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. Howev er, 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 bipha sic stimuli to better understand the inherent tradeoffs of these stimuli. U sing guinea pig and cat animal models, we compared the auditory nerve respo nses to both 40 mus monophasic and 30 mus/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 auditor y nerve fiber. In all cases, our stimuli were cathodic monophasic and catho dic-first biphasic pulses. As expected, monophasic stimuli provided lower t hresholds relative to biphasic stimuli. They also evoked responses with rel atively longer latencies. We also examined responses to charge-balanced bip hasic pulses composed of two phases of differing duration (i.e. pseudomonop hasic stimuli). The first phase was fixed at 40 mus, while the second phase was systematically varied from 40 to 4000 Irs. With a relatively long seco nd phase, we hypothesized that these stimuli would provide some of the bene ficial 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 mu s. Consideration of single-fiber data and computer simulations suggest that these results are consistent with the neural membrane acting as a leaky in tegrator. The computer simulations also suggest that the integrative proper ties at least partially account for the difference between our monophasic-b iphasic results and previously published data. Our results apply to cathodi c-leading stimuli; due to differing patterns of membrane depolarization, th ey may not be applicable to situations using anodic-leading stimuli. Finall y, we observed differences between the guinea pig and cat response patterns . Compared to cats, guinea pigs produced smaller monophasic vs. biphasic th reshold differences. This interspecies disparity may be due to differences in cochlear anatomy. (C) 2001 Elsevier Science B.V. All rights reserved.