Perception of body weight and body mass at twice earth-gravity acceleration levels.

On Earth, when standing on two feet, we experience particular patterns of force and pressure on the soles of our feet. As we lift one foot and balance on the other, little or no increase in force or pressure is perceived on the sole of the stance foot even though the contact forces of support on that foot have doubled. The failure to perceive this increase is actually an illusion resulting from the operation of spatial constancy mechanisms serving to preserve feelings of near constant force and pressure on the support surface(s) of the body. On Earth, body weight and body mass are perceived as remaining constant regardless as to whether we are standing on two feet or one and whether we are carrying large objects. In the high force phase(2 g acceleration) of parabolic flight, body weight is perceived as doubling, and a great increase in force is perceived on the soles of our feet if we are standing. When shifting balance from two feet to one, an increase in force of approximately 0.5 mg is felt on the sole of the stance foot. The actual increase in force is 1.0 mg but perceptual compensation is only being made for a 0.5 mg increase such as would be characteristic of shifting balance on Earth; accordingly an additional 0.5 mg (1.0-0.5 mg) residue is perceived. These findings indicate that body weight is dependent on the magnitude of the gravitoinertial forces acting on the body. Variations in the contact forces supporting the body due to passive or active locomotion of the body or to objects that are being carried are monitored and disregarded in computing apparent body weight. When stepping up and down from a low platform during the high force phases of parabolic flight, aberrant motion of the body and the aircraft is experienced. These illusory motions result because the doubling of body weight in a 2 g force background alters the normal relationship between patterns of alpha and gamma activation of antigravity muscles, muscle spindle activity, and the movements of the body. Accordingly, sensory-motor control and perceptual and postural stability on Earth are dependent on an active calibration to a 1 g background force level.