Some aspects of temporal coding for single-channel electrical stimulation of the cochlea

Estimates of the useful frequency range for single-channel electrical stimulation of the cochlea range from 400 to 4000 Hz. Psychophysical studies in single-channel implant patients are relevant not only to the practical problem of designing stimulation strategies, but also to questions of temporal processing of pitch in the normal auditory nervous system. Patients with single-channel extracochlear devices participated in several experiments involving stimuli differing in fine temporal structure. Stochastic pulse trains, in which the probability of pulse delivery (p) for a given cycle was less than 1.0, were readily discriminated from ordinary pulse trains. Frequency discrimination using stochastic pulse trains differing only in fine temporal structure (but identical average pulse rates) was as good as with ordinary pulse trains or sinusoids for P greater than or equal to 0.5, but deteriorated rapidly for P less than 0.5. Discrimination of triangular and trapezoidal waveforms from square waves was surprisingly good: rise-times (for 0 to maximum current) as low as 0.08 ms could be discriminated. Conversely, detection of jitter in pulse trains was almost an order of magnitude worse. The results show that frequency discrimination for single-channel electrical stimulation of the cochlea is based on discrimination of inter-pulse periods, and that pulse rates which would be unnatural for acoustically-evoked VIIIth nerve activity - up to 750 Hz - are more useful for coding mid-range frequencies than low-rate stochastic stimulations of normal VIIIth nerve firing patterns. The waveform discriminations reported would be obscured by low-pass filtering even at 2000 Hz, and probably depend on changes in relative synchrony among an array of VIIIth nerve units with different thresholds. In general, these results support the use of analog coding schemes with relatively large bandwidth.