What we do and don’t know about perceiving figures and segmenting objects

6. What we do and don’t know about perceiving figures and segmenting objects • What we know • Simple and complex cells can do something like edge or boundary detection. • What we don’t know • How to integrate local edges into global figures? • How to “fill in” edges where no image discontinuities are found?

7. Phenomenal window into visual processing of contours: • illusory contours and amodal completion

8. Illusory Contours

10. Amodal Completion

12. Illusory contours and amodal completion are flip sides of the same coin • Amodal completion - Filling in boundaries of objects behind occluders. • Illusory contours - Filling in boundaries OF occluders. • The appearance of illusory contours usually coincides with the appearance of amodally completed boundaries.

13. Kanizsa Triangle

15. Perception is inference • Visual system finds most likely explanation of image in terms of surfaces and objects. • Illusory contours appear because visual system infers the presence and shape of an occluding figure.

16. Perception as Inference • Visual system selects most likely interpretation of scene. What scene “explains” the image data best? • Illusory contours appear when • occluding object interpretation fits the image data. • occluding object interpretation is simplest explanation of the image data. • Alternative interpretations involve accidental configurations of objects.

19. Amodal Completion Examples

20. Underlying Mechanisms • von der Heydt has found cells in Area V2 of visual cortex which are sensitive both to real and imaginary edges. • Cells fire when a real edge appears in its receptive field. • Cells fire when an illusory edge appears in its receptive field, even when the inducing edges are outside its receptive field.