The Visual System

1. The Visual System

2. Light • Enters the eye as electromagnetic radiation • Travels in a wave that vary in amplitude (height) and wavelength (between peaks) • Amplitude= brightness • Wavelength= color (hue)

3. Humans see a mixture of several wavelengths • Varies purity • Small portion of the spectrum • Animals- what do they see?

4. Parts of the eye

5. Parts of the Eye • Cornea (1) transparent covering/protection • Pupil and Iris (2) bright light/contracts, dim light/relax • Lens (3) focuses light on the retina, accommodation • Retina (4) neural tissue that absorbs light • Optic disk & Blind Spot (5) • Optic Nerve (6) • Fovea (7) • Blind Spot Activity!!

6. The Retina • Millions of receptor cells • 10 % of light • Rods- 100 to 125 million • Cones- 5-6 million • Expt- What Color is it??

7. Light and Dark Adaptation

8. Light and Dark Adaptation

9. Afterimage

11. Form Perception • Perceptual Set - The influence of prior assumptions and expectations on perceptual interpretations

12. Perceptual Set • What do you see in the center picture: a male saxophonist or a woman’s face? Glancing first at one of the two unambiguous versions of the picture is likely to influence your interpretation.

13. Feature Analysis • Bottom Up Processing & Top Down Processing I cdnuolt blveiee taht I cluod aulaclty uesdnatnrd waht I was rdanieg. It dseno’t mataetr in what oerdr the ltteres in a word are, the olny iproamtnt tihng is taht the frsit and lsat ltter be in the rghit pclae.

14. Gestalt Principles (p105) • Figure Ground • Proximity • Closure • Similarity • Simplicity • Continuity

15. Phi Phenomenon • The illusion of motion when fixed lights are turned on and off in a sequence • Stare at the X in the middle and notice what happens. Is the Green Dot Moving? • http://www.weeville.com/eyetest.htm

16. Depth Perception • Binocular depth cues • Retinal disparity- objects 25ft project images to different locations on the right & left retina

17. Motion Parallax Monocular Depth Cues • Linear Perspective

18. Interposition Monocular Depth Cues • Texture gradient

19. Height in a plane Monocular Depth Cues • Relative Size

20. Size Constancy • People are the same size even though their image sizes differ • The depth cues such as linear perspective and relative size help judge the size accurately

21. Size Distance Relationship

22. Size Distance Relationship

23. Size Distance Relationship

24. Shape Constancy • The understanding that an object’s shape remains the same even though the angle of view makes the shape appear changed

25. Shape Constancy • It is hard to tell if the figure on the upper right is a trapezoid or a square slanted backward. • If we add texture, the texture gradient helps us see that it is actually a square

26. Brightness Contrast • Perceived lightness stays roughly constant as long as the context or surroundings stay the same. • When the context changes you may perceive the color as changing. • Want to see more of these perceptions? Goto http://www.skidmore.edu/~hfoley/Perc4.htm#lightcon

27. Context & Culture • What is above the woman’s head? In one study, nearly all the East Africans who were questioned said the woman was balancing a metal box or can on her head and that the family was sitting under a tree. Westerners, for whom corners and boxlike architecture are more common, were more likely to perceive the family as being indoors, with the woman sitting under a window.

28. Illusions • When we misperceive the true characteristics of an object or image. • Help researchers understand how sensation and perception normally works

29. Müller-Lyer Illusion • Perceptual psychologists have hypothesized that the top horizontal line looks longer because it also looks farther away • Specifically, the inward pointing arrows signify that the horizontal line is closest to you, and the outward pointing arrows signify the opposite case

30. Müller-Lyer Illusion

31. Müller-Lyer Illusion Most people think segment AB equals BC. In reality AB is much longer than BC.

32. Müller-Lyer Illusion

33. Müller-Lyer Illusion

34. Müller-Lyer Illusion

35. Ponzo Illusion • Converging lines indicate that top line is farther away than bottom line

36. The interplay between perceived size and distance (a) The monocular cues for distance make the pursuing monster look larger than the pursued. It isn’t. (b) This visual trick, called the Ponzo illusion, is based on the same principle as the fleeing monsters. The two red bars cast identical-sized images on our retinas. But experience tells us that a more distant object can create the same-sized image as a nearer one only if it is actually larger. As a result, we perceive the bar that seems farther away as larger.

37. Moon Illusion • Moon appears larger when it is on the horizon than when it is directly overhead. • Objects on the horizon are perceived as farther away than those above us • The moon appears to be behind those objects on the horizon. Since it is bigger than those object it is perceived as huge! (click on box below for explanation) Click Below to View an Explanation: Moon Illusion

38. Ames Room Illusion: Secret Revealed When in fact it is a trapezoid! We perceive the room to be as we are used to, a perfect square or rectangular.

39. Poggendorff Illusion

40. Impossible Figures • These grouping principles help us construct reality but perceptual contradictions can lead us astray See how this and others like it are done

41. More Impossible Figures

42. More Impossible Figures

43. Escher’s Impossible Scenes

44. Chrysanthemum Is this 3-D?

45. Water or Monks? Heads or Houses?

46. Rocks or Horses? In or out of the picture?