The vestibular evoked response to linear, alternating, acceleration pulses without acoustic masking as a parameter of vestibular function

In this study, short latency vestibular evoked potentials (VsEPs) were recorded in five guinea pigs in response to alternating linear acceleration pulses with and without acoustic masking. A steel bolt was implanted in the skull and coupled to a shaker. Linear acceleration pulses (n = 400) in upward, downward or alternating directions were given, with a peak acceleration of 4g after 0.5 msec. Tests were repeated with acoustic masking, after modiolus destruction and after application of KCl in the vestibule. Stimuli of the vestibular nerve were recorded with a platinum electrode in the bony facial nerve canal in the bulla. Unilateral linear acceleration showed a shallow plateau at 0.5 msec, which disappeared with alternating acceleration impulses and after modiolus destruction. Therefore all further tests were done with alternating impulses. After a latency time of 0.8 msec a multiwave response was seen, with a first positive peak P1 at 1.16 ms. These were followed by other positive and negative peaks (N1, P2, N2, P3, N3). With the elimination of cochlear influences by using acoustic masking, P1 remained stable, while subsequent peaks were altered or eliminated. After modiolus destruction, the P1 peak remained, although with a smaller amplitude due to vestibular damage. After application of a saturated KCl solution in the vestibule all responses, including P1, disappeared, thus confirming the vestibular origin of these responses. We conclude that the onset latency of the VsEP and the peak latency and level of the first positive peak P1 in response to alternating linear acceleration pulses without acoustic masking, measured in the facial canal, are good and stable parameters of vestibular function in guinea pigs.