Sensation & Perception

1. Sensation & Perception On chapters 7 & 8 Gray 4th edition

2. Sensation - Topics • Basics • Taste and Smell • Touch and Pain • Hearing • The ear and cochlea • Getting aural information to the brain • Vision • The eye and retina • Getting visual information to the brain

3. Perceptual Basics Thresholds Top down & bottom up processing

4. The picking up of information from the environment and the body The transformation of input signals into action potentials We experience 5 modalities of external sensation Coming aware of information from the environment The transformation of action potentials from multiple modalities into a coherent representation of the environment Sensation ≠ Perception

5. Basics - Absolute thresholds • The limits of perception • The lowest level of a stimulus, X, that provokes a response from the sensory system • Vision: the faintest detectable light • Sound: the quietest detectable sound

6. Difference thresholds • The limits to the perception of change • The smallest increment of a stimulus, DX, that provokes a response from the sensory system • Weber’s Law • Pressure: • X = the weight of some object • DX = the smallest weight that makes that object feel heavier Q = Dc/c

7. Another threshold • Vision: We see 24 “frames” per second • Anything faster appears continuous • Movies project at 32 (?) frames per second and convince us the motion is continuous

8. Subliminal signals • Signals that are below threshold • Question: Can we be influenced by signals that are below threshold?

9. From the whole to the parts Perception is driven by context or expectation From the parts to the whole Perception is driven by features of the stimulus Top Down vs. Bottom Up Top Down Bottom Up

10. What is this? • Describe it

11. Please read this aloud THE CAT IN THE HAT

12. An ancient Buddhist parable • It was six men of Indostan, to learning much inclined, • Who went to see the elephant (though all of them were blind), • That each by observation might satisfy his mind. • The first approached the elephant, and, happening to fall • Against his broad and burly side, at once began to call: • "I see," said he, "the elephant is very like a wall!" • The second, feeling of the tusk, cried, "Ho! What have we here • So very round and smooth and sharp? To me 'tis mighty clear • This wonder of an elephant is very like a spear!"

13. Top down or bottom up ? • The third approached the animal, and, happening to take • The squirming trunk within his hands, thus boldly up and spake, • "I see," said he, "the elephant is very like a snake!" • The fourth reached out his eager hand and felt about the knee: • "What most this wondrous beast is like is mighty plain, " said he, • "'Tis clear enough the elephant is very like a tree!" • The fifth, who chanced to touch the ear, said, "E'en the blindest man • Can tell what this resembles most. Deny the fact who can, • This marvel of an elephant is very like a fan!"

14. End of the parable • The sixth no sooner had begun about the beast to grope, • Than, seizing on the swinging tail that fell within his scope, • "I see," said he, "the elephant is very like a rope!" • And so these men of Indostan disputed loud and long, • Each in his own opinion exceeding stiff and strong, • though each was partly in the right, and all were in the wrong! • So oft in group endeavors, the members of the team • Rail on in utter ignorance of what each other mean, • As if it were an elephant not one of them has seen.

15. Taste and Smell The chemical senses Gatekeepers to the insides

16. Taste and Smell • Intimately related • Sensory interaction • Most of the flavor of food is sensed not in the mouth but in the nose • Pinch your nose and apple = potato • Smell is unique • No thalamic waystation

17. Signal Receptors Spontaneously regenerate every two weeks Neural pathway Stuff that enters your body Taste buds on tongue Sweet Salt Bitter Sour Thalamus to the medulla Taste

18. Signal Receptors Neural pathway Stuff that enters your body Little hairs in the back of the nose Directly to the temporal lobe: no thalamic waystation Smell

19. Smell • Each receptor responds to a limited number of chemicals • There are thousands of receptors & thousands of chemicals • There is no one-to-one dedicated circuit

20. Smell • The receptors in your nose are every bit as good as those in your dog’s nose • Why does your dog have a better sense of smell? • The dog has a thousand times as many sensors

21. Pheromones • Chemical communication • Sexual attraction / receptivity

22. Touch and Pain

23. Touch ≠ Pain • Touch is for exploring the environment • Pain is a warning signal that something is wrong • Leprosy is a disease that, among other things, kills the nerves that carry the pain signal

24. Touch • The skin has 4 different kinds of pressure sensors specialized to sense different type of pressure • Broad, diffuse pressure • Tapping • Stretching • Vibration • Thermoreceptors are different sets of cells

25. Touch • The density of pressure sensors varies • Why? • Lots on the tips of fingers, lips • Few on the calf

26. Gate Control Theory of Pain • Small fibers carry the pain signal • Activity in the small fibers opens a “gate,” and you sense pain • Large fibers carry all other pressure signals • Activity in the large fibers closes the “gate,” suppressing the pain

27. Hearing From Sound Waves to the Cortex

28. Ear Workings • Sound waves • Ear drum (tympanic membrane) • Middle ear • Hammer, anvil, stirrup • Inner ear • Cochlea • Basilar membrane • Hair cells

29. Wavelength The distance between adjacent pressure peaks or troughs Pitch or tone or notes Which notes travel further? High or low? Low notes Why? The lower the note, the longer the wavelength Properties of Sound Waves

30. Amplitude The height between a pressure peak and the adjacent trough Loudness Is there such a thing as too loud? Yes Why? The receptors - hair cells in the cochlea - are very sensitive and can be easily damaged Properties of Sound Waves

31. How do we … ? • Determine Loudness • Encoded by the number of neurons firing • Localize Sounds • Two ears: • Sound reaches the two ears differently • time differences • loudness differences

32. Hearing Pitch - 2 Theories Place Theory Frequency Theory

33. Place theory

34. Frequency theory for pitch • Neurons fire at same rate as the frequency of the sound. • Example: • If a sound wave has 100 pressure peaks per second (100 Hz), then the action potentials will fire 100 times per second.

35. So which is right?Place Th. or Frequency Th.? • High Frequencies • Place theory • V. Low Frequencies • Frequency theory • Everything else • Some combination of the two Both!

36. Signal Signal processing apparatus Receptors/ transducers Neural pathway Soundwave Ear Eardrum Middle ear bones Inner ear, cochlea Hair Cells Auditory nerve Hearing, Summary

37. Vision & Hearing • Both the optic and auditory nerves project to the thalamus • Information from the right (left) side goes to the left (right) thalamus and to the left (right) hemisphere of the cortex • Perceptual information is processed on the contralateral side • The auditory cortex is in the temporal lobe • The visual cortex in the occipital lobe

38. Vision From Photons to Action Potentials

39. The visual stimulus • Photons • Light that reflects off a surface in the environment and then passes through the lens of your eye

40. Eyeball

41. Pupil Iris Lens Humors Retina Fovea Optic nerve Blind spot Distribution of photoreceptors Receptive fields Parts of the eye Topics

42. The Retina • The back of the eye is called the retina • The retina is composed of photoreceptors: • rods and cones • and neurons: • bipolar • horizontal • ganglia The retina is part of the brain

43. Rods and Cones • Rods and cones are photoreceptors • They transduce (= convert) light into action potentials • They convert photons into neural activity

44. Structure of the retina

45. How the transduction works • A photon stimulates a rod or cone • The rod or cone generates a cascade of chemical reactions that produce a voltage • The voltage triggers an action potential in the attached neuron (a bipolar cell)

46. Visual information • The action potential is information of the existence of light at a specific location on the retina • Each location on the retina provides information about a specific location in the environment • The action potential is information about surfaces in the environment

47. The transmission of information • The bipolar cell sends a signal to a ganglion cell (another type of neuron) • The ganglion cell sends the signal via the optic nerve to the thalamus (the sensory way station)

48. Differences betweenrods and cones • Sensitivity vs. Resolution: • Rods share ganglion cells • high sensitivity, low resolution • Cones do not • low sensitivity, high resolution • Color • Rods are insensitive to color • There are 3 types of cones, sensitive to 3 different parts of the color spectrum

49. Color Vision - 2 Theories

50. Tri-Chromatic Theory - Helmholtz • Herr Helmholtz (1860s) claimed human vision uses three color receptors • sensitive to either red, green, blue • and constructed all intermediate colors from combinations of the basic three