Multi-unit recording from regenerated bullfrog eighth nerve using implantable silicon-substrate microelectrodes

Multi-microelectrode silicon devices were developed for extracellular recording from multiple axons in regenerated eighth cranial nerves of American bullfrogs. Each includes a photolithographically defined array of holes and adjacent metal microelectrodes. A device is implanted within a transected eighth nerve; regenerating fibers grow through the holes en route to the brainstem. Multiple spike trains were recorded from two animals at up to 21 weeks after implantation. Single units were tracked for over 8 h. Some responded to sound with tuning typical of fibers innervating the amphibian and basilar papillae. Units of vestibular origin also were recorded. Action potentials were 30-140 μV P-P amid noise of 5-10 μV RMS, an adequate signal-to-noise ratio for spike detection and sorting. Histology confirmed that bundles of myelinated fibers grew through holes near electrodes that recorded activity. The implantation success rate was low, due to surgical morbidity, device extrusion, and lack of nerve regeneration through some devices. Future designs will address these issues and incorporate transistor amplifiers on devices to increase signal-to-noise ratios. The potential of implanted silicon devices to simultaneously record from many axons offers an opportunity for multicellular studies of auditory, vestibular and seismic signal processing in the vertebrate inner ear.