1. INTRODUCTION TO HEARING

2. WHAT IS SOUND?

3. amplitude Intensity measured in decibels.

6. SAME PITCH DIFFERENT TIMBRE

8. * * * All points on this curve have the same perceived loudness as the standard (*) EQUAL LOUDNESS CURVES

10. SUMMARY Sound is variation in pressure Frequency, pitch Intensity (SPL, “sound pressure level”), loudness, decibels Timbre, harmonics Equal loudness Hearing thresholds

11. THE EAR

16. Amplification: Bones 1.3x Area 17 x Total = 22.1x Density: Air: 1.2 kg/m 3 Water: 1000 kg/m 3 Ratio:

19. LESS STIFF STIFF HIGH FREQUENCIES LOW FREQUENCIES

22. SUMMARY Outer, middle, inner ear Oval window, round window, Basilar membrane, tectorial membrane, hair cells

23. THEORIES OF HEARING

24. PLACE THEORY (which fibres, labelled lines) Von Békésy (Nobel prize 1961) 1 - Travelling wave; stiffness varies 2 - one place most active for a given frequency 3 - Tonotopic code; coded as place PERIODICITY THEORY (how they are firing, temporal code) 1 - sound coded as pattern

26. MODEL OF THE BASILAR MEMBRANE Varies in stiffness… RESONANCE Traveling wave:

27. WHERE THE WAVE HAS ITS HIGHEST AMPLITUDE DEPENDS ON ITS FREQUENCY

29. Evidence against place -- Missing fundamental -- which can be masked -- some animals have no basilar membrane Evidence against periodicity -- cells can’t fire fast enough -- diplacusis Evidence for place -- physiology (basilar membrane) (cells tuned for frequencies) -- masking Evidence for periodicity -- multiple cells could do it -- phase locking of cells

32. MASKING

33. Evidence against place -- Missing fundamental -- which can be masked -- some animals have no basilar membrane Evidence against periodicity -- cells can’t fire fast enough -- diplacusis Evidence for place -- physiology (basilar membrane) (cells tuned for frequencies) -- masking Evidence for periodicity -- multiple cells could do it -- phase locking of cells

34. PHASE LOCKING

35. Temporal coding up to about 4,000 hz… but each spike takes about 2ms… therefore only up to around 500 hz…..

36. But could share it out over several cells One cell might only be able to follow every 4 th cycle….. … but others can share the task

37. Evidence against place -- Missing fundamental -- which can be masked -- some animals have no basilar membrane Evidence against periodicity -- cells can’t fire fast enough -- diplacusis Evidence for place -- physiology (basilar membrane) (cells tuned for frequencies) -- masking Evidence for periodicity -- multiple cells could do it -- phase locking of cells

38. THE CASE OF THE MISSING FUNDAMENTAL Pitch determined by fundamental….

39. So what happens if we remove the fundamental? What does it sound like?

40. Play it

41. Training a goldfish... CAN AN ANIMAL WITH NO BASILAR MEMBRANE DISTINGUISH FREQUENCIES?

42. Evidence against place -- Missing fundamental -- which can be masked -- some animals have no basilar membrane Evidence against periodicity -- cells can’t fire fast enough -- diplacusis Evidence for place -- physiology (basilar membrane) (cells tuned for frequencies) -- masking Evidence for periodicity -- multiple cells could do it -- phase locking of cells

43. Place theory sound coded as place Periodicity theory sound coded as pattern Duplicity below 1-4 kHz, coded by periodicity above 1-4 kHz, coded by place

44. AUDITORY SYSTEM

46. Auditory cortex Auditory thalamus Superior colliculus Inferior colliculus cochlea Cochlear nucleus Superior olive The Auditory System

47. The Auditory System (cortical route) Cochlear nucleus Inferior colliculus thalamus cortex

48. The Auditory System (sub-cortical route) Cochlear nucleus Superior olive Inferior colliculus Superior colliculus

49. The Auditory System

51. Cells in primary auditory cortex: -- tonotopically arranged -- respond to more complex features (eg howler monkey calls) primary auditory cortex

52. The Superior Colliculus

53. AUDITORY LOCALIZATION

54. Auditory localization 1 inter-aural time of arrival differences -- circle of confusion 2 inter-aural intensity differences 3 pinnae (up/down front/back etc..) 4 head movements

55. 1 inter-aural time of arrival differences

57. 2 inter-aural intensity differences

59. 3 pinnae (up/down front/back etc..)

60. 4 head movements

62. AUDITORY SCENE ANALYSIS