1. Reading Assignment Read chapter by Pinker for Wed. of next week.

2. Shape from Shading

3. Investigating Perception What is Science?

4. Investigating Perception What is Science? Science is a method for understanding how nature works

5. Science What isn’t Science? Headlines on www.reuters.com Science Page: –Juan Strengthens Into Hurricane, Misses Bermuda

6. Science What isn’t Science? Headlines on www.reuters.com Science Page: –Juan Strengthens Into Hurricane, Misses Bermuda –Japan's Mt Fuji Shows Subsidence, No Eruption Danger

7. Science What isn’t Science? Headlines on www.reuters.com Science Page: –Juan Strengthens Into Hurricane, Misses Bermuda –Japan's Mt Fuji Shows Subsidence, No Eruption Danger –Official Report: French Heat-Wave Death Toll 15,000

8. Science What isn’t Science? Headlines on www.reuters.com Science Page: –Juan Strengthens Into Hurricane, Misses Bermuda –Japan's Mt Fuji Shows Subsidence, No Eruption Danger –Official Report: French Heat-Wave Death Toll 15,000 –Man Questioned in Australia Over Mutilated Whale Carcass

9. Science Science is like a set of procedures: Identify a question

10. Science Science is like a set of procedures: Identify a question Make a Hypothesis

11. Science Science is like a set of procedures: Identify a question Make a Hypothesis Make a Prediction

12. Science Science is like a set of procedures: Identify a question Make a Hypothesis Make a Prediction Test It

13. Science Science is like a set of procedures: Identify a question Make a Hypothesis Make a Prediction Test It Prediction holds Prediction Fails

14. Example: The horizontal-vertical illusion

15. Question: Hypothesis: Prediction: Test:

16. Example: The horizontal-vertical illusion Question: Why does the vertical line seem longer? Hypothesis: Prediction: Test:

17. Example: The horizontal-vertical illusion Question: Why does the vertical line seem longer? Hypothesis: Wundt -> vertical and horizontal eye movements scan the image differently Prediction: Test:

18. Example: The horizontal-vertical illusion Question: Why does the vertical line seem longer? Hypothesis: Wundt -> vertical and horizontal eye movements scan the image differently Prediction: Stabilized image should eliminate the illusion Test:

19. Example: The horizontal-vertical illusion Question: Why does the vertical line seem longer? Hypothesis: Wundt -> vertical and horizontal eye movements scan the image differently Prediction: Stabilized image should eliminate the illusion Test: Illusion is still present in afterimage!

20. Muller - Lyer Illusion

21. What theory does Gregory take up?

22. Muller-Lyer Illusion What theory does Gregory take up? –Thiery / Woodworth : Muller-Lyer figure “suggests perspective” –implicit is that the brain shrinks things that are perceived as near and expands things that are perceived as far away

23. Size Constancy Brain shrinks things that are near and expands things that are far The Ponzo Illusion:

24. Size Constancy The lines of perspective in the Ponzo Illusion make the top line appear farther away Why isn’t it this simple for the Muller-Lyer Illusion ?

25. Size Constancy The lines of perspective in the Ponzo Illusion make the top line appear farther away Why isn’t it this simple for the Muller-Lyer Illusion ? –Because the Muller-Lyer figure looks flat !

26. Paradox of Pictures What does Gregory find so paradoxical about pictures?

27. Paradox of Pictures What does Gregory find so paradoxical about pictures? What does he do to remove the paradox so that the Muller-Lyer figure no longer looks flat?

28. Paradox of Pictures

29. Removing the Background With background removed, Muller-Lyer figure actually looks 3D What prediction can you make about how the perceived size and the perceived distance of the center line are related? How did Gregory test that?

30. Removing the Background Perceived size and perceived distance are tightly related

31. Size Constancy in Motion What does Gregory wonder about size constancy and motion? How does he test that?

32. Size Constancy in Motion Gregory finds that motion does invoke size constancy in certain cases:

33. Size Constancy in Motion Gregory finds that motion does invoke size constancy in certain cases: –only when observer is accelerating

34. General Theory of Illusions What’s the more general theory about why illusions occur ?

35. General Theory of Illusions What’s the more general theory about why illusions occur ? That illusions occur because of perceptual mechanisms evolved to handle vision under normal circumstances

36. General Theory of Illusions What is Gregory’s theory about how the brain accomplishes this?

37. General Theory of Illusions What is Gregory’s theory about how the brain accomplishes this? “it is as though the brain entertains alternative hypotheses of what object the eye’s image may be representing”

38. Modern “Two-System” Model of Vision What Gregory called “Control System” vs. “Lookup System” we often now call “Bottom-Up” vs. “Top-Down” or “Stimulus-Driven” vs. “Goal Driven”

39. Two-System model of Vision Bottom-up System –incoming stimulus is “processed” by successively higher stages until its identity is obtained –Percept is controlled directly by the retinal image Pattern on Retina brightness and line orientation Surfaces and background identity

40. Surfaces and background Two-System model of Vision Top-Down System –incoming stimulus triggers a comparison between a top-down “hypothesis” and the actual image Pattern on Retina brightness and line orientation COMPARE possible identities Brain “tests” different representations until it finds a match identity

41. Surfaces and background Two-System model of Vision Top-Down and Bottom-Up systems work together Pattern on Retina brightness and line orientation COMPARE possible identities identity BOTTOM-UPTOP-DOWN

42. Top-Down System at Work Your previous experience with this image helps you identify the object much faster

43. Two models of Vision Bottom-Up System: What it’s telling you: Pattern on Retina brightness and line orientation Surfaces and background identity Blobs big blobs and little blobs nope just blobs modern art

44. nope just blobs Two models of Vision Top-Down System What It’s Telling You: Blobs Big blobs and little blobs COMPARE cat? bird? A beer? none of the above = modern art

45. Two models of Vision Top-Down System Blobs Big blobs and little blobs COMPARE cat? bird? A beer? nope just blobs none of the above = modern art “it’s a dog”

46. Two models of Vision Top-Down System Blobs Big blobs and little blobs COMPARE Dog? nope just blobs none of the above = modern art “it’s a dog”

47. Two models of Vision Top-Down System Blobs Big blobs and little blobs COMPARE Dog? blobs that look a bit like a dog DOG ! “it’s a dog”

48. Testing the Two-Systems Model Have we talked about any phenomena that cannot be explained (easily) by a strictly bottom-up model of vision?

49. Testing the Two-Systems Model Have we talked about any phenomena that cannot be explained (easily) by a strictly bottom-up model of vision? Once a stimulus enters an auditory or visual pathway, it should be unaffected by subsequent stimuli (according to the bottom-up model).

50. Single Target - Identify shape

52. Single Shape/ Lingering Dots Lingering dots do not interfere with percept of shape

53. Single Shape/ Lingering Dots Lingering dots do not interfere with percept of shape Let’s overload the system and see what happens.

54. Single Shape/ Lingering Dots Lingering dots do not interfere with percept of shape Let’s overload the system and see what happens. What would a strictly bottom-up model predict?

55. Many targets - Identify shape inside dots

57. Object Substitution Masking Bottom-up info gets changed before top- down process completes all that’s left in the bottom-up signal is the four dots

58. Next Time: Read Pinker for next Tuesday! More about 3D Vision