Recording of short-latency vestibular evoked potentials induced by acceleration impulses in experimental animals: current status of the method and its applications.

The short-latency vestibular evoked potential (VsEP) induced by angular acceleration impulses (maximal amplitude 30,000 deg/sec2, rise time 2-3 msec) was recorded by skin electrodes in intact cats after various surgical and pharmacological procedures. The normal VsEP consists of 5-8 waves, several microvolts in amplitude, during the first 10 msec. The latency of the first wave (P1) is about 2 msec with respect to the start of head acceleration. The first and the second waves (P1 and P2) were shown to originate from the vestibular nerve and nucleus, respectively. The VsEP disappears permanently after bilateral labyrinthectomy, excision of the 8th nerves, or administration of large doses of gentamicin. Temporary disappearance is caused by anoxia induced for a brief period of time or injection of lidocaine (4%) into the vestibular nerve or into the inner ear after contralateral labyrinthectomy. The VsEPs in the intact cat are similar whether clockwise or counterclockwise stimuli are used and are not affected by changing the position of the head. Unilaterally labyrinthectomized animals, however, show asymmetric response whereby excitatory stimulation of any of the intact semicircular canals evokes prominent P1 and P2 waves which are absent with inhibitory stimulation. The rate and input-output intensity functions of the VsEP are described. The threshold of the VsEP was found to be 1000-1500 deg/sec2. In addition to the neurogenic waves, 2 other potentials appear occasionally in the response: large-amplitude and longer-duration waves with latencies of 8-20 msec, which are of myogenic origin, and smaller waves with shorter latency which probably represent vestibular microphonics and generator potentials. Extracellular recordings of the responses of single second-order neurons in the vestibular nuclei to the same acceleration impulses confirmed that the kinetic vestibular neurons can respond to these stimuli with a latency as short as 3.5 msec. This method for inducing and recording VsEPs has proved to be a powerful tool for the evaluation of vestibular function in experimental animal models.