Sensitivity to Pulse Rate and Amplitude Modulation in an Animal Model of the Auditory Brainstem Implant (ABI).

The auditory brainstem implant (ABI) is an auditory neuroprosthesis that provides hearing by electrically stimulating the cochlear nucleus (CN) of the brainstem. Our previous study (McInturff et al., 2022) showed that single-pulse stimulation of the dorsal (D)CN subdivision with low levels of current evokes responses that have early latencies, different than the late response patterns observed from stimulation of the ventral (V)CN. How these differing responses encode more complex stimuli, such as pulse trains and amplitude modulated (AM) pulses, has not been explored. Here, we compare responses to pulse train stimulation of the DCN and VCN, and show that VCN responses, measured in the inferior colliculus (IC), have less adaption, higher synchrony, and higher cross-correlation. However, with high-level DCN stimulation, responses become like those to VCN stimulation, supporting our earlier hypothesis that current spreads from electrodes on the DCN to excite neurons located in the VCN. To AM pulses, stimulation of the VCN elicits responses with larger vector strengths and gain values especially in the high-CF portion of the IC. Additional analysis using neural measures of modulation thresholds indicate that these measures are lowest for VCN. Human ABI users with low modulation thresholds, who score best on comprehension tests, may thus have electrode arrays that stimulate the VCN. Overall, the results show that the VCN has superior response characteristics and suggest that it should be the preferred target for ABI electrode arrays in humans.