Electrophysiological Assessment and Validation of the Auditory Midbrain Implant (AMI)

Based on several clinical and neurophysiological findings, the inferior colliculus central nucleus (ICC) is an appropriate target for prosthetic stimulation of the central auditory pathway (Lenarz et al., ARO MWM 2004, Lim and Anderson, Proc. IEEE EMBS Conf. Neural Eng., 2003, 193-196). The AMI (Cochlear Ltd.) is a central auditory prosthesis designed for penetrating microstimulation of the human ICC. In order to investigate the electrophysiological properties of the AMI and validate its ability to provide low-threshold, frequency-specific stimulation, a set of acute experiments based on electrical stimulation of the ICC and multiunit cortical recordings was performed in guinea pigs.

In these experiments, a multichannel Michigan electrode was positioned along the tonotopic gradient of the primary auditory cortex (A1) and the AMI electrode was inserted along the tonotopic gradient of the ICC. The best frequency (BF) for each site in A1 and ICC was determined in response to pure tone acoustic stimuli to ensure both electrodes were aligned along similar frequency ranges. Each ICC site was stimulated with single, biphasic, charge-balanced, monopolar pulses (200 μs/phase) and the corresponding neural activity was recorded in A1. Based on the results, frequency-specific stimulation is achievable using the AMI. Stimulation threshold levels to evoke A1 activity ranged between 10 to 40 μA, which is well within the safe limits suggested for stimulation of the central nervous system (Agnew and McCreery, 1990). These findings suggest that the AMI may provide an alternative approach for hearing restoration in the patients with bilateral retrocochlear deafness.

This work was supported by Cochlear Ltd. Australia, NIH/NIDCD T32 DC00011 and NIH/NIBIB P41 EB2030.