Effects of electrical stimulation of the inferior colliculus on auditory cortical activity: Implications for an auditory midbrain implant (AMI).

Abstract

More than 560 million people worldwide have some form of hearing impairment. For individuals with conductive hearing loss, hearing aids or surgical intervention can restore sufficient auditory function for daily necessities. Cochlear implants (CIs) have been effective for patients with sensorineural hearing loss. However, for individuals without functional auditory nerves, effective remedies are almost nonexistent. The goal of this dissertation was to propose a new solution for hearing restoration based on electrical stimulation of the inferior colliculus central nucleus (ICC), which is a midbrain auditory structure. An auditory midbrain implant (AMI) would be similar in concept to CIs, except that it would stimulate an auditory region beyond the cochlea and would not require a functioning auditory nerve. To investigate potential stimulation effects of an AMI, we performed experiments in ketamine-anesthetized guinea pigs. We electrically stimulated the ICC and recorded the corresponding auditory cortical activity using multi-site silicon-substrate probes. We observed that ICC stimulation achieved low activation thresholds, large dynamic ranges, and localized, frequency-specific activation in the primary auditory cortex (Al). These properties have shown to be important for achieving intelligible speech perception with low energy requirements. The fact that these properties were better than those achieved by CI stimulation suggests that an AMI has potential to improve hearing performance over CIs. We also observed that location of stimulation within the ICC could affect various sound-coding properties, which may correlate with certain perceptual effects (i.e. threshold, level discrimination, frequency discrimination, temporal acuity) and could dictate where to implant an AMI. In addition to activation of neural elements ascending from the ICC to higher auditory regions, stimulation of the ICC also activated descending fibers projecting from Al to the ICC. These projections are thought to modulate spectral processing within the ICC; thus their activation may affect AMI performance. Overall we have demonstrated that ICC stimulation elicits cortical activation properties favorable for an auditory prosthesis, and have provided a detailed description of how stimulation within different ICC regions may affect AMI performance. These findings provide the groundwork for future studies aimed at the development and implementation of the AMI in deaf patients.