The speed at which it travels is a function of the density of the conducting medium.

Transcription

1

2 Sound is a compression wave which (unlike light) must have a medium to conduct it. If the medium is uniform in density, the sound will spread at as a uniform ring of circles (actually spheres). The speed at which it travels is a function of the density of the conducting medium.

3 Taking air as the medium of conduction, the sound waves consist of zones of compressed air alternating with zones of decompressed air.

4 The air molecules are randomly spaced before a sound source moves. To generate a sound wave, the source will alternately compress the air molecules which lie next to it, and then decompress them. The compression zone brings about compression of the next zone of air -- like dominos -- so the compression zone itself seems to move away from the source.

5

6 A sound source oscillates more than once, of course, to generate a series of compression waves which travel away from the source like ripples on a pond. We commonly diagram the wave using a squiggly line.

7 Most sound sources generate very complex waves. But some sources (such as a tuning fork) vibrate in a very regular manner, producing what are called The shape of the wave approximates a

8

9 The unit of frequency is which means the same thing as cycles per second. Some pure-tone sources will oscillate more rapidly than others. How many cycles are completed in one second specifies the of the sound. [ frequency is related to pitch, as will be discussed later ]

10

11 But also note that the sound waves can have the same frequency, but differ in amplitude. The amplitude difference is the major factor in specifying how loud it will be (as will be discussed later).

12

13 The sound enters the ear, vibrates the eardrum and a set of connected bones called the ossicles, and then enters the fluid chambers of the cochlea. (A lot of anatomical detail is being left out.)

14

15 The cochlea is separated into two chambers by the organ of Corti, which attaches to bony spurs on each side of the cavity.

16

17

18 The organ of Corti consists of a number of structures, the main ones being labeled here.

19

20 Hair cells get their name because they have cilia growing from the top of the cell. They are actually specialized neurons, which respond to the sound waves (as detailed below) by changes in membrane potential and the release of transmitter substance to neurons which will carry the information to the brain.

21 To further discuss the transmission of the sound wave through the fluid chambers of the inner ear, it is useful to consider the cochlea as being unrolled.

22 The sound enters the cochlea through the oval window. It then travels down the length of the cochlea in the upper chamber, passes through the basilar membrane by bending it (it is elastic), and then escapes from the lower chamber by passing out the round window.

23

24 The cilia of the hair cells brush against the tectorial membrane, which is fairly rigid. The sound causes the basilar membrane to oscillate, which moves the hair cells and causes the cilia to bend. This brings about changes in their membrane potential, with release of transmitter and generation of spikes in the neurons of the auditory nerve.

25 Dendrites from neurons in the spiral ganglion receive the information from the hair cells, and the axons of these neurons form the auditory nerve.

26 It is called the spiral ganglion because has an elongated shape which spirals along beside the cochlea. Each portion of the spiral ganglion has neurons which serve the hair cells along the length of the basilar membrane.

27 superior temporal lobe medial geniculate nucleus of thalamus cochlear nucleus olives inferior colliculus Here are the key brain structures for audition.

28

29

30 The connections to and among these structures is rather complex!

31 We ll discuss the olives and the superior colliculus but the main point for now is that auditory messages from each ear flow to both sides of the brain through the cochlear and medial geniculate nuclei.

32 There are more nerve fibers carrying information to the contralateral hemisphere. Though auditory information goes to both hemispheres, the contralateral side gets the stronger message. Most people process the spoken word better if it is presented to the right ear, because this sends it to the left side, which is better specialized for language.

33

34 1500s -- Venturi studies the basis for sound localization. large open field Venturi rang a bell at various points all around the perimeter of the field. The blindfolded assistant pointed to where he thought the sound was coming from.

35 He found that the assistant was most accurate when the source was directly to the right or left, and least accurate when it was straight ahead or behind.

36 The logical basis for this effect was that the sound struck one ear with greater intensity than the other (because the head produces something analogous to a shadow...

37 Much later, namely at the start of WWI, Wertheimer wanted to help the Germans deal with a new war machine -- the airplane. Mostly they used for surveillance, but they were hard to shoot down, because of echoes from hillsides. Wertheimer decided to study how sounds are localized.

38 Wertheimer thought it likely that the sound was localized on the basis of time of arrival at the two ears. So he used instruments which could deliver clicks to the ears of the subjects. He delivered a click to one ear, following shortly thereafter with a click to the other. If the time difference was greater than 2 milliseconds, the subject hear two separate clicks.

39

40 The two milliseconds corresponds to the time it takes for a sound to travel from one side of the head to the other (at sea level).

41 With one millisecond of separation the sound appeared to come from an oblique angle, approximately half way toward the front or back of the subject.

42 With simultaneous presentation of the clicks, the sound appeared to come from straight ahead or from straight behind. The smallest difference which can be perceived is about 1/10 millisecond, wherein the sound seems to come from just off the midline.

43 About mid-century, Licklidder proposed a neural mechanism for how the time differences would be detected.

44 The arrival of a sound at one ear would send a spike which would travel down all branches to an array of neurons.

45 The spike converges upon each successive neuron in the array, but the neurons don t respond because two simultaneous spikes are needed to generate a response. [The change in color is just for illustration.] But when the spike arrives from the left ear to add with the one coming from the right, one of the neurons will fire.

46 right ear leads by 2 ms simultaneous input to ears left ear leads by 2 ms The basic concept is that the time of travel down the branches carrying the message from each ear is exactly right to activate only one of the cells in the array. Which one depends upon the difference in time of arrival of the sound, and thus the time difference between when the spikes begin their journey.

47 The inferior colliculi also have a role in sound localization. Subsequent work in the 70s and 80s indicates that this kind of circuit exists in the olivary complex. Exact details of mechanism don t fully match the Licklidder model.

48 So, was Wertheimer completely right, and Venturi completely wrong about the basis for localizing the source of a sound.

49 intensity cue ,000 Frequency timing cue

50 amplifier

51 base of cochlea apex of cochlea high frequency low frequency In the late 1800s Helmholtz drew attention to the basilar membrane, and suggested that it might resonate at different locations as a function of the pitch of a note. This came to be known as

52 An alternative view is known as -- (your text calls it the frequency theory ) amplifier

53 The initial thinking was that the auditory nerve would conduct an analog signal to the brain which was an electrical copy of the sound wave. This was later replaced by the idea that the frequency of spikes indicates the frequency of the sound.

54 One problem is that neurons fire no faster than about 1000 spikes per second. A solution is proposed by Volley Theory, which has neurons operating as a pool to encode the sound. combined total of 5000 spikes/sec 5000 hz tone

55 300 hz

56 Georg von Békésy In 1961, he was awarded the Nobel Prize in Physiology of Medicine for his research on the function of the cochlea in the mammalian hearing organ

57 He put a dusting of carmine particles on the basilar membrane, and observed where they would dance. The hard part, of course, was to collect data at various tilts of the cochlea so that one could think of it as lying flat. 300 hz

58 base apex 50 hz Von Bekesy reported that the maximum vibration was closest to the apex for low frequency sounds, and progressively got closer to the oval window at higher frequencies. 200 hz 400 hz 1600 hz Distance from stapes in millimeters

59 Further, the location of hair cell loss with loud sounds was a function of the frequency of the sound.

60 Base Apex He argued that the pitch of a sound was known on the basis of where the membrane vibrated the most. At low frequencies the maximal response was near the apex, and at high frequencies it was near the base of the cochlea.

61 time 1 time 2 time 3 But Von Bekesy emphasized the dynamic aspects of the membrane response, and called his view the

62

63

64 It is now known that nerve fibers in the auditory nerve will fire just as predicted by volley theory. That is, they fire at the peaks of the sound wave, dropping out as necessary because of the speed at which they are coming, but firing as often as possible on the peaks of the waves. It appears that they can follow in this probabilistic way up to several thousand hertz.

65

66 What can be said about the task of identifying a complex sound? It is not composed of a single frequency, so how can it be identified?

67 As you might guess, the favorite concept is that the nervous system (some invoke the cochlea) breaks the sound down into its harmonic components. The sound can then be identified by the relative amplitude of these components.

68 The harmonic components of a high pitched instrument..... would be different from that of a low pitched instrument.

69 The problem -- of course -- is that we are often presented with a mixture of sounds, all coming at once.

70 The sound waves combine, so the ear is presented with a very complex mixture.

71 What is ICA? Blind Source Separation: X = AS Unknown sources S as latent variables and unknown mixing matrix A Estimate S and unmixing matrix W=A -1 from only X (observations). Mixing matrix A s 1 Sources s 2 n sources, m observations x 1 Observations x 2 X= AS The cocktail party problem: based only on x 1 and x 2 we need to recover s 1 and s 2.

72

73 The primary auditory cortex is the first region of cerebral cortex to receive auditory input. Perception of sound is associated with the right posterior superior temporal gyrus (STG). The superior temporal gyrus contains several important structures of the brain, including Brodmann areas 41 and 42, marking the location of the primary auditory cortex, the cortical region responsible for the sensation of basic characteristics of sound such as pitch and rhythm. The auditory association area is located within the temporal lobe of the brain, in an area called the Wernicke's area, or area 22. This area, near the lateral cerebral sulcus, is an important region for the processing of acoustic signals so that they can be distinguished as speech, music, or noise.

74 If Wernicke s area is damaged in the non-dominant hemisphere, the inability to perceive the pitch, rhythm, and emotional tone of speech. Broca s aphasia, the deficit in language production.

75

76

77