Auditory nerve responses to monophasic and biphasic electric stimuli

Charge-balanced, biphasic stimulus pulses are commonly used in implantable cochlear prostheses as they can be safely delivered to living tissue. However, monophasic stimuli are more efficient (i.e. producing lower thresholds) and likely provide more spatially selective excitation of nerve fibers. We examined the neural responses to monophasic, 'pseudomonophasic', and biphasic stimuli to better understand the inherent tradeoffs of these stimuli. Using guinea pig and cat animal models, we compared the auditory nerve responses to both 40 micros monophasic and 40 micros/phase biphasic stimuli using both electrically evoked compound action potential and single-fiber recordings. We also made comparisons using a computational model of the feline auditory nerve fiber. In all cases, our stimuli were cathodic monophasic and cathodic-first biphasic pulses. As expected, monophasic stimuli provided lower thresholds relative to biphasic stimuli. They also evoked responses with relatively longer latencies. We also examined responses to charge-balanced biphasic pulses composed of two phases of differing duration (i.e. pseudomonophasic stimuli). The first phase was fixed at 40 micros, while the second phase was systematically varied from 40 to 4000 micros. With a relatively long second phase, we hypothesized that these stimuli would provide some of the beneficial features of monophasic stimuli. Both the gross-potential and single-fiber data confirmed this and indicate that the largest incremental effects of changing the second-phase duration occur for durations less than 500 micros. Consideration of single-fiber data and computer simulations suggest that these results are consistent with the neural membrane acting as a leaky integrator. The computer simulations also suggest that the integrative properties at least partially account for the difference between our monophasic-biphasic results and previously published data. Our results apply to cathodic-leading stimuli; due to differing patterns of membrane depolarization, they may not be applicable to situations using anodic-leading stimuli. Finally, we observed differences between the guinea pig and cat response patterns. Compared to cats, guinea pigs produced smaller monophasic vs. biphasic threshold differences. This interspecies disparity may be due to differences in cochlear anatomy.