Chapter 3: Visual Perception

1. Ted talk: beau lotto: optical illusions 2009 Chapter 3: Visual Perception

2. Some Questions of Interest • What is sensation vs. perception? • What are two fundamental approaches to explaining perception? • What happens when people with normal visual sensations cannot perceive visual stimuli?

3. Distal vs. proximal stimuli • Photon absorption • Conduction to basilar membrane • Absorption in olfactory epithelium • Contact with taste buds • Stimulation of dermis receptor cells • Light waves • Sound waves • Chemical molecules • Chemical molecules • Pressure/vibration

5. Our Visual System Electromagnetic light energy is converted into neural electrochemical impulses

6. Our Visual System Three main layers of retina • Ganglion cells • Amacrine cells, horizontal cells, bipolar cells • Photoreceptors • Rods and cones

8. Visual Pathways in the Brain “what” • Temporal lobe lesions in monkeys • Can indicate where but not what “where” • Parietal lobe lesions in monkeys • Can indicate what but not where

9. Sensation does not equal perception!

10. Perceptual Basics • Sensory adaptation • Occurs when sensory receptors change their sensitivity to the stimulus • Our senses respond to change • Ganzfeld effect

11. Perceptual Constancy • Object remains the same even though our sensation of the object changes • Size constancy vs. shape constancy

12. Depth Perception • Monocular vs. binocular depth cues

13. Monocular Depth Cues • Texture gradients • Grain of item • Relative size • Bigger is closer • Interposition • Closer are in front of other objects • Linear perspective • Parallel lines converge in distance • Aerial perspective • Images seem blurry farther away • Motion parallax • Objects get smaller at decreasing speed in distance

15. Binocular Depth Cues • Binocular convergence • Binocular disparity

16. Perceptual Illusions • Sometimes we cannot perceive what does exist • Sometimes we perceive things that do not exist • Sometimes we perceive what cannot be there

17. Perceptual Illusions http://www.michaelbach.de/ot/#history

18. Visual illusions: brain function

19. Theories of Perception • Bottom-up vs top down theories

20. 0 Bottom-Up Processing Theories • Direct perception • Template theories • Feature-matching theories • Recognition-by-components theory

21. Gibson’s Theory of Direct Perception • The information in our sensory receptors is all we need • No complex thought processes • EX: Use texture gradients as cues for depth and distance • Mirror neurons start firing 30-100 ms after a visual stimulus

22. 0 Template Theories compare to templates in memory until a match is found • Problem of imperfect matches

23. 0 Feature-Matching Theories • Recognize objects by features • Detect elements and assemble them into more complex forms • Brain cells respond to specific features • lines and angles

24. Pandemonium Model • Four kinds of demons • Image demons • Feature demons • Cognitive demons • Decision demons

25. 0 Physiological Evidence for Features • Hubel & Wiesel (1979) • Simple cells – bars/edges • Complex cells – bars/edgesdetect bars of particular orientation • Hypercomplex cells -- particular colors (simple and complex cells), bars, or edges of particular length or moving in a particular direction

30. CogLab: Visual Search • Features vs. objects • The role of attention (next chapter)

31. 0 Top-Down Processing (Constructive Approach) • Perception is not automatic from raw stimuli • making inferences • guessing from experience

32. “Finished files are the result of years of scientific study combined with the experience of many years.”

33. 0 Evidence for Top-Down Processing • Context effects

34. Configural-Superiority Effect • Objects presented in context are easier to recognize than objects presented alone • Task: Spot the different stimuli, press button

35. Configural-Superiority Effect Measure reaction time Target alone = 1884 Composite = 749 Target spotted faster in a context! Target Composite

36. Gestalt’s View of Perception “The whole is more than a sum of its parts” • Law of Prägnanz • Individuals organize their experience in as simple, concise, symmetrical, and complete manner as possible

37. Gestalt’s Principles of Visual Perception

38. Gestalt’s Principles of Visual Perception • Figure-ground • Organize perceptions by distinguishing between a figure and a background • Proximity • Elements tend to be grouped together according to their nearness • Similarity • Items similar in some respect tend to be grouped together

39. Gestalt’s Principles of Visual Perception • Continuity • Based on smooth continuity, which is preferred to abrupt changes of direction • Closure • Items are grouped together if they tend to complete a figure • Symmetry • Prefer to perceive objects as mirror images B A D C

40. Pattern Recognition Systems • Feature analysis system • Recognize parts of objects • Assemble parts into wholes • Configurational system • Recognize larger configurations

41. Evidence for Separate Systems • Tanaka & Farah (1993) • Participants studied • Faces and names • Pictures of homes and home owner’s names • At test, given only a piece of face (e.g., nose), whole face, whole home, or a piece of the home (e.g., window) • Asked to recall names

42. Tanaka & Farah (1993) • People have more difficulty recognizing parts of faces than parts of houses

43. Fusiform Gyrus in Temporal Lobe • pattern recognition • facial recognition • high expertise in any item (birds, cars) recognition • Expert individuation hypothesis

44. Evidence for Separate Systems • Prosopagnosia • Inability to recognize faces after brain damage • Ability to recognize objects is intact • Associative agnosia • Difficulty with recognizing objects • Can recognize faces

45. Deficits in Perception • Disruption of the “what” pathway • Agnosia: Inability to recognize and identify objects or people • Disruption of the “what” pathway • Simultagnosic • Normal visual fields, yet act blind • Perceives only one stimulus at a time—single word or object • Prosopagnosia • Inability to recognize faces, including one's own

46. Visual Pathways in the Brain “what” • Temporal lobe lesions in monkeys • Can indicate where but not what “where” • Parietal lobe lesions in monkeys • Can indicate what but not where

47. Deficits in Perception • Disruption of the “how” pathway • Optic ataxia • Cannot use vision to guide movement • Unable to reach for items

48. Synesthesia Michael: “Flavors have shape,” he started, frowning into the depths of the roasting pan. “I wanted the taste of this chicken to be a pointed shape, but it came out all round. … When I taste something with an intense flavor,” Michael continued, “the feeling sweeps down my arm into my fingertips. I feel it—its weight, its texture, whether it’s warm or cold, everything. I feel it like I’m actually grasping something.” (p. 4)

49. Synesthesia Victoria: In response to a pager going off, said: “Oh, those blinding red jaggers! Turn that thing off,” she snapped. … “Your beeper made me see three red lightning bolts, brilliant red going up to the left.” She kept rubbing her head. “It’s usually not that strong, but it’s given me a splitting pain,” she continued. (pp. 47-48). “What makes you see things?” I asked. “Give us some details.”“Sharp, shrill sounds always do it,” she said, “like your beeper, or ambulance sirens, crashes, screeching tires.” … “Sudden sounds like that. Sometimes music will do it, if it’s loud enough and high enough in pitch,” she continued. … “I once heard a Chihuahua who drove me crazy with the sound of white spikes. But that can’t be the correct explanation, because words and names sometimes have color too.” (p. 48)

50. Perceptual Illusions and “construction” • Phantom limb • synesthsia