SIMULATED MICROGRAVITY AND CORTICAL INHIBITION - A STUDY OF THE HEMODYNAMIC-BRAIN INTERACTION

The present study was carried out to determine the inhibitory cortical processes induced by changes in hemodynamics. Previous experiments in humans conducted in our laboratory have shown that there is a close r elationship between posture and delta and theta EEG activity. The most pronounced effects were obtained during the 6 degrees head-down tilt (HDT) position. In space medicine the HDT procedure is very frequently employed to simulate micro-gravity and to determine the neurohormonal counter-regulations evoked by the expansion of central volume. Twenty male subjects spent 23 h in bed in 6 degrees HDT and 23 h in 6 degree s HUT (head-up tilt) positions during which EEG (frontal, central, par ietal, occipital), startle responses, and reaction-times were measured every 2 h (from 10:00 h till 20:00 h). The effects of cardiovascular deconditioning (CD) regularly occurring after HDT were assessed by exa mining orthostatic tolerance and the physical work capacity (bicycle e rgometry). As expected, 23 h HDT led to more pronounced CD than HUT. S pectral power analyses of EEG revealed increases in delta and theta fr equency bands similar to those found during HDT in previous EEG studie s. In addition, subjects responded more slowly (S1-S2 reaction-time ta sk) during HDT as compared with HUT bedrest. The influence of HDT on s tartle response, however, was not in keeping with the initial hypothes is (i.e. dampening of reflex activity). The EEG data and the sensorimo tor performance indicated that the body fluid shift towards the thorac ic cavity induced by HDT resulted in signs of cortical inhibition. In addition to neural mechanisms, other processes must be postulated whic h are closely related to the counter-regulation evoked by the varying body positions.