VULNERABILITY AND ADAPTATION OF DISTORTION-PRODUCT OTOACOUSTIC EMISSIONS TO ENDOCOCHLEAR POTENTIAL VARIATION

The endocochlear potential (EP) was reversibly decreased in adult gerb ils by the intraperitoneal injection of furosemide, while cochlear fun ctioning was monitored by measurement of distortion production otoacou stic emissions (DPE) at a range of stimulus intensities. Stimulus freq uencies for DPEs were f1 = 6.8 and f2 = 8 kHz (f2/f1 = 1.18). Emission s monitored in the ear canal and scala media were 2 f1 - f2, 3 f1 - 2 f2, 2 f2 - f1, and f2 - f1. Typically, the EP decreased smoothly, reac hed a minimum one-half hour after injection, then recovered slowly ove r several hours. Emissions at 2 f1 - f2 and 3 f1 - 2 f2 at low stimulu s levels were particularly vulnerable to the change in EP. These vulne rable emissions showed characteristic trajectories in which the amplit udes changed little with the initial EP decrease, then dropped sharply as the EP continued to decrease. However, the amplitudes then began t o recover even before the EP reached minimum, and recovered completely while the EP remained subnormal. The trajectories of the other odd or der emissions were similar, but lacked the abrupt decrease. The variat ion of the even order (f2 - f1) component was completely different, bu t appeared related to the odd order trajectories in a complex fashion. During the initial decrease for the vulnerable components, the decrea se in emission amplitude (in dB) was found to be proportional to the s quare of the change in EP (in mV). The recovery with a subnormal EP wa s interpreted as an adaptive effect with a time constant of about 15 m in.