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NPB+112+11+auditory+_+language+lecture+notes - NPB 112 Feb...

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1 NPB 112, Feb. 21-23, 2011 OUTLINE Hearing (Ch. 13) I. Sound II. Cochlear transduction of sound III. Cochlear hair cells IV. Frequency discrimination A. cochlear mechanics B. place principal C. auditory nerve fiber tuning curves D. phase locking and volley principal V. Central Auditory Pathway I. Sound: Fig. 13.1 A. Sound: disturbance of air caused by vibrating object. 1. Sinusoidal pressure wave, consisting of alternating areas of compression and rarefaction of air molecules. 2. FREQUENCY of sound pressure wave (cycles per second = Hz) determines pitch. Humans hear from 100 to about 20,000 Hz (bats: higher) B. Amplitude of sinusoid = intensity or loudness C. Loudness is measured with decibel scale: Sound Pressure Level (SPL) = 20 log P test/ P reference in DECIBELS (P reference= 20 μN/sq. m) 1. P reference = sound that is just noticed. 2. Just noticeable difference between two tones increases logarithmically as tones get louder (easier to tell 2 soft sounds apart than 2 loud sounds). 3. Normal conversation = 65 dB, electro-house band = 110 decibels D. SUMMARY: frequency of sounds is pitch, amplitude is loudness. II. Cochlear Transduction of sound: Fig. 13.3 A. Sound waves travel in air, but they must influence cochlear hair cells that are bathed in saltwater. When sound in air hits a water interface, most of the energy is reflected (why one can't hear people talking when underwater). B. Cochlea of inner ear: long coiled tube filled with fluid (perilymph) with basilar membrane running down middle (FIG. 13.3)
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2 1. Is long fluid-filled tube with 2.5 spirals. FIG. 13.5 shows it unspiraled and stretched out C. How are air pressure waves transmitted into fluid medium of cochlea? 1. air pressure waves channeled through auditory canal to tympanic membrane (eardrum). 2. Most sound energy is reflected due impedance mismatch between air and tympanic membrane, but some energy causes eardrum to vibrate. 3. Amplitude of vibration of eardrum is low, must be amplified. 4. Eardrum attached to 3 middle ear bones (FIG 13.3) 5. Stapes contacts oval window of cochlea 6. Vibration of eardrum moves bones so that the stapes vibrates against the oval window, pushing it in and out with each cycle of vibration. 7. Vibration of a large area- eardrum- is transferred to a small area- the oval window-to amplify vibrations (like focusing body weight onto a small area when wearing spike-heel shoes) 8. Vibration of oval window causes pressure waves in the cochlear fluid. 9. Since fluid is incompressible, round window (at other end of cochlea) bulges out to relieve pressure 10. Fluid pressure wave in cochlea sets up traveling wave in flexible basilar membrane. 11. cochlear hair cells resting on basilar membrane ride up and down: how it this transduced into electrical potentials? III. Cochlear hair cells A. Auditory hair cells in organ of Corti sit on the basilar membrane: see Fig. 13.4.
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