PSY101 Sensation Perception, Lecture 3

1. PSY101Sensation & Perception,Lecture 3

4. So far� Perception as a problem Features Cues to depth

5. Next Putting features into meaningful groups�

6. Perceiving form i.e. what goes with what a first choice is what is figure and which is ground�

11. The gestalt grouping principles

17. Gestalt principles and graphic design

18. Common fate

21. So far� Perception as problem solving Features Example: Depth cues Grouping

22. assumptions & expectations Cues don�t mean anything unless you make some assumptions about what they imply� e.g. shape from shading assumes shading indicates shadows, combined with the additional assumption that light comes from up, we can infer bumps or divets. More generally, what we are expecting to see powerfully guides perception

23. �top down� vs �bottom up� a.k.a. knowledge driven vs data driven

28. Top-down influence in auditory perception: the electronic voice phenomenon Another one bites the dust My name is� � baby one more time� http://jeffmilner.com/backmasking.htm

29. �One shot learning� Immediate changes to top-down influences Cf. more subtle changes, �cultivated perception�

30. perceptual constancy The same object can project many different images on to the retina e.g. as distance varies, or lighting A variety of mechanisms compensate for this variation in image characteristics, so that the perceptual effect is (what is assumed to be) closer to the true characteristics of the object

31. Size constancy go! http://www.michaelbach.de/ot/sze_shepardTerrors/ by �unconscious inference� our visual system adjusts the size things appear according to how far away it thinks they are

33. Brightness constancy Brightness is psychological: how bright things appear Luminance is physical: how much light they reflect So: a sheet of white paper in a dim cellar will reflect less light than a lump of coal in daylight � but the paper still appears white and coal black. That�s brightness constancy (what you see is adjusted by the surrounding context)

34. Adelson�s checkershadow illusion

35. Adelson�s checkershadow illusion

39. The biology of visual perception

41. and in the visual cortex there�s a complex web of prcoessing modules�

44. Two processing streams �What� form, objects (�ventral�, to inferior temporal lobe) �Where� Motion (�dorsal�, to superior parietal lobe) Ungerleider, L.G. & Mishkin, M. (1982). Two cortical visual systems. In D.J. Ingle, M.A. Goodale & R.J.W. Mansfield (Eds). Analysis of Visual Behavior. Cambridge, MA: MIT Press. pp.549-586.

45. The Stepping Feet illusion go! http://www-psy.ucsd.edu/~sanstis/Foot.html Huh? your dorsal/motion stream is insensitive to colour without colour information, the motion of the blocks is disguised by the columns the effect is strongest in peripheral vision (where magno cells predominate)

49. Book recommendation Goodale, M. & Milner, D. (2004). Sight Unseen: An Exploration of Conscious and Unconscious Vision. Oxford: Oxford University Press

50. Summary Perception as active construction of a meaningful scene from a retinal image primitives: motion, depth, colour etc grouping & segregation cues, assumptions & expectations illusions can be revealing of these Biology of perception: two stream theory illusions can be revealing of this too

51. Perception and attention Where you look, and what you are looking for affects what you see Attention is a series of filters on what we process, up to the point at which we do just one thing interacts with visual primitives (e.g. motion grabs attention), expectations, previous learning�and intention!

52. Example: sex differences in attention http://viscog.beckman.uiuc.edu/flashmovie/15.php

54. Example: inattentional blindness illustrates the power of attention, and the active nature of seeing Simons, D. J., & Chabris, C. F. (1999). Gorillas in our midst: Sustained inattentional blindness for dynamic events. Perception, 28, 1059-1074

56. Questions? t.stafford @ shef.ac.uk