Illusions of postural, visual, and aircraft motion elicited by deep knee in the increased gravitoinertial force phase of parabolic flight. Evidence for dynamic sensory-motor calibration to earth gravity force levels.

Illusions of self motion and aircraft motion are experienced when executing deep knee bends in the high force phases of parabolic flight. The occurrence of such illusions indicates that skeletomotor control is actively calibrated to a 1 g reference level and that departures from this level affect the execution and appreciation of voluntary movements. The origin of the illusory patterns is shown to be understandable in terms of "mismatches" between efferent control signals and expected patterns of associated muscle spindle activity. It is shown, too, that spindle activity is interpreted within an entire context of spatial information about ongoing and intended motion of the body and whether the body is laden.