Basic Principles of Sensation and Perception

1. Basic Principles of Sensation and Perception

2. Sensation vs. Perception • Basketball Study: http://www.youtube.com/watch?v=vJG698U2Mvo • Inattentional Blindness = inability to see an object or a person in our midst. Intuitively, we think that as long as our eyes are open, we are seeing. Research beginning with Ulrich Neisser’s “basketball study,” has clearly indicated that visual perception is not like a videotape.

3. Sensation vs. Perception Sensation /Bottom-Up Processing Perception/Top-Down Processing The process by which the brain organizes and interprets the data received from the senses, enabling us to recognize meaningful objects and events. Interpreting what we sense based on prior knowledge or context • The stimulation of sensory receptors (cilia in ears, rods/cones in vision, taste buds) by the properties of the stimulus (sound waves, light energy, chemicals) and the transmission of sensory information into the central nervous system Psychophysics = study of the relationship between physical characteristics of stimuli and our psychological experience of them (light  brightness, sound  volume, pressure  weight, taste  sweetness)

4. Sensation vs. Perception Bottom-Up Processing Analysis of the stimulus begins with the sense receptors and works up to the level of the brain and mind. Picture is really black and white blotches broken down into features by the brain Top-Down Processing Information processing guided by higher-level mental processes as we construct perceptions, drawing on our experience and expectations. Based on expectations, context or “love of dogs”, we make meaning out of the black and white blotches and perceive a dog. What is this picture?

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

6. Sensation/Perception • Transduction: sensory receptors (eyes, ears, nose, skin, tongue) convert the stimulus into neural impulses which are sent to the brain • EX: Receptor cells in the inner ear convert sound waves/vibrations into electrochemical signals. These signals are carried by neurons to the brain

7. Process of Sensation and Perception • Any change in energy in environment creates stimuli (light waves, sound vibrations, pressure)… • which activates receptor cells of sense organs (eye, ear) to trigger electrical signals or impulses… • which are transformed by the brain into… • sensations or meaningless bits of sensory information… • to which experience automatically adds meanings, feelings, and memories… • which result in meaningful patterns or images known as perceptions.

8. Sensation & Perception Don’t “Just Happen” Sensation • Light bounces off Lemon • Light forms image of Lemon on retina (upside down) • Image generates electrical signals in receptors • Signals travel along nerve fibers to the brain... Perception Signals are processed and you “perceive” Lemon

9. Detection Absolute Threshold Intensity No No No Yes Yes Observer’s Response Detected Tell when you (the observer) detect the light (50% of the time).

10. 100 Percentage of correct detections 75 50 Subliminal stimuli 25 0 Low Absolute threshold Medium Intensity of stimulus Absolute Thresholds

11. Sensing the World: Basic Principles • Absolute Threshold = the lowest amount of stimulus needed to notice it 50% of the time. • Measure absolute threshold by recording the stimulation needed for us to pinpoint its appearance 50% of the time • As stimulus intensity increases, subjects’ probability of responding to stimuli gradually increases • There is no single stimulus intensity at which the subject jumps from no detection to completely accurate detection  not really “absolute” or constant. Threshold varies within a person over time – due to changing psychological states, hormone levels, sensory adaptation, etc • EX: You turn down the radio to a point where you only hear the faint sound half the time. Then that loudness (decibel) is your absolute threshold for sound. • EX: The level of heat on a car heat warmer to feel it half the time. • EX: Lemon Lab – the number of lemons it takes to smell the scent of lemons in a room half the time

12. Sensing the World: Basic Principles • Difference Threshold (just noticeable difference or jnd) = the lowest difference between two stimuli that person can detect 50% of the time. • EX: A musician must detect minute discrepancies in an instrument’s tuning • EX: A wine taster must detect the slight flavor difference between two vintage wines • EX: Parents must detect the sound of their own child’s voice amid other children’s voices • EX: Lemon Lab – students must detect the slight weight, firmness, size, etc between the lemons.

13. Difference Threshold Difference Threshold: Minimum difference between two stimuli required for detection 50% of the time, also called just noticeable difference (JND). Difference Threshold No Yes No Observer’s Response Tell when you (observer) detect a difference in the light. (50% of the time) Light intensity – the two light bulbs must differ by 8% (Weber’s Law)

14. Sensing the World: Basic Principles • Weber’s Law = regardless of magnitude, two stimuli must differ by a constant proportion for the difference to be noticeable. • Light intensity – 8% • Tone frequency - .3% • Weight – 2% • EX: Lemon Lab – if you lemon weighs 6 oz then the next lemon will have to weigh .12 oz heavier or .12 oz lighter in order to detect the difference between lemon • JND varies according to the strength or intensity of the original stimulus. The greater the stimulus the greater the change necessary to produce JND • EX: If a farmer grows giant lemons, a greater difference threshold will be needed to determine a change from a 500 oz lemon, such as a change of 10 oz versus .12 oz with a 6 oz lemon.

15. Sensing the World: Basic Principles • Fechner’s Law – larger and larger increases in stimulus intensity are required to produce perceptible increments in the magnitude of sensation. Constant increments in stimulus intensity produce smaller and smaller increases in perceived magnitude of sensation. • Scene #1: dark room – add one light bulb – difference in light is striking • Scene #2: same room – add a second light bulb – the amount of light is doubled but the room does not seem twice as bright • Scene #3: same room – add a third light bulb, it adds just as much light as the second, but you barely notice the difference • Three equal increases in stimulus intensity produces progressively smaller differences in the magnitude of sensation

16. Sensing the World: Basic Principles • Sensory Adaptation = lowered sensitivity due to constant exposure from a stimulus. After constant exposure to a stimulus, our nerve celss fire less frequently • EX: when you go into someone’s house you notice an odor…but this only lasts for a little while because sensory adaptation allows you to focus your attention on changing environment • EX: forget your sunglasses are on the top of your head. • EX: Lemon Lab – students toward the end had a harder time detecting their lemon

17. Sensing the World: Basic Principles • Signal Detection Theory – predicting when we will notice a weak stimulus (signal). Detecting a weak signal depends on: • Signal’s strength • Our internal psychological state (experience, motivation, and fatigue)  Absolute threshold is not really “absolute”! Absolute Threshold varies depending on the level and nature of ongoing sensory stimulation; differs moment to moment and person to person • EX: exhausted parents of a newborn will notice the faintest whimper from the cradle, while failing to notice louder, unimportant sounds. • EX: On a dark night, on a lonely street, a twig snapping might trigger a stimulus that wouldn’t fire if it were light and busy. • EX: Lemon Lab – fatigue, embarrassment, motivation among students can influence the detection of the lemon

18. 100 Percentage of correct detections 75 50 Subliminal stimuli 25 0 Low Absolute threshold Medium Intensity of stimulus Sensing the World: Basic Principles • A stimulus is Subliminal if it is below your absolute threshold, you detect it less than 50% of the time. Not always about unconscious processing, just means below absolute threshold (consciously detect a weak stimulus some of the time)

19. Vision

20. Visual Processing: SENSATION->light waves cornea pupil (iris) lens retina (rods and cones – Begin Colortrichromatic theory bipolar ganglion – 1st stage of Coloropponent process) optic nerve (blind spot) thalamus occipital lobe (visual cortex – end of Coloropponent process ) feature detectors abstraction (cells in parietal and temporal lobe combine info from feature detectors) PERCEPTION

22. Acuity = sharpness of vision- Nearsightedness = nearby objects seen more clearly; lens focuses image of distant objects in front of retina- Farsightedness = faraway objects seen more clearly; lens focuses near objects behind retina • Farsighted Nearsighted Normal Vision Vision Vision

23. Nearly a million messages can be sent by the optic nerve at once, through nearly 1 million ganglion fibers. One cone often synapses onto one bipolar and ganglion cell, while the axons of many rods have to share one bipolar and ganglion cell  allows cones to be more senstivie to detail.

24. Union of Opposites: Rods and cones are responsible for transduction- the transformation of stimulus energy (sights, sounds, smells) into neural impulses.

25. Blind Spot: Point where the optic nerve leaves the eye because there are no receptor cells located there. Blind Spot Activity – Cover one eye and hold up a finger at arm’s length. Have them focus straight ahead and move the finger about two palm widths to the side until it disappears.

27. Hermann Grid

28. Hermann Grid 4 bright patches in the inhibitory surround  inhibits cell, less neural activity so seems less bright 2 bright patches in the inhibitory surround  less inhibition so more neural activity Look directly at intersection, the OFF and ON regions are so small that both fit within the width of a strip. Thus, all the cells around the region of fixation give the same response, whether in the intersection or not

29. (-) (+) (-) Motion Aftereffects • Waterfall Illusion: http://www.michaelbach.de/ot/mot_adapt/index.html • Fixing gaze  sensory adaptation (over-stimulate cells that detect outward movement) • Shift gaze  when the outward-movement detectors stop firing, there is a tendency for inward-movement detectors to start firing for a few seconds *Motion Blindness: • http://www.michaelbach.de/ot/mot-mib/index.html Activated by outward-movement Activated by inward-movement

30. Parallel Processing • simultaneous processing of several aspects of a problem simultaneously • the brain divides a visual scene into subdivisions such as color, depth, form, movement, etc. • Feature Detectors • nerve cells in the visual cortex respond to specific features • shape • angle • movement

31. Visual Information Processing Retinal Processing Rods & Cones èBipolar Cells èGanglion Cells Feature Detection Detector cells respond to elementary features Abstraction High-level cells respond to combined info from feature-detector cells Recognition Brain matches the constructed image with stored images

32. From Sensation to Recognition

33. Is a lemon “yellow”? Light has no color – brain constructs color from the variations in light waves reflected from objects NO! Is a chili pepper “red”? Color Vision Do objects possess color?

34. Two basic types of color mixing: Psychophysics! • subtractive color mixture - combining different color paints • Different pigments subtract different wavelengths: red subtracts all • but red, blue all but blue, green subtracts blue and red, etc… • 2. additive color mixture - combining different color lights. • By combining lights of different wavelengths we can create the perception of new colors. Examples: red + green = yellow; red + blue = purple; green + blue = cyan; red + blue + green = white • http://www.michaelbach.de/ot/col_mix/index.html

35. Hue (color) = dimension of color determined by the wavelength of light (the distance from the peak of one wave to the peak of the next wave). Visible light has wavelengths from about 400nm to 700nm • Intensity (brightness) = amount of energy in a wave determined by the amplitude. • Saturation = richness or purity of light determined by the smoothness or complexity of the waves Most humans can distinguish 7 million different color shades

36. Wavelength (hue) - different wavelengths of light result in different colors. Violet Green Orange Red Indigo Blue Yellow 400 nm 700 nm Short wavelengths Long wavelengths Intensity (brightness) - Blue color with varying levels of intensity. As intensity increases or decreases, blue color looks more “washed out” or “darkened.”

38. Color Perception pg 418 • Humans are able to discriminate 7 million different hues. • Colors convey important information: • Ripeness of food • Danger signals • Trichromatic theory (1st stage, occurring at the level of cones) • Eye contains 3 different color sensitive elements • Blue, green or red elements • Trichromatic theory accounts for color mixing of lights. • Opponent-Process theory (2nd stage, occurring further on in the visual system • Visual system is organized into red-green, blue-yellow and black-white units. • Theory can account for negative color afterimages.

39. Helmholtz 1852 Trichromatic Theory of Color Vision Human eye has 3 types of cone receptors sensitive to different wavelengths of light. Short Medium Long People see colors because the eye does its own “color mixing” by varying ratio of cone neural activity

40. Theories of Color Vision: Trichromatic Theory Wavelength Input Cone Signal to Brain “Blue” Blue “Green” Equal Parts Red and Green = Yellow “Red”

42. Theories of Color Vision: Trichromatic Theory • Trichromatic Theory can explain some aspects of colorblindness: • most of us are trichromats • someone missing one of the three cone types is a dichromat • dichromats have only two primaries: any color they can see can be matched with differing proportions of the two wavelengths to which they are sensitive • most common is deuteranopia (~3% of men, <1% of women) - missing “green” cones • cannot see color difference between reds and greens - but they can see luminance difference • someone missing two is a monochromat • someone missing all cone types is called a rod monochromat (very poor vision!) People who suffer red-green blindness have trouble perceiving the number within the design

43. Some Views With and Without Color Vision Link Jay and Maureen Neitz Color Vision Page

45. Theories of Color Vision: Opponent-Process Theory

46. Theories of Color Vision: Opponent-Process Theory G+R- Y+B- B+Y- R+G- R+G- B+Y- Y+B- G+R- Red/Green Blue/Yellow Double Opponent Cells in V1

47. Opponent Process- Afterimage Effect 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.

48. Explaining Complementary Afterimages • white normally stimulates the red and green cells equally • exposure to green fatigues the green cell while the red cell rests • exposure to white NOW causes red receptor to respond but green receptor is “tired” • we see red instead of white

49. Opponent Process – Afterimage Effect

50. Color Constancy • Perceiving familiar objects as having consistent color, even if changing illumination alters the wavelengths reflected by the object • Visual Pathway Review Activity