Effects of Rectilinear Acceleration and Optokinetic and Caloric Stimulations in Space

Abstract

During the flight of Spacelab 1 the crew performed a number of experiments to explore changes in vestibular function and visual-vestibular interactions on exposure to microgravity. Measurements were made on the threshold for detection of linear oscillation, vestibulo-ocular reflexes elicited by angular and linear movements, oculomotor and posture responses to optokinetic stimulations, and responses to caloric stimulation. Tests were also conducted on the ground, during the 4 months before and on days 1 to 6 after flight. The most significant result was that caloric mystagmus of the same direction as on the earth could also be evoked in the weightless environment.

The Vestibular System

“…However, caloric nystagmus also occurs in astronauts under conditions of weightlessness ( Fig. 34.14), in the same direction as on earth [39]. Therefore, another mechanism must also be involved, because absence of gravitational force is accompanied by absence of thermodynamic streaming of the endolymph.…”

“…This assists the eyeballs in performing compensatory movements, preventing a shift of the visual field on the retina when the body moves, thus maintaining continual visual awareness of the environment. The contribution of the maculae to visual control was investigated in the spacelab with the help of astronauts in weightlessness [34,39,44). On earth the astronauts readily identified a target by means of the vestibulo-ocular reflex, but in space this ability deteriorated by about 20%.…”

Contribution of Endolymphatic Fluid Shift to Caloric Response in Plugged Semicircular Canals

“…Thermoconvective fl ow is considered to be the most plausible mechanism involved. However, there have been reports that the caloric nystagmus is observed even under microgravity and in monkeys with plugged canals [1,2] . The endolymphatic fl uid shift effect [3] and the direct thermal effect on the vestibular receptor [4] have also been proposed as possible mechanisms.…”

Vestibular system physiology and space motion sickness: An introduction.

“…These results are not what one would expect on the basis of the convection current explanation of caloric tests. Convection currents are caused by the movements of different density fluids due to gravity (14).…”