1. Careers for Psychology and Neuroscience Majors Oct. 19th5-7pm in SU 300 Ballroom B

2. WebCT and Your Exam Your grade as it currently appears on WebCT might be off by up to 2 points. WebCT programmers are working to fix the bug. Don’t panic.

3. “Top-Down” and “Bottom-Up” Processes in Perception Light & Dark Line Orientation Surfaces and Background Identity/Meaning A “Bottom-Up” System Why should this model be called into question?

4. “Top-Down” and “Bottom-Up” Processes in Perception Light & Dark Line Orientation COMPARE Possible Interpretations Surfaces and Background Identity/Meaning -- Sensory System -- “Top-Down” or “Look-up” System A Top-Down System

5. Shape Identification Requires Interaction between Top-Down and Bottom-Up Processes

6. Single Target - Identify shape

8. Single Shape/Brief Dots Shape is identified by top-down system before information in the bottom-up system gets changed

9. Single Shape/Brief Dots Shape is identified by top-down system before information in the bottom-up system gets changed Let’s push the system: overload top-down system AND change the signal in the bottom-up system

10. Single Shape/Brief Dots Shape is identified by top-down system before information in the bottom-up system gets changed Let’s push the system: overload top-down system AND change the signal in the bottom-up system What would you predict of a strictly bottom- up system?

11. Many targets - Identify shape inside dots

13. Object Substitution Strictly bottom-up system should still identify the shape! but top-down model accounts for the phenomenon: Bottom-up info gets changed before top- down process completes all that’s left in the bottom-up signal is the four dots

14. More Depth Cues Pictorial Depth Cues Physiological Depth Cues Motion Parallax Stereoscopic Depth Cues

15. Physiological Depth Cues – Two Physiological Depth Cues 1. accommodation 2. convergence

16. Physiological Depth Cues – Accommodation

17. Physiological Depth Cues – Accommodation – relaxed lens = far away – accommodating lens = near – What must the visual system be able to compute unconsciously?

18. Physiological Depth Cues – Convergence

19. Physiological Depth Cues – Convergence – small angle of convergence = far away – large angle of convergence = near – What two sensory systems is the brain integrating? – What happens to images closer or farther away from fixation point?

20. Physiological Depth Cues – Convergence and accommodation are reflexively linked Under what circumstances might this be a problem?

21. Motion Depth Cues – Motion 1. Parallax

22. Motion Depth Cues – Parallax

23. Motion Depth Cues – Parallax – points at different locations in the visual field move at different speeds depending on their distance from fixation

24. Motion Depth Cues – Parallax

25. Seeing in Stereo

26. It’s very hard to read words if there are multiple images on your retina

27. Seeing in Stereo It’s very hard to read words if there are multiple images on your retina But how many images are there on your retinae?

28. Binocular Disparity Your eyes have a different image on each retina –hold pen at arms length and fixate the spot –how many pens do you see? –which pen matches which eye?

29. Binocular Disparity Your eyes have a different image on each retina –now fixate the pen –how many spots do you see? –which spot matches which eye?

30. Binocular Disparity Binocular disparity is the difference between the two images

31. Binocular Disparity Binocular disparity is the difference between the two images Disparity depends on where the object is relative to the fixation point: –objects closer than fixation project images that “cross” –objects farther than fixation project images that do not “cross”

32. Binocular Disparity Corresponding retinal points

33. Binocular Disparity Corresponding retinal points

34. Binocular Disparity Corresponding retinal points

35. Binocular Disparity Corresponding retinal points

36. Binocular Disparity Points in space that have corresponding retinal points define a plane called the horopter The Horopter

37. Binocular Disparity Points not on the horopter will be disparate on the retina (they project images onto non-corresponding points)

38. Binocular Disparity Points not on the horopter will be disparate on the retina (they project images onto non-corresponding points) The nature of the disparity depends on where they are relative to the horopter

39. Binocular Disparity points nearer than horopter have crossed disparity points farther than horopter have uncrossed disparity

40. Binocular Disparity Why don’t we see double vision?

41. Binocular Disparity Why don’t we see double vision? Images with a small enough disparity are fused into a single image

42. Binocular Disparity Why don’t we see double vision? Images with a small enough disparity are fused into a single image The region of space that contains images with close enough disparity to be fused is called Panum’s Area

43. Binocular Disparity Panum’s Area extends just in front of and just behind the horopter

44. Stereopsis Our brains interpret crossed and uncrossed disparity as depth That process is called stereoscopic depth perception or simply stereopsis

45. Stereopsis Stereopsis requires that the brain can encode the two retinal images independently

46. Stereopsis Primary visual cortex (V1) has bands of neurons that keep input from the two eyes seperate

47. Stereopsis If the brain only gets normal signals from one eye early in life, that eye’s neurons crowd out the other eye’s neurons

48. Amblyopia Amblyopia is a visual deficit in which one eye has poor vision because the brain never developed the ability to use signals from that retina

49. Amblyopia Amblyopia is a visual deficit in which one eye has poor vision because the brain never developed the ability to use signals from that retina Usually caused by –strabismus - when eyes don’t lock onto the same point –anisometropia - when one eye has very bad optics and the other is normal

50. Next Time: More about stereo vision MagicEye stereograms Pinker chapter