rules • Perceptual committees almost always agree on a single unambiguous interpretation • Exceptions that prove the rule PSY 295 - Grinnell College - Fall 2012

  13. The accidental viewpoint • Our perceptual committees avoid interpreting the scene as viewed from one exact location PSY 295 - Grinnell College - Fall 2012

  14. Accidental viewpoint demo • http://sites.sinauer.com/wolfe3e/chap4/ambiguityF.htm PSY 295 - Grinnell College - Fall 2012

  15. Edges belong to objects • World is comprised of objects which `own’ edges • Which object do we assign edge to? • Figure-ground assignment PSY 295 - Grinnell College - Fall 2012

  16. Border ownership cells in V2 PSY 295 - Grinnell College - Fall 2012

  17. Principles for figure-ground assignment • Surrounded-ness • Size • Symmetry • Parallelism PSY 295 - Grinnell College - Fall 2012

  18. Extremal edges PSY 295 - Grinnell College - Fall 2012

  19. Occlusion complicates things PSY 295 - Grinnell College - Fall 2012

  20. Occlusions • When is something occluded interpreted as a single object? • Concept of relatability PSY 295 - Grinnell College - Fall 2012

  21. T-junctions signal occlusions PSY 295 - Grinnell College - Fall 2012

  22. Mid-level vision • Use prior knowledge about statistical regularities in the natural environment to work backwards from 2-D image to correct 3-D object interpretation PSY 295 - Grinnell College - Fall 2012

  23. Object recognition & neural codes PSY 295 - Grinnell College - Fall 2012

  24. Template matching PSY 295 - Grinnell College - Fall 2012

  25. Impractical – need a lot of templates! PSY 295 - Grinnell College - Fall 2012

  26. Alphabet of complex features PSY 295 - Grinnell College - Fall 2012

  27. 2D shape cells in V4 PSY 295 - Grinnell College - Fall 2012

  28. 3D shape tuning in IT PSY 295 - Grinnell College - Fall 2012

  29. Human cortical specialization PSY 295 - Grinnell College - Fall 2012

  30. Structural Description PSY 295 - Grinnell College - Fall 2012

  31. Structural description • One way to get around problems with template matching is to use the fact that objects share a common structure • Match image to the structural description, i.e. specify in terms of parts and relationships. • Biederman’s “recognition-by-components” theory PSY 295 - Grinnell College - Fall 2012

  32. Geons • Alphabet of geometric primitives from which objects build • Structural description theory suggests object recognition should be view-point invariant PSY 295 - Grinnell College - Fall 2012

  33. Viewpoint dependence • Experiments show object recognition is viewpoint dependent • Faster for familiar viewpoints PSY 295 - Grinnell College - Fall 2012

  34. Monkey experiments PSY 295 - Grinnell College - Fall 2012

  35. Top-down influences • Debate whether object recognition feed-forward or top down • Object substitution masking is when perception of a briefly flashed object is blocked by a subsequently presented object • Suggests top-down re-entrant processing PSY 295 - Grinnell College - Fall 2012

  36. Web activity • http://sites.sinauer.com/wolfe3e/chap4/objectsubF.htm PSY 295 - Grinnell College - Fall 2012

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