The dynamic range of inner hair cell and organ of Corti responses

Inner hair cell (MC) and organ of Corti (OC) responses are measured from th e apical three turns of the guinea pig cochlea, allowing access to regions with best, or most sensitive, frequencies at approximately 250, 1000, and 4 000 Hz. In addition to measuring both ac and de receptor potentials, the av erage value of the half-wave rectified response (AVE(HR)) is computed to be tter reflect the signal that induces transmitter release. This measure faci litates comparisons with single-unit responses in the auditory nerve. Altho ugh MC ac responses exhibit compressive growth, response magnitudes at high levels depend on stimulus frequency. For example, IHCs with moderate and h igh best frequencies (BF) exhibit more linear responses below the BF of the cell, where higher sound-pressure levels are required to approach saturati on. Because a similar frequency dependence is observed in extracellular OC responses, this phenomenon may originate in cochlear mechanics. At the most apical recording location, however, the pattern documented at the base of the cochlea is not seen in IHCs with low BFs around 250 Hz. In fact, more l inear behavior is measured above the BF of the cell. These frequency-depend ent features require modification of cochlear models that do not provide fo r longitudinal variations and generally depend on a single stage of saturat ion located at the synapse. Finally, behavior of de and AVEHR responses sug gests that a single IHC is capable of coding intensity over a large dynamic range [Patuzzi and Sellick, J. Acoust. Sec. Am. 74, 1734-1741 (1983); Smit h et al., in Healing-Physiological Bases and Psychophysics (Springer, Berli n, 1983); Smith, in Auditory Function (Wiley, New York, 1988)] and that inf ormation compiled over wide areas along the cochlear partition is not essen tial for loudness perception, consistent with psychophysical results [Vieme ister, Hearing Res. 34, 267-274(1988)]. (C) 2000 Acoustical Society of Amer ica. [S0001-4966(00)02303-1].