Eye movements in response to electric stimulation of the human posterior ampullary nerve.

OBJECTIVES: The concept of a vestibular implant to restore balance, similar to that of a cochlear implant to restore hearing in deaf patients, has been investigated in animal models. It remains to be shown, however, that electric stimulation of the human end organ or its vestibular nerve branches is capable of eliciting a nystagmic eye movement response.
METHODS: Three subjects were given electric stimulation of their posterior ampullary nerve, which was surgically exposed under local anesthesia, by a procedure developed by Gacek. The stimulus was a multiphasic, charge-balanced train of electric pulses.
RESULTS: In all subjects, a pulse repetition rate of 200 pulses per second produced a robust vertical nystagmus without any apparent change in the slow component velocity of the preexisting horizontal nystagmus.
CONCLUSIONS: We have been able to replicate in humans a finding somewhat similar to that of Suzuki and Cohen in monkeys for electric stimulation of the posterior semicircular canal. The similarity is an eye movement with a large, predominant vertical component. The difference is that we saw no horizontal response component, and were not able to measure a torsional response, because we used 2-dimensional video methods. In addition, we found a robust nystagmus with slow component velocities that are large enough to compensate for vertical head movements. This is an essential step in demonstrating the feasibility of a vestibular prosthesis using electric stimulation.