Chronic Auditory-Nerve Implant Enhances Brainstem Phase Locking to Electric Pulse Trains

Present-day cochlear implants (CIs) can deliver usable speech reception in quiet surroundings. Most CI users, however, show impaired sensitivity to temporal fine structure, which hampers their use of pitch contours and spatial cues to segregate competing talkers. In previous short-term animal studies, we used intraneural (IN) electrodes to stimulate pathways originating from various cochlear turns. Neurons in the inferior colliculus synchronized to apical stimulation at higher rates than to stimulation of the middle-to-basal pathways that are stimulated primarily by today's CIs. Here, we use non-invasive recordings to test the safety and efficacy of up to 6 months of IN implantation and stimulation in cats.

Deafened cats (ten female, two male) were implanted with IN and/or conventional CI electrodes. The IN electrodes were single activated-iridium shanks that targeted apical-turn fibers. Scalp recordings were made from sedated animals at 2-3-week intervals. Auditory brainstem responses to single electrical pulses (eABR) tracked sensitivity and growth of responses. Frequency following responses to electrical pulse trains (eFFR) assessed brainstem temporal transmission at varying pulse rates.

Thresholds for eABR were lower for IN than for CI stimulation, dynamic ranges were wider, and (by inference) spread of activation was more restricted. The eFFR evaluated at latencies comparable to those of inferior-colliculus spikes synchronized at maximum pulse rates averaging > 360 pulses/s for IN compared to ~ 240 pulses/s for CI stimulation. The eABR thresholds and eFFR cutoff rates were stable out to 6 months after implantation.

The results demonstrate the safety and efficacy of chronic IN stimulation in an animal model. In a future clinical device, an IN electrode could augment cochlear-implant performance by enhancing temporal acuity, thereby improving speech reception amid competing sounds.