Inferior Colliculus Responses to Multichannel Microstimulation of the Ventral Cochlear Nucleus: Implications for Auditory Brainstem Implants

Abstract

Multichannel techniques were used to assess the frequency specificity of activation in the

central nucleus of the inferior colliculus (CIC) produced by electrical stimulation of localized

regions within the ventral cochlear nucleus (VCN). Data were recorded in response to pure

tones from 141 and 193 multiunit clusters in the rat VCN and the CIC, respectively. Of 141

VCN sites, 126 were individually stimulated while recording responses in the CIC. A variety

of CIC response types were seen with an increase in both electrical and acoustic stimulation

levels. The majority of sites exhibited monotonic rate level types acoustically while spike rate

saturation was achieved predominantly with electrical stimulation. In 20.6% of the 364

characteristic frequency aligned VCN-CIC pairs, the CIC sites did not respond to stimulation.

In 26% of the 193 CIC sites, a high correlation was observed between acoustic tuning and

electrical tuning obtained through VCN stimulation. A high degree of frequency specificity

was found in 58% of the 118 lowest threshold VCN-CIC pairs. This was dependent on

electrode placement within the VCN as a higher degree of frequency specificity was achieved

with stimulation of medial, central and postero-lateral VCN regions than more antero-lateral

regions. Broadness of acoustic tuning in the CIC played a role in frequency-specific

activation. Narrowly tuned CIC sites showed the lowest degree of frequency specificity upon

stimulation of the antero-lateral VCN regions. This data provides significant implications for

Auditory Brainstem Implant electrode placement, current localization, power requirements

and facilitation of information transfer to higher brain centers.