Characteristics of detection thresholds and maximum comfortable loudness levels as a function of pulse rate in human cochlear implant users.

The ability of an implanted ear to integrate multiple pulses, as measured by the slopes of detection threshold level (T level) versus pulse rate functions, may reflect cochlear health in the cochlea, as suggested by previous animal studies (Kang et al., 2010; Pfingst et al., 2011). In the current study, we examined the slopes of T level versus pulse rate functions in human subjects with cochlear implants. Typically, T levels decrease as a function of pulse rate, consistent with a multipulse integration mechanism. The magnitudes of the slopes of the T level versus pulse rate functions obtained from the human subjects were comparable to those reported in the animal studies. The slopes varied across stimulation sites, but did not change systematically along the tonotopic axis. This suggests that the slopes are dependent on local conditions near the individual stimulation sites. The characteristics of these functions were also similar to those found in animals in that the slopes for higher pulse rates were steeper than those for the lower pulse rates, consistent with a combined effect of multipulse integration and cumulative partial depolarization mechanisms at rates above 1000 pps. The maximum comfortable loudness level (C level) versus pulse rate functions were also examined to determine the effect of level on the slopes. Slopes of C-level functions were shallower than those for the T-level functions and were not correlated with those of the T-level functions, so the mechanisms underlying these two functions are probably not identical. The slopes of the T- or C-level functions were not dependent on stimulus-current level. Based on these results, we suggest that slopes of T level versus pulse rate functions might be a useful measure for estimating nerve survival in the cochlea in regions close to the stimulation sites.