Introduction to Sensation and Perception: Vision Module 11

1. Introduction to Sensation and Perception: VisionModule 11

2. Sensation & Perception How do we construct our representations of the external world? To represent the world, we must detect physical energy (a stimulus) from the environment and convert it into neural signals. This is a process called sensation. When we select, organize, and interpret our sensations, the process is called perception.

3. Bottom-up Processing Analysis of the stimulus begins with the sense receptors and works up to the level of the brain and mind. Letter “A” is really a black blotch broken down into features by the brain that we perceive as an “A.”

4. Top-Down Processing Information processing guided by higher-level mental processes as we construct perceptions, drawing on our experience and expectations. THE CHT

5. Top Down Processing • Aoccdring to rscheearch at CmbridgeUinvertisy. It deosn’tmttaer in what oredr the ltteers in a word are, the olnyiprmoetnttihng is that the frist and lsatltteer be at the rghitpclae. The rset can be a total mses and you can still raed it wouthit a porbelm. This is bcusawe the huamnmniddeos not raederveylteter by istlef, but the word as a wlohe.

6. Likelihood Principle • Herman von Helmholtz • We will perceive the object that is most likely to be the cause of our sensory stimulation. That is, if a number of different objects could have caused a specific pattern of light and dark on the retina, we will perceive the object that is most likely to occur in that particular situation.

7. Making Sense of Complexity Our sensory and perceptual processes work together to help us sort out complex images. “The Forest Has Eyes,” Bev Doolittle

8. Sensing the World Senses are nature’s gift that suit an organism’s needs. A frog feeds on flying insects; a male silkworm moth is sensitive to female sex-attractant odor; and we as human beings are sensitive to sound frequencies that represent the range of the human voice.

9. Exploring the Senses What stimuli cross our threshold for conscious awareness?

10. Psychophysics A study of the relationship between physical characteristics of stimuli and our psychological experience with them.

11. Proportion of “Yes” Responses 0.00 0.50 1.00 0 5 10 15 20 25 Stimulus Intensity (lumens) Thresholds Absolute Threshold:Minimum stimulation needed to detect a particular stimulus 50% of the time.

12. ABSOLUTE THRESHOLDS • Vision: a single candle flame from 30 miles on a dark, clear night. • Hearing: The tick of a watch from 20 feet in total quiet. • Smell: 1 drop of perfume in a 3 room apt. • Taste: 1 tsp. sugar in two gallons of water. • Touch: The wing of a bee on your cheek, dropped from 1 cm.

13. Subliminal Threshold Subliminal Threshold: When stimuli are below one’s absolute threshold for conscious awareness. http://www.scentair.com/index.html http Kurt Scholz/ Superstock

14. Signal Detection Theory • Predicts when we will detect new weak signals. • Why do people respond differently to the same stimuli? • You’re driving a bus with 12 passengers. At your first stop, 6 passengers get off. At the second stop, 3 get off. At the third stop, 2 more get off the but 3 get on. What color are the bus driver’s eyes?

15. Weber’s Law Two stimuli must differ by a constant minimum percentage (rather than a constant amount), to be perceived as different. Weber fraction: k = dI/I. • Difference thresholds grow with the magnitude of the stimulus. • (quarter example)

16. Sensory Adaptation Diminished sensitivity as a consequence of constant stimulation. Put a band aid on your arm and after awhile you don’t sense it. We perceive the world not exactly as it is, but as it is useful for us to perceive it.

17. Now you see, now you don’t

18. VISION • What is the energy that we see as visible light? • Transduction: In sensation, the transformation of stimulus energy (sights, sounds, smells) into neural impulses.

19. The Stimulus Input: Light Energy ElectroMagnetic Spectrum Both Photos: Thomas Eisner

20. The Stimulus Input: Light Energy • What strikes our eye is not color but pulses of electromagnetic energy that our visual system perceives as color.

21. Physical Characteristics of Light that help our determine our sensory experience of them. • Wavelength (hue/color): distance from one wave peak to the next • Intensity (brightness)-the amount of energy in the light wave –determined by the height or magnitude.

22. Wavelength (Hue) Hue (color)is the dimension of color determined by the wavelength of the light. Wavelengthisthe distance from the peak of one wave to the peak of the next.

23. Wavelength (Hue) Violet Green Orange Red Indigo Blue Yellow 400 nm 700 nm Long wavelengths Short wavelengths Different wavelengths of light result in different colors.

24. Intensity (Brightness) Intensity:Amount of energy in a wave determined by the amplitude. It is related to perceived brightness.

25. Intensity (Brightness) Blue color with varying levels of intensity. As intensity increases or decreases, blue color looks more “washed out” or “darkened.”

26. The Eye

27. Parts of the eye • Cornea: Transparent tissue where light enters the eye. • Iris: Muscle that expands and contracts to change the size of the opening (pupil) for light. • Lens: Focuses the light rays on the retina. • Retina: Contains sensory receptors that process visual information and sends it to the brain.

28. The Lens Lens:Transparent structure behind the pupil that changes shape to focus images on the retina. Accommodation: The process by which the eye’s lens changes shape to help focus near or far objects on the retina.

29. Retina Retina: The light-sensitive inner surface of the eye, containing receptor rods and cones in addition to layers of other neurons (bipolar, ganglion cells) that process visual information.

30. Optic Nerve, Blind Spot & Fovea Optic nerve: Carries neural impulses from the eye to the brain. Blind Spot: Point where the optic nerve leaves the eye because there are no receptor cells located there. Fovea: Central point in the retina around which the eye’s cones cluster. http://www.bergen.org

31. Test your Blind Spot Use your textbook. Close your left eye, and fixate your right eye on the black dot. Move the page towards your eye and away from your eye. At some point the car on the right will disappear due to a blind spot.

32. Photoreceptors E.R. Lewis, Y.Y. Zeevi, F.S Werblin, 1969

33. Bipolar & Ganglion Cells Bipolar cells receive messages from photoreceptors and transmit them to ganglion cells, which converge to form the optic nerve.

34. Visual Information Processing Optic nerves connect to the thalamus in the middle of the brain, and the thalamus connects to the visual cortex.

35. Feature Detection Feature detector nerve cells in the visual cortex of the occipital lobe respond to specific features, such as edges, angles, and movement. They then pass the info. on to supercells in other cortical areas that respond to more complex problems. Ex. Temporal Lobe-enables you to perceive faces. Ross Kinnaird/ Allsport/ Getty Images

36. Shape Detection Specific combinations of temporal lobe activity occur as people look at shoes, faces, chairs and houses. Ishai, Ungerleider, Martin and Haxby/ NIMH

37. Visual Information Processing Processing of several aspects of the stimulus simultaneously is called parallel processing. The brain divides a visual scene into subdivisions such as color, depth, form, movement, etc. (unlike a computer that does “Serial processing”)

38. Motion Aftereffects • http://www.michaelbach.de/ot/mot_adaptSpiral/index.html • Are caused by the adaptation of motion-specific detectors that are tuned to the direction of the movement of the stimuli being viewed.

39. From Sensation to Recognition Tim Bieber/ The Image Bank

40. Color Vision Trichromatic theory: Young and von Helmholtz suggested that the eye must contain three receptors that are sensitive to red, blue and green colors. http://phet.colorado.edu/simulations/sims.php?sim=Color_Vision#topics Standard stimulus Comparison stimulus Max Low Medium Blue Red Green

41. Any color can be created by combining the lightwaves of 3 primary colors –red, blue, green. • When we stimulate combinations of these cones, we see other colors. • For example, there are no receptors especially sensitive to yellow—when both red and yellow cones are stimulated-we see yellow.

42. Color Blindness Genetic disorder in which people are blind to green or red colors. This supports the Trichromatic theory. Ishihara Test

43. COLOR BLINDNESS • Most people with color –deficient vision are not color blind. They simply lack functioning red or green sensitive cones.

44. OPPONENT PROCESS THEORY • Trichromatic Theory does not solve all issues with color vision. • For example, we see yellow when mixing red and green light. • BUT those blind to red and green can often still see yellow? • Clue found by Hering (a physiologist) when looking at afterimages.

45. Opponent Colors Gaze at the middle of the flag for about 30 Seconds. When it disappears, stare at the dot and report whether or not you see Britain's flag.

46. WHY? • When you stare at a green square for a while, then look at a white sheet of paper—you see RED—its opponent color. • Said there must be two additional processes-one responsible for green vs. red and one responsible for blue vs. yellow (later also white and black).