CHANGES IN COCHLEAR MECHANICS DURING VOCALIZATION - EVIDENCE FOR A PHASIC MEDIAL EFFERENT EFFECT

The mustached bat, Pteronotus p. parnellii, has a finely tuned cochlea that rings at its resonant frequency in response to an acoustic tone pip. The decay time (DT) and frequency of these damped oscillations ca n be measured from the cochlear microphonic potential (CM) to study ch anges in cochlear mechanics. In this report, we describe phasic change s that occur in synchrony with communication sound vocalizations of th e bat. Three animals with chronically implanted electrodes were studie d. During the experiments, 1-2 ms tone pips were emitted from a speake r every 200 ms. This triggered a computer analysis of the resulting CR I to determine the DT and cochlear resonance frequency (CRF) of the r inging. The time relative to vocalizations was determined by monitorin g the output of a microphone placed near a bat's mouth. Similar result s were obtained from all three bats tested. In a representative case, the average DT was 2.33 +/- 0.25 ms while the bat was quiet: but it de creased by 46% to 1.26 +/- 0.75 during vocalizations, which indicates a greater damping of the cochlear partition. Sometimes, DT started dec reasing immediately before the bat vocalized. After the end of a vocal ization, the return to baseline values varied from rapid (milliseconds ) to gradual (1-2 seconds). The CRF also changed from baseline values during vocalization, although the amount and direction of change were not predictable. When gentamicin was administered to block the action of medial olivocochlear (MOC) efferents, DT reduction was still eviden t during vocalization but less pronounced. We conclude that phasic cha nges in damping occur in synchrony with vocalization, and that the MOC system plays a role in causing suppression. Since suppression can beg in prior to vocalization, this may be a synkinetic effect, mediated by neural outflow to the ear in synchrony with neural outflow to the mid dle ear muscles and the muscles used for vocalization. (C) 1998 Elsevi er Science B.V. All rights reserved.