Postural illusions experienced during Z-axis recumbent rotation and their dependence upon somatosensory stimulation of the body surface.

A blindfolded recumbent subject experiences a variety of postural illusions when rotated about his Z axis. Initially, during the acceleratory phase of rotation turning about his Z axis is experienced; but, as rotary velocity increases, a spiraling of the body outward in the direction opposite to true rotation is experienced as well. Above 15-20 rpm, only orbital motion of the body is experienced, with the subject feeling that he is always facing in the same direction. One cycle of the apparent orbit is completed each time the subject actually rotates 360 degrees. The reverse sequence of illusory motion is experienced during deceleration. The illusory motion all subjects experience during Z-axis recumbent rotation is shown to depend upon the touch- and pressure-stimulation of the body surface generated by contact forces of support.