Effects of gravity on the functions of the central nervous system.

The sensitivity of the mammalian central nervous system to gravitational influences involves both direct and indirect factors. Gradual loss of cerebral circulation with increasing acceleration beyond 5 G has been shown to evoke changes in patterns of brain electrical activity, with epileptiform discharges triggered primarily in the hippocampal system of the temporal lobe, and spreading into other brain systems. The relationship of these structures to judgment and discriminative functions is discussed. Complete loss of cerebral circulation is associated with electrical silence in the brain. The possible effects of weightlessness on intracranial fluid distribution are reviewed. Tests of vibrational stimuli in the monkey have disclosed "driving" of electrical brain rhythms at the shaking frequency, particularly in the range from 11 to 15 cycles per second. These effects are unrelated to photic stimulation, and are abolished by anesthesia or death. Tests of discriminative capability at frequencies producing maximum driving have shown increased response latencies and increased errors. The neurophysiological basis for adaptive phenomena in recurrent vestibular stimulation has been found to reside partly in the vestibular nuclei of the medulla, and not to require integrity of connections with higher vestibular centers. The potentiation of responsiveness to vestibular stimulation in the weightless state has been shown to have a basis in the exaggerated responsiveness of medullary vestibular centers following loss of proprioceptive influxes from the spinal cord. The possible contribution of weightlessness to anomalous psychophysiological functions is reviewed. This area presents a major challenge in experimental design, and may require evaluation of such phenomena as distortion of the body image, modified sleep patterns and changes in optimal sleep-work cycles.