FIG. 4.

A comparison of cat ANF rate-level functions (A) and stimulus levels used for human prosthetic stimulation (B) is made by the use of ECAP data obtained from both species. The top graph shows rate-level functions for all ANFs based on spike rates obtained in the first 50-ms epoch and levels referenced to each cat's ECAP threshold (see text). The boundaries of the four “R” rate categories used in this paper are indicated by the horizontal dashed lines. The bottom histogram shows the distribution of maximum comfort (or “C”) levels for 43 human ears implanted with the Nucleus 24 M device, with each ear providing an average of 4.9 data from different stimulus electrodes of each implanted array. The histogram indicates that, in most cases, C levels exceed ECAP threshold, while the vertical dashed lines indicate that the upper limit for C levels (6 dB re: ECAP threshold) encompasses the lower range of ANF rate-level functions. However, it is important to note that ANFs within that lower range of levels can achieve high spike rates. This comparison leads to the conclusion that it is likely that human prosthetic stimulation typically evokes activity from low-threshold fibers that, in some cases, can achieve high spike rates that cover all four “R” groups of this study.