ARE INNER OR OUTER HAIR-CELLS THE SOURCE OF SUMMATING POTENTIALS RECORDED FROM THE ROUND WINDOW

The relative contribution of inner hair cells (IHCs) and outer hair ce lls (OHCs to the production of the summating potential (SP) is unresol ved in the literature. Since OHCs in the base of the cochlea have been reported to produce little de receptor potential except at very high sound pressure levels [I. J. Russell and P. M. Sellick, J. Physiol (Lo ndon) 284, 261-290 (1983)], the IHCs appear to be the dominant source of the SP. However, results of intracochlear recordings are conflictin g, although deriving from measurements in different turns of the cochl ea [e.g., I. J. Russell and P. M. Sellick, J. Physiol. (London) 284, 2 61-290 (1983) versus P. Dallos and M. A. Cheatham, Sensory Transductio n (1992)]. To determine which type of hair cells is the dominant sourc e of the SP recorded at the round window, we used carboplatin to selec tively destroy IHCs or a combination of IHCs and OHCs in the chinchill a. Related work, using measurements of distortion product otoacoustic emissions and cochlear potentials to assess the functional status of t he OHCs served to validate this animal model [Trautwein et al., Hearin g Res. 96(1-2), 71-82 (1996)]. The SP, cochlear microphonic (CM), and compound action potential (CAP) were recorded from the round window, a nd cochleograms were determined using well-established histological me thods. The results were reasonably distinctive among three groups of e ars-control (from untreated normal chinchillas), IHC-loss (extensive I HC loss with minor or no loss of OHCs), and IHC-OHC loss(total IHC los s plus extensive loss of OHCs over the basal half of the cochlea). Ear s of chinchillas in the IHC loss group had a decrease of over 50% in S P output compared to control ears with the exact reduction depending s omewhat upon the stimulus conditions. Ears in the IHC + OHC loss group , nevertheless, showed even further reduction in SP output which was c learly attributable to destruction of OHCs in the cochlear base. It wa s concluded that, although the IHCs are responsible for a greater cont ribution of de-receptor potential to the SP recorded at the round wind ow, a significant contribution is made by the OHCs, as well. The resul ts suggest, specifically, that the round window ''sees'' SP output rou ghly in inverse proportion to the IHC:OHC. Lastly, the complexity of S P production, as recorded from the round window, precludes a completel y straightforward interpretation of the SP:CAP in clinical ECochG. (C) 1998 Acoustical Society of America.