VESTIBULOOCULAR REFLEX GAIN AS A MEASURE OF VESTIBULAR FUNCTION IN GUINEA-PIGS WHILE IN A RECOMPRESSION CHAMBER - APPARATUS DESIGN AND EFFECTS OF NITROGEN NARCOSIS

There are several mechanisms whereby alteration of barometric pressure can produce vertigo in divers or aviators. Development of a reliable measure of vestibular function in an animal model is the first require ment for further study of these mechanisms. This report presents the d evelopment of a rotatory table device capable of evoking the vestibulo -ocular reflex (VOR) of a guinea pig while in a hyperbaric chamber. To assess the reproducibility of this response, eight animals were monit ored by electronystagmography during rotations at three table velociti es (62.4, 83.3, and 100 degrees/s). Two test sessions were performed o n each animal with a 6-hour interval between sessions. The VOR gain wa s calculated by dividing the average peak velocity of the slow phase c omponent of the nystagmus by the peak stimulus velocity. At least eigh t observations per test speed were averaged; calibration of eye moveme nt was performed prior to each session by forced ocular abduction. Mul tifactorial analysis of variance revealed no significant differences ( p > .05) between the differing rotation speeds nor between test sessio ns for individual animals. However, there was a significant difference in VOR gain between animals (p < .002). The VOR gain was then measure d, using the same techniques, in another group of seven animals before , during, and after an air dive to the equivalent of 200 feet of seawa ter (7.06 atmospheres absolute) to assess the effects of nitrogen narc osis. Pre- and post-dive VOR gains were significantly greater than tho se measured at depth (p < .05). These results are consistent with the slow processing model of nitrogen narcosis and the controversial theor y that central nervous system depressants decrease the VOR gain. The r esults also demonstrate the ability of this inexpensive apparatus to p rovide a sensitive measure of pressure-induced changes of vestibular f unction in guinea pigs.