Differential sensitivity to rotation measured on potentials evoked by electrical stimulation of the guinea-pig ear

Responses to electrical stimulation of the ear applied between round-window and vertex electrodes were recorded in awake guinea-pigs from the same electrodes or from separate vertex/mastoid subdermal needle electrodes. They were averaged during opposite phases of sinusoidal rotation or before and after constant velocity rotation. In both cases the responses were subtracted from each other and yielded differential per- or post-rotatory "electrovestibular" responses. For comparison, responses were also recorded in the same animals and conditions of electrical stimulation during silence and during silence and during presentation of a broad-band noise. The difference yielded "electroacoustic" responses. In round-window records, electrovestibular and electroacoustic responses presented typical compound nerve action potential patterns. Electrovestibular responses could be recorded for head angular velocities as low as 3 degrees sec-1 at 0.1 Hz. Response amplitude showed a logarithmic relation to head velocity. Changes in amplitude, as a function of time after rotation, were comparable to those reported for vestibular nerve fibre responses. In vertex/mastoid records, electroacoustic responses presented a sequence of peaks similar to the click-evoked auditory brain-stem responses, and electrovestibular responses presented two peaks, presumably representing contributions of central vestibular structures. Such "electrovestibulography" permits the study of an individual ear and makes available to the investigator a large range of vestibular stimulation conditions.