Electrically induced potentiation of eighth nerve responses

Whole-nerve potentials were recorded from a total of 42 anesthetized cats using a 25-micron electrode in the 8th nerve as it exits from the internal auditory meatus, in response to acoustic clicks across an 80-db range. Biphasic electric pulses, 0.5 msec in duration, 100/sec for 5-10 min, at 100 microamps, were delivered by the 8th nerve electrode. Potentiation of responses to acoustic clicks was observed in amplitude, latency, and duration when comparing sets of pre- vs post-stimulation output. High consistency was achieved. These data were corroborated by recording output also from the round window, and by auditorily evoked brainstem far field potentials with a subcutaneous electrode. Control experiments such as ablation of AI, AII, EP, and IT, resection of the inferior colliculus, the crossed olivocochlear bundle, and middle-ear muscles, determined that electric potentiation was a local effect. Furthermore, the usual electric pulses applied to an electrode on the round window had no potentiating effect. Electric stimulation had a significant effect at the .001 level in increasing amplitude of response. Latency of N1 was shortened by 0.2 msec. Potentiation persisted for up to 3 hrs. An unexpected result was potentiation of 8th nerve and brainstem nuclei responses after acoustic stimulation of the contralateral ear; electric stimulation may induce a more active involvement of efferent activity originating in the superior olive or more rostrally. The interpretation of how such changes might affect perception after, for example, long term stimulation of neurons by a prosthesis is difficult to make without behavioral corroboration. The amplitude change of the whole-nerve action potentials are rough indicators of relative loudness. Thus an accentuated loudness may be experienced over time as equivalent intensities tend, as in these cats, to elicit greater response. The long time constancy of the decay, on the other hand, could tend to maintain a consistency of such potentiated responses and thus create no real variability in perception over time. The shortened latency could mean an involvement of higher frequency perception, but how this might be translated into a change in, for example, intelligibility of speech it is not possible to infer at this time.