Multisensory integration in the human vestibular velocity storage mechanism?

To test the hypothesis that auditory inputs can interact with the optokinetically or vestibularly-charged human VSM, we examined the effects of sound on rotational nystagmus and HOKAN. Subjects were rotated at 60 degrees/s, in either clockwise (CW) or counterclockwise (CCV) direction, for a total of 3 min. After 1 min of constant rotation, the optokinetic surround (7 ft dia., 2 degrees stripes at 18 degrees intervals) was illuminated for 60 s and the ensuing HOKAN was recorded for 60 s (standard DCEOG). The chair was stopped 60 s later and the post-rotatory nystagmus (PRN) was recorded. The acoustic stimulus (4/s, 10 ms pulses) was presented to the subject by a loudspeaker in 4 randomized test conditions: (1) fixed to the surround-on during OKN, off during OKAN (2) fixed to the surround-off during OKN, on during OKAN (3) rotating with the subject-on during OKN, off during OKAN (4) rotating with the subject-off during OKN, on during OKAN. In all conditions, the sound was on during per- and post-rotatory nystagmus. Each test was performed on a different day and consisted of 4 randomized control and test trials (CW and CCW). Under none of the above conditions was the rotatory or optokinetic decay affected by the presence of stationary or rotating sound. This absence of alteration of PRN by sound cues contradicts that reported by Kollar et al. (1988). Our evidence that neither HOKAN nor post rotatory nystagmus decay is affected by sound cues supports the conclusion that, under our conditions, multisensory interaction does not occur in the human VSM.