Responses of inferior colliculus neurons to amplitude-modulated intracochlear electrical pulses in deaf cats

Current cochlear prostheses use amplitude-modulated pulse trains to encode acoustic signals. In this study we examined the responses of inferior colli culus (IC) neurons to sinusoidal amplitude-modulated pulses and compared th e maximum unmodulated pulse rate (Fmax) to which they responded with the ma ximum modulation frequency (maxFm) that they followed. Consistent with prev ious results, responses to unmodulated pulses were all low-pass functions o f pulse rate. Mean Fmax to unmodulated pulses was 104 pulses per second (pp s) and modal Fmax was 60 pps. Above Fmax IC neurons ceased responding excep t for an onset burst at the beginning of the stimulus. However, IC neurons responded to much higher pulse rates when these pulses were amplitude modul ated; 74% were relatively insensitive to carrier rate and responded to all modulated carriers including those exceeding 600 pps. In contrast, the resp onses of these neurons (70%) were low-pass functions of modulation frequenc y, and the remaining (30%) had band-pass functions with a maxFm of 42 and 3 4 Hz, respectively. Thus temporal resolution of IC neurons for modulated fr equencies is significantly lower than that for unmodulated pulses. These tw o measures of temporal resolution (Fmax and maxFm) were uncorrelated (r(2) = 0.101). Several parameters influenced the amplitude and temporal structur e of modulation responses including modulation depth, overall intensity and modulation-to-carrier rate ratio. We observed distortions in unit response s to amplitude-modulated signals when this ratio was 1/4 to 1/6. Since most current cochlear implant speech processors permit ratios that are signific antly greater than this, severe distortion and signal degradation may occur frequently in these devices.