Re: mechanical cochlear model ("reinifrosch@xxxxxxxx" )


Subject: Re: mechanical cochlear model
From:    "reinifrosch@xxxxxxxx"  <reinifrosch@xxxxxxxx>
Date:    Wed, 10 Mar 2010 12:40:53 +0000
List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>

Dear David, Last night, you have written: >Many theories have been advanced to explain the details of the peak region >of the cochlear response but perhaps we can try to agree on some fundamental >principals. I would argue that a traveling wave, involving transverse >motion of the basilar membrane, passively propagates acoustic energy to the >peak region. This passive propagation behaves in a manner described by >Peterson and Bogert (1950). When this energy reaches the beginning of the >peak region, some process, usually called the "cochlear amplifier" and >involving outer hair cells, takes over and shapes the peak response. We may >question whether the cochlear amplifier results from somatic motility or >from hair bundle motility and we may debate questions of how outer hair cell >forces get coupled back into basilar membrane and inner hair cell responses >but does anyone question whether the classical traveling wave plays a role >in at least passive propagation of acoustic energy to the best place? I for one do not question that. I think that there is a travelling wave both in the spatial "active-peak" and "passive-peak" regions. What I mean by these terms is illustrated, e.g., in Fig. 14 of the review "Mechanics of the Mammalian Cochlea" by Robles and Ruggero (2001), Physiological Reviews 81, 1305-1352. In that Fig. 14, based on published measurements, by Russell and Nilsen (1997), of BM motion at various places in a guinea-pig cochlea during 15-kHz tones of many different sound-pressure levels, the active peak (maximum at SPL < 60 dB) is at 14.45 mm from apex (~4.35 mm from base), and the passive peak [clearly visible second peak at 55 dB < SPL < 100 dB] is at 15.85 mm from apex (~2.95 mm from base). So the distance between the two peaks is 1.4 mm, i.e., ~0.5 one-octave distance. At 55 dB SPL, the BM displacement at the passive peak is seen to be lower than that at the active peak by a factor of 0.1, i.e., by 20 dB. The passive-peak BM displacement is seen to be proportional to the sound-pressure of the stimulus. In the lower panel of the mentioned Fig. 14, the phase is seen to drop steeply from 14.9 mm to 14.0 mm from apex, indicating a travelling wave in the spatial region of the active peak. At 14.9 mm, the phase is -180 degrees and falling steeply, so a travelling wave is indicated also for the spatial passive-peak region. Reinhart. Reinhart Frosch, Dr. phil. nat., r. PSI and ETH Zurich, Sommerhaldenstr. 5B, CH-5200 Brugg. Phone: 0041 56 441 77 72. Mobile: 0041 79 754 30 32. E-mail: reinifrosch@xxxxxxxx .


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