Determinants of orientation in microgravity.

During two parabolic flight campaigns, one with the NASA-KC-135, the second with the ESA Caravelle, human spatial orientation in an altered gravitational environment was studied by measuring the subjective visual vertical (SVV) by means of a luminous line, and by asking the subjects to give a report, with eyes closed, about their orientation to apparent vertical. The inflight data are compared with baseline data measurements of the subjective horizontal body position (SHP) at normogravity (1g) and at 2g. Pertinent theoretical alternatives to modelling subjective static orientation are developed and compared to the data. It turns out that a good fit to the baseline results and a satisfactory prediction of the perceived orientation in microgravity can be obtained if the otolithic output is assumed to be normalized, but that of the somatic gravity sensors is not.