DERIVED WHOLE NERVE ACTION POTENTIALS IN RESPONSE TO LOW-FREQUENCY STIMULI

Whole nerve action potentia Is were recorded from the round window of a single cat in an effort to obtain responses from the apical region of the cochlea. A 500-Hz signal was either 2 or 10 msec in duration, and was either presented at a low or high intensity. A high-pass masking noise, with cut-offfrequencies of 1000, 3000, and 6000 Hz, was used in an attempt to eliminate activity from different frequency regions of the basilar membrane. The results suggest that particularly when a low-level, JO-msec duration signal was used, activity from the apical region predominated the response pattern. INTRODUCTION There has been considerable interest in using electrocochleography and brain-stem recordings to assess the integrity of the human auditory system, especially when conventional audiometric techniques are unsuccessful. However, the frequency specificity of these methods has been questioned (Davis, 1976). In particular, it is difficult to obtain electrical responses from low-frequency regions of the cochlea without stimulating high-frequency regions as well. Placing an electrode on the round window allows us to measure the whole-nerve actionpotential (AP), which consists of a summation of stimulus-evoked electrical activity. When high-frequency signals or clicks are used as stimuli, the traveling wave and resulting electrical activity are confined to the base of the basiIar membrane (Teas, Eldredge, Davis, 1962). When a low-frequency sinusoid is presented to the cochlea, a traveling wave begins at the base and reaches a maximum near the apex (Bekesy, 1960). Particularly at high intensities, the entire length of the basilar membrane is set in motion. This is congruent with single fiber recordings from animal auditory nerves which show that fibers with high characteristic frequencies (originating from the base) also respond to low frequency stimuli (Kiang and Moxon, 1974). One would expect that the low-frequency evoked AP reflects activity from the entire basilar membrane. The importance of obtaining low-frequency information has prompted recent authors to devise methods to accompJlsh this task. We shall use a high-pass masking paradigm. first employed by Teas et al. (1962). With this method, AP responses are obtained in the presence of high-pass masking noises of different cut-off frequencies. Suppose we obtain AP's in the presence of two high-pass noises, with 1000 and 3000-Hz cut-off frequencies. The AP obtained with the 1000-Hz high-pass noise reflects activity from regions representing frequencies less than 1000 H z. When the 3000-H z high-pass noise is used, the AP reflects activity from regions representing frequencies less than 3000 Hz. If we subtract, point by point, the AP waveform obtained in the presence of the IOOO-Hz high-pass noise from the AP obtained with the 3000-Hz high-pass noise, the resulting waveform should be representative offrequency regions between 1000 and 3000 H z. The underlying assumption is that the high-pass noise has eliminated synchronous activity from frequency regions higher than the cut-off frequency. We shall examine these derived waveforms in a cat by measuring the AP to 500-Hz signals.