Effects of waveform shape on human sensitivity to electrical stimulation of the inner ear

Psychophysical measures of the electrically stimulated human auditory system were obtained for different types of symmetric and asymmetric charge-balanced waveforms. Absolute detection thresholds of biphasic, pseudomonophasic, and 'alternating monophasic' current waveforms delivered by a bipolar intra-cochlear electrode pair were determined for four subjects implanted with the LAURA device. Thresholds for alternating monophasic stimuli, in which anodic and cathodic phases alternated every 5 ms, were 5-8 dB lower than for the biphasic waveforms for all four subjects. For two of the four subjects, thresholds for the pseudomonophasic waveforms were also significantly lower than for the biphasic waveforms. These pseudomonophasic thresholds were greatly affected neither by a 500-μs gap inserted between the two phases, nor by whether the shorter phase preceded or followed the longer one. Loudness balancing measures performed at the most comfortable levels also showed that, for equal loudness, alternating monophasic stimuli required a lower level than biphasic and pseudomonophasic waveforms. For three of the four subjects, the dynamic ranges of the pseudomonophasic (but not alternating monophasic) waveforms were greater than those of the biphasic waveforms. The results demonstrate that thresholds and dynamic ranges of human cochlear implant users can be controlled by manipulating the way in which the charge produced by the initial phase of an electrical pulse is recovered.