Effect of stimulus level on the temporal response properties of the auditory nerve in cochlear implants
Electrically evoked compound action potentials (ECAPs) have been used to examine temporal response patterns of the auditory nerve in cochlear implant (CI) recipients. ECAP responses to individual pulses in a pulse train vary across stimulation rates for individual CI users. For very slow rates, auditory neurons have ample time to discharge, recover, and respond to each pulse in the train. As the pulse rate increases, an alternating ECAP-amplitude pattern occurs. As the stimulation rate increases further, the alternating pattern eventually ceases and the overall ECAP amplitudes are diminished, yielding a relatively stochastic state that presumably reflects a combination of adaptation, desynchronization, and facilitation across fibers. Because CIs operate over a range of current levels in everyday use, it is important to understand auditory-nerve responses to pulse trains over a range of levels. The effect of stimulus level on ECAP temporal response patterns in human CI users has not been well studied. The first goal of this study was to examine the effect of stimulus level on various aspects of ECAP temporal responses to pulse-train stimuli. Because higher stimulus levels yield more synchronous responses and faster recovery, it was hypothesized that: (1) the maximum alternation would occur at slower rates for lower levels and faster rates at higher levels, (2) the alternation depth at its maximum would be smaller for lower levels, (3) the rate that produces a stochastic state (‘stochastic rate’) would decrease with level, (4) adaptation would be greater for lower levels as a result of slower recovery, and (5) refractory-recovery time constants would be longer (slower) for lower levels, consistent with earlier studies. The second goal of this study was to examine how refractory-recovery time constants relate specifically to maximum alternation and stochastic rate. Data were collected for 12 ears in 10 CI recipients. ECAPs were recorded in response to each of 13 pulses in an equal-amplitude pulse train ranging in rate from 900–3500 pps for three levels (low, medium, high). The results generally supported hypotheses 1–4; there were no significant effects of level on the refractory-recovery time constants (hypothesis 5). When data were pooled across level, there was a significant negative correlation between alternation depth and refractory recovery time. Understanding the effects of stimulus level on auditory-nerve responses may provide further insight into improving the use of objective measures for potentially optimizing speech-processing strategies.