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An experimental study of the electrical stimulation of the guinea pig cochlea is made using an electrode on the round window for both stimulation and recording. The neural response is separated from the electrical artifact with a masking procedure combined with a low amplification, "statistical" averaging method [Charlet de Sauvage et al., Hear. Res. 2, 343-346 (1980)]. The high electrical impedance required for recording physiological responses implies the use of a current pulse generator. Monitoring of evoked potentials from the auditory cortex provides evidence that the effects of electrical stimulation (and of masking noise) are of auditory origin. The electrically evoked round window response is of very short latency (less than 0.2 ms). There is a response threshold for both electrical stimulus and masking noise. The response amplitude varies monotonically as a function of masking noise or electrical stimulus intensity. Experiments with high-pass noise masking suggest that the electrical stimulus is mainly acting on basal fibers. The response latency and waveform are independent of electrical stimulus intensity, repetition rate, masker level, or spectrum. Little intersubject variation is noted. Our experiments (reciprocal forward masking by electrical and acoustic stimuli) suggest that a direct, instantaneous electrical stimulation of the fibers occurs. We believe that this response to electrical stimulation represents the mean unit response of the auditory nerve fibers. This approach may be useful in the separate study of cochlear and VIIIth nerve functions and in the analysis (deconvolution) of the acoustically evoked compound AP.

Measures of cochlear selectivity can be obtained from compound responses using tone-on-tone masking procedures [Dallos and Cheatham, J. Acoust. Soc. Am. 59, 591--597 (1976)]. For the normal guinea pig, cochlear fiber tuning is sharper by a factor of 1.8 than AP tuning curves using simultaneous masking (threshold criterion = 25% N1 amplitude reduction). Anesthesia does not appear to affect AP tuning. In pathological cochleas, AP tuning is broadened by a factor of 2--3, and differences between forward and simultaneous masking curves are reduced. Tuning changes can sometimes occur without threshold elevation. AP tuning curves were obtained from humans during transtympanic electrocochleography. For subjects with near normal thresholds, Q10dB values (simultaneous masking) are approximately 2.3 at 2 kHz, 3.6 at 4 kHz, and 4.7 at 8 kHz. Using the relationship between cochlear fiber tuning and AP tuning in the guinea pig, estimates of human cochlear fiber tuning are 4.2 at 2 kHz, 6.5 at 4 kHz, and 8.5 at 8 kHz. Patients with threshold elevations of more than 30 dB resulting from cochlear deafness have AP tuning curves less sharply tuned by a factor of 2--3.