Object Vision 1

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

2. Vision beyond V1 PSY 295 - Grinnell College - Fall 2012

3. Many visual areas beyond V1 • Complicated hierarchy • Process particular aspects of visual stimuli like motion, color, complex form and shape PSY 295 - Grinnell College - Fall 2012

4. Organization • Ventral stream – ‘What’ pathway specialized for objects • Dorsal stream – ‘Where’ pathway specialized for space PSY 295 - Grinnell College - Fall 2012

5. Ventral stream • V1 V2  V4  IT • Neurons sensitive to features useful for object recognition border-ownership neuron PSY 295 - Grinnell College - Fall 2012

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

7. Lesion studies • In humans, people with lesions of temporal lobe see things without knowing what they are seeing (agnosia) • Prosopagnosia is the inability to recognize faces PSY 295 - Grinnell College - Fall 2012

8. Feed-forward and feed-back processes • Both feed-forward and feed-back projections • ERP studies show that different signal develops to animal and non-animal pictures after 150 msec • Suggests feed-forward mechanisms are at least good enough for simple object discrimination Thorpe et al (1996) PSY 295 - Grinnell College - Fall 2012

9. Mid-level vision PSY 295 - Grinnell College - Fall 2012

10. Mid-level vision • We know a lot about neurons which are sensitive to simple features like bars and spots, and complex things like faces • We know very little about how one gets neurons sensitive to complex features from neurons sensitive to simple features • This is the problem of intermediate or middle vision PSY 295 - Grinnell College - Fall 2012

11. Object perception is hard • Different views of same object may be retinally very different • Views of different objects may be retinally quite similar (for instance, different faces) PSY 295 - Grinnell College - Fall 2012

12. Perceptual organization • Detecting feature conjunctions allows one to detect objects • Some feature conjunctions belong to different objects • How do we group features? PSY 295 - Grinnell College - Fall 2012

13. Finding edges • One way to detect objects is to find their edges • However, not all edges correspond to object boundaries • Output of computer vision edge-detector PSY 295 - Grinnell College - Fall 2012

14. Filling in missing information

15. Illusory Contours • Your visual system assumes that something is blocking or occluding the circles and infers there is an object PSY 295 - Grinnell College - Fall 2012

16. Illusory contours • Usually when something stops abruptly, it is blocked by something • Weak evidence for edges is integrated to perceive contours PSY 295 - Grinnell College - Fall 2012

17. Gestalt Grouping Rules PSY 295 - Grinnell College - Fall 2012

18. Schools of early psychology • Structuralists believed that perceptions were built up of atoms of sensation • Gestalt school argued that the perceptual whole is greater than the sum of its parts (gestalt = ‘form’) • Gestalt psychologists proposed rules for how the visual system groups features into perceptual wholes PSY 295 - Grinnell College - Fall 2012

19. Good continuation • Similarly oriented lines are seen as part of the same contour • Reflects the structure of the natural sensory environment PSY 295 - Grinnell College - Fall 2012

20. Good continuation PSY 295 - Grinnell College - Fall 2012

21. Similarity • Different image regions have different statistical properties • Group together regions with similar properties PSY 295 - Grinnell College - Fall 2012

22. Similarity PSY 295 - Grinnell College - Fall 2012

23. Proximity • Nearby object tend to be grouped together • Note horizontal rather than vertical grouping PSY 295 - Grinnell College - Fall 2012

24. Over-ruling proximity PSY 295 - Grinnell College - Fall 2012

25. Parallelism and Symmetry PSY 295 - Grinnell College - Fall 2012

26. Camouflage • The goal of camouflage is to prevent accurate feature grouping so that you cannot perceive animal PSY 295 - Grinnell College - Fall 2012

27. Web Activity: Gestalt Grouping • http://sites.sinauer.com/wolfe3e/chap4/gestaltF.htm PSY 295 - Grinnell College - Fall 2012