Electrical stimulation of the auditory nerve: II. Effect of stimulus waveshape on single fibre response properties

To investigate the generation of action potentials by electrical stimulation we studied the response of auditory nerve fibres (ANFs) to a variety of stimulus waveforms. Current pulses were presented to longitudinal bipolar scala tympani electrodes implanted in normal and deafened cochleae. Capacitively coupled monophasic current pulses evoked single ANF responses that were more sensitive to one phase (the 'excitatory' phase) than the other. Anodic pulses produced a significantly shorter mean latency compared with cathodic pulses, indicating that their site for spike initiation is located more centrally along the ANF. The fine temporal structure of ANF responses to biphasic pulses appeared similar to that evoked by monophasic pulses. An excitatory monophasic pulse evoked a significantly lower threshold than a biphasic current pulse having the same polarity and duration leading phase, i.e. the addition of a second phase leads to an increase in threshold. Increasing the temporal separation of the two phases of a biphasic pulse resulted in a moderate reduction in threshold which approached that of an excitatory monophasic pulse for interphase gaps > 100 micros. Greater threshold reductions were observed with narrower current pulses. There was a systematic reduction in threshold with increasing pulse width for biphasic current pulses, reflecting the general charge-dependent properties of ANFs for narrow pulse widths. Chopped biphasic current pulses, which uniformly delivered multiple packets of charge (2 x 30 micros, 3 x 20 micros or 6 x 10 micros) with the same polarity over a 120 micros period, followed by a similar series in the reverse polarity, demonstrated the ability of the neural membrane to integrate sub-threshold packets of charge to achieve depolarisation. Moreover, thresholds for these current pulses were approximately 1.5 dB lower than 60 micros/phase biphasic current pulses with no interphase gap. Finally, stimulation using charge-balanced triphasic and asymmetric current pulses produced systematic changes in threshold and latency consistent with the charge-dependent properties of ANFs. These findings provide insight into the mechanisms underlying the generation of action potentials using electrical stimuli. Moreover, a number of these novel stimuli may have potential clinical application.