Sense of Body Position in Parabolic Flight a

Under terrestrial conditions we normally take for granted the perceptual continuity and stability of our own orientation and that of our surroundings. For example, if we close our eyes, we expect to continue to experience the same orientations of our bodies. Not only do we expect our sense of the direction of up and down not to be influenced by whether our eyes are open, we also expect to continue to have a sense of up and down if we close our eyes. However, some years ago in the course of studying gravitoinertial force influences on otolithic function, we became aware of the complexity of the sensory and motor factors that determine perceived orientation in a weightless environment.’ This made us realize that perceived orientation under terrestrial conditions which we so take for granted is also the result of quite complex processes. In these early experiments, subjects were being rotated in barbecue spit fashion (see FIGURE 1) at constant velocity and their eye movements and their sense of position within a rotary cycle were being continuously monitored. At constant velocity after the semicircular canal response to acceleration had decayed, their otolith organs were continuously reoriented in relation to the force of gravity. In addition, there was a continuously changing pattern of touch and pressure stimulation of the body surface, with the “down” side of the body being stimulated by the contact forces of support provided by the restraint harness and body mold. If a subject’s eyes were open, he also received visual information about his changing spatial orientation in relation to the test chamber. At constant velocities of rotation above 10-12 rpm (sometimes as low as 6 rpm), subjects did not experience rotation about their z-axis if their eyes were Instead, most subjects experienced themselves to be going through an orbital motion while always facing in the same direction, up or down. The direction of orbital motion was opposite to that of the actual direction of rotation. With full vision, subjects correctly perceived themselves to be turning in the true direction of rotation. FIGURE 2 illustrates the eyes-closed situation. We had the opportunity to test subjects in this situation in parabolic flight maneuvers during which there are periods of increased and decreased force level. The flight profile of the Boeing KC-135 aircraft used in our experiments is illustrated in FIGURE 3. Periods of high force, approximately 1.8 G peak, and free fall, 0 G, each lasting about 25 seconds alternate. During the 1.8-G period, the forces acting on the otolith organs nearly double as do the contact forces of support acting on the subject’s body. In free fall, the net force on the otolith organs is effectively zero, and the contact forces of support are virtually zero as well. Subjects rotating at constant velocity in straight and level flight experienced the