Acoustic stimulation vibrates the cochlear basilar membrane, initiating a wave of displacement that travels toward the apex and reaches a peak over a restricted region according to the stimulus frequency. In this characteristic frequency region, a tone at the characteristic frequency maximally excites the sensory hair cells of the organ of Corti, which transduce it into electrical signals to produce maximum activity in the auditory nerve. Saturating, nonlinear, feedback from the motile outer hair cells is thought to provide electromechanical amplification of the travelling wave. However, neither the location nor the extent of the source of amplification, in relation to the characteristic frequency, are known. We have used a laser-diode interferometer to measure in vivo the distribution along the basilar membrane of nonlinear, saturating vibrations to 15 kHz tones. We estimate that the site of amplification for the 15 kHz region is restricted to a 1.25 mm length of basilar membrane centered on the 15 kHz place.

Electromotile outer hair cell (OHC) feedback provides the sensitivity and sharp frequency tuning of the cochlea. Basilar membrane displacements in response to characteristic frequency (CF) tones were measured with an interferometer at up to 15 locations across the basilar membrane width in the basal turn of the guinea pig cochlea. For CF tones, basilar membranes vibrations were largest beneath the OHCs; these phase-led vibrations beneath outer pillar cells and adjacent to the spiral ligament by approximately 90 degrees. Post mortem, responses measured beneath the OHCs were reduced by up to 65 dB, and the basilar membrane moved with similar phase across its entire width. We suggest OHCs amplify basilar membrane responses to CF tones when the basilar membrane moves at maximum velocity.

In the mammalian cochlea, the basilar membrane's (BM) mechanical responses are amplified, and frequency tuning is sharpened through active feedback from the electromotile outer hair cells (OHCs). To be effective, OHC feedback must be delivered to the correct region of the BM and introduced at the appropriate time in each cycle of BM displacement. To investigate when OHCs contribute to cochlear amplification, a laser-diode interferometer was used to measure tone-evoked BM displacements in the basal turn of the guinea pig cochlea. Measurements were made at multiple sites across the width of the BM, which are tuned to the same characteristic frequency (CF). In response to CF tones, the largest displacements occur in the OHC region and phase lead those measured beneath the outer pillar cells and adjacent to the spiral ligament by about 90°. Postmortem, responses beneath the OHCs are reduced by up to 65 dB, and all regions across the width of the BM move in unison. We suggest that OHCs amplify BM responses to CF tones when the BM is moving at maximum velocity. In regions of the BM where OHCs contribute to its motion, the responses are compressive and nonlinear. We measured the distribution of nonlinear compressive vibrations along the length of the BM in response to a single frequency tone and estimated that OHC amplification is restricted to a 1.25-to 1.40-mm length of BM centered on the CF place.cochlear amplifier ͉ outer hair cell ͉ frequency selectivity ͉ laser-diode interferometry v on Békésy (1) discovered that sound-induced stapes movement causes a wave of basilar membrane (BM) displacement to travel from the base of the cochlea to the apex because of a pressure difference set up across the cochlear partition. The length of the BM is graded in stiffness, and the cochlear partition can be modeled as a series of weakly coupled sections, each section comprised of a rigid mass connected to the sides of the cochlea by springs that decrease in stiffness toward the apex (2). The BM will resonate when the mechanical impedance caused by the stiffness of the springs cancels that caused by the mass of the section (1). At a point just basal to the resonant place, a peak of displacement will occur on the BM. At a point just apical to the resonant place, the BM impedance is insufficient to maintain a pressure difference across the cochlear partition, and the traveling wave dies out. The location of the displacement peak depends on stimulus frequency, so that each segment of the BM is tuned to a characteristic frequency (CF). von Békésy measured BM movement under postmortem conditions and found that the mechanical responses were broadly tuned and insensitive. Subsequent measurements made from sensitive cochleae in vivo have revealed that, in fact, BM displacement responses are sharply tuned and very sensitive at low sound-pressure levels (3-6).Contributions from the electromotile outer hair cells (OHCs) (7-10) appear to be essential for cochlear sensitivity and frequency tuning (11,12). Many current models of cochlear functi...

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