Origins of eighth nerve unit response pattern in round window cap recordings

An experimental technique based on a combined electrical and acoustical stimulation of the cochlea (R. Charlet de Sauvage et al., 1983, J. Acoust. Soc. Am. 73, 616-627) allowing to record a close estimation of single unit contribution to the VIIIth nerve CAP has been recently proposed. D.C. Teas et al.'s (1962, J. Acoust. Soc. Am. 24, 1431-1459) theory about this pattern is that, due to conduction time in the internal auditory meatus, the propagated depolarisation partly differentiates in far-field recordings, giving rise to a specifically diphasic pattern. In order to evaluate Teas's hypothesis, 83 unit waveforms collected in 10 guinea pigs are analysed. Several reproducible peaks are identified. Latency and slope measurements are performed on these peaks. This data is processed, after identifying two homologous components which could combine in accordance with Teas's theory. The schematic pattern of a primary waveform is actually determined. Its relative amplitude on the two electrodes and the delay in the meatus are inferred. Results are in good agreement with published data. This is taken as an indirect validation of Teas's hypothesis.