Changes over time in thresholds for electrical stimulation of the cochlea.

The purpose of this paper is to better characterize changes over time that occurred in psychophysical detection thresholds for electrical stimulation of the cochlea. Threshold changes observed in nonhuman primates implanted with cochlear electrode arrays can be divided into at least three types based on the patterns of change over time. Short-term increases and subsequent decreases in threshold were commonly observed during the first months after implantation and were often followed by periods of long-term threshold stability. Long-term slow increases in thresholds and more rapid increases after a period of threshold stability have also been observed. The threshold changes may be divided into at least two classes based on their dependence on the waveforms used for the threshold measurements. Some changes occurred primarily in thresholds for long phase-duration signals while other changes were equal in magnitude (in decibels) for all tested stimuli. This suggests that at least two mechanisms underlay these threshold changes. The observed changes in thresholds have implications for experimental studies of electrical stimulation and for clinical application of auditory prostheses.