Evaluation of the vestibular evoked myogenic potential during parabolic flight in humans.

The purpose of this study was to investigate how gravity affects the vestibular evoked myogenic potential (VEMP). Eight healthy subjects (seven men, one woman; age range 19-45 years) participated in experiments in which three different gravity levels [microgravity (MG), normal gravity (NG), and hypergravity (HG)] were imposed during a parabolic flight procedure. The VEMP was evoked in response to an intense mono-aural click while the subjects kept the sternocleidomastoid (SCM) muscle contracted bilaterally. Background electromyographic activity of the SCM during the test was corrected. The p13-n23 amplitude was significantly greater under MG than under NG or HG. There was no difference in p13 latency between the three gravity levels. Possible mechanisms related to this phenomenon are discussed.