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Chapter 10

Chapter 10. The Ear and Auditory System. Sound waves. If a tree falls in the woods and no one is around to hear it …. Distinguish between perceptual qualities and physical qualities. The physical quality relevant for hearing is mechanical disturbances in particular media.

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Chapter 10

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  1. Chapter 10 The Ear and Auditory System

  2. Sound waves • If a tree falls in the woods and no one is around to hear it …. • Distinguish between perceptual qualities and physical qualities

  3. The physical quality relevant for hearing is mechanical disturbances in particular media. • This is acoustic energy. • Robert Boyle shows that typical sounds are vibrations in the molecules of air.

  4. Speed of sound • 343 meters/second in air • 1,500 meters/second in water • 5,000 meters/second in steel Speed is constant in any given medium, although sounds fade with increasing distance. Signal strength declines with the square of the distance. (double distance = 4x reduction in sound)

  5. Echoes • Sound, unlike light, can propagate around and through objects. This makes it more difficult to block out sounds than light. • Objects do reflect sound waves as echoes. • These echoes can be used in sonar (sound navigation ranging)

  6. Echoes Direction of reflections make a vast difference in acoustic properties of rooms, e.g. concert halls. Plaster or tile absorbs about 3% of incident sound waves. Carpet absorbs about 25% of incident sound waves.

  7. Echoes • Anechoic chambers use foam wedges to eliminate all echoes. • These rooms have a “dead” feel to them. • Echoes do appear to enable humans to navigate and can be used to identify materials.

  8. Nature of sound waves • Sound waves are variations in the density of molecules in the air.

  9. Sounds levels • Measured in decibels (dB). • Decibel scale is logarithmic. • dB = 20 log (p1/p0) • 20 μPa ≈ softest sound humans can hear.

  10. The Auditory System: The Ear

  11. Outer Ear Trivia • Pinna “colors” the sounds we hear. • Ear canal is about 2.5 cm long x 7mm in diameter. • Ear drum (tympanic membrane) can detects sounds using a displacement of one millionth of a centimeter. • Ear drum has surface area of 68mm2 • Gerbil area = 15 mm2 • Elephant area = 450 mm2

  12. Middle Ear Each bone is about the size of a letter on a printed page. • Ossicles vary in size across species: • Human: 28.5 mg • Gerbil: 1.15 mg • Elephant: 335 mg Note: Error in Figure 10.9, p. 362, Labeling of middle ear.

  13. Why have the ossicles? • Sound waves in air do not transmit well to sound waves in water. • The ossicles decrease the loss in signal strength. • The Eustachian tube maintains air pressure on both side of the ear drum.

  14. The acoustic reflex • The tensor tympani connects to the ear drum. • The stapedius connects to the stapes. • These muscles flex during loud noises to reduce the response of the ossicles. • This reduces intensity of sound transmission by the equivalent of 30 dB.

  15. The Acoustic Reflex • Is more effective at damping low frequencies than at damping high frequencies. • Takes roughly 1/20 of a second to take effect. • Perhaps reduces ability to hears one’s own voice.

  16. The Inner Ear

  17. Cochlea • Three chambers: • Vestibular canal (aka scala vestibuli) • Cochlear duct (aka scala media) • Tympanic Canal (aka scala timpani) http://www.vimm.it/cochlea/index.htm

  18. Hair cell trivia • ~20,000 total hair cells • ~4,500 Inner hair cells • ~15,500 outer hair cells • OHC arranged in “v”s • IHC are linearly arranged. • OHC attached to tectorial membrane; • IHC are not attached. • OHC amplify signals of IHC.

  19. Background to Bekesy • Temporal theory: Basilar membrane vibrates at frequency of incoming sound waves. • Place theory: Basilar membrane vibrates at a place corresponding to the frequency of incoming sound.

  20. Temporal theory • Temporal theory: Basilar membrane vibrates at frequency of incoming sound waves. • Rutherford: A 500 Hz sound would cause basilar membrane to vibrate at 500 Hz; a 1,200 Hz sound would cause basilar membrane to vibrate at 1,200 Hz.

  21. Problems for temporal theory • Basilar membrane varies in width and stiffness over its length, so cannot vibrate uniformly over its length. • Nerve cells cannot fire faster than about 1,000 Hz, but sounds are much high pitched than this. • Maybe two or more neurons act together to coarse code frequency information. (Volley theory)

  22. Place Theory • Frequency information is encoded by the place along the basilar membrane disturbed by the fluid vibration. Hermann von Helmholtz

  23. Problems for Place Theory • Basilar membrane is not composed of fibers along its width. It is a continuous strip. • Basilar membrane is not under tension.

  24. Bekesy & traveling waves • 1920’s Georg von Bekesy could not image the cochlea or basilar membrane in action. • So, von Bekesy built a model of the cochlea.

  25. Waves travel along the basilar membrane. • A wave reaches a peak, then quickly dissipates. • This peak is the peak sensitivity. • Lower tones travel farther. • This yields tonotopic representation (cf. retinotopic representation).

  26. Loudness • Louder noises correspond to sound waves of higher amplitude. • In the ear, this leads to waves of greater amplitude in the basilar membrane. • In the IHC, this leads to a larger neural response.

  27. Cochlear emissions • Sound in air • Movement of ear drum • Movement of ossicles • Movement of oval window • Fluid-borne pressure waves • Displacement of basilar membrane • Stimulation of hair cells (cf. p. 374).

  28. Cochlear emissions • Typically have a narrow band of frequencies. • Roughly 66% of those tested display cochlear emissions. • Frequency of emission is idiosyncratic. • More prevalent and stronger in women. • Dogs, cats, and birds have cochlear emissions.

  29. Cochlear emissions • Can be induced by clicks near the ear. • Can be diagnostic of early ear damage.

  30. Tinnitus • Ringing in the ears • Occurs in about 35% of people at some point in their lives. • Can be caused temporarily by large dose of aspirin. • Appears to have a cortical basis.

  31. Auditory system: The auditory pathways • Feedforward: auditory nerve, superior olive, medial geniculate nucleus and inferior colliculus, auditory cortex. • Feedback: • Medial portion of superior olivary complex to OHCs. • Lateral portion of superior olivary complex to auditory nerve.

  32. Auditory system: The auditory pathways

  33. Auditory system: The auditory pathways • The auditory nerve • The “what” pathway • The “where” pathway • These last two are analogous to those found in vision.

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