Auditory midbrain implant (AMI)

The auditory midbrain implant targets the next station up, the inferior colliculus (IC), where the tonotopic map is arranged in orderly laminae that a penetrating shank array can, in principle, access more systematically than a surface ABI can reach the cochlear nucleus. The AMI was developed largely for NF2 patients who do poorly with an ABI, on the hypothesis that a deeper, better-organized, penetrating target could yield finer place coding.

First-in-human results (single-shank AMI) were disappointing: recipients gained loudness percepts and some speech-tracking and lip-reading benefit but generally not the hoped-for open-set speech, and pitch percepts did not map cleanly onto the IC's frequency laminae — reflecting the IC's complex, not-purely-tonotopic organization and the difficulty of placing a single penetrating array correctly. Multi-shank designs and revised targeting are under investigation. The AMI (and penetrating ABI, PABI) illustrate a general lesson of central auditory prosthetics: moving up the pathway trades surgical access and safety against increasingly transformed, harder-to-drive neural representations.