The radial pattern of basilar membrane motion evoked by electric stimulation of the cochlea

Electric current applied to the cochlea can evoke in situ electromotile res ponses of the organ of Corti. These nonsound-generated responses can give i nsight into the mechanics of the organ as the putative forces produced by o uter hair cells (OHC) must couple to the modes of vibration of the basilar membrane (BM), In this study, platinum-iridium wire electrodes were positio ned into the scala vestibuli and scala tympani of the first cochlear turn i n the guinea pig. Current (1.5 ms rectangular-shaped pulses) was applied to these electrodes at levels to 500 mu A peak. A laser Doppler velocimeter w as used to record the velocity or displacement of the basilar membrane at t he tonotopic 18 kHz place via an opening into the scala tympani of the firs t cochlear turn. Beads were positioned across the width of the BM so that t he velocity or displacement of the BM could be studied in the radial direct ion. It was found that the current pulses evoked linear displacements of up to 2 nm for current levels of 500 CIA (higher levels were damaging to the organ of Corti). The pattern of motion across the width of the BM was such that maximum displacement and velocity was located near the first row of OH Cs and the position of the outer pillar cell footplate. The BM motion was b iphasic in that the zona arcuata moved in the opposite direction to that of the zona pectinata. The results of this study demonstrate that the level o f force produced by OHCs is effective in moving the BM and that the distrib ution of force within the organ of Corti leads to a multimodal motion patte rn of the BM for this experimentally artificial means of evoking OHC motion . (C) 1999 Elsevier Science B.V. Al rights reserved.