Contribution of ankle, knee, and hip joints to the perception threshold for support surface rotation

The purpose of the present experiment was to investigate the extent to whic h subjects can perceive, at very slow velocities, an angular rotation of th e support surface about the medio-lateral axis of the ankle, knee, hip, or neck joint when visual cues are not available. Subjects were passively disp laced on a slowly rotating platform at .01, .03, and .05 deg/sec. The subje cts' task was to detect movements of the platform in four different postura l conditions allowing body oscillations about the ankle, knee, hip, or neck joint. In Experiment 1, subjects had to detect backward and forward rotati on (pitching). In Experiment 2, they had to detect left and right rotations of the platform (rolling). In Experiment 3, subjects had to detect both ba ckward/forward and left/right rotations of the platform, with the body fixe d and the head either fixed or free to move. Overall, when the body was fre e to oscillate about the ankle, knee, or hip joints, a similar threshold fo r movement perception was observed. This threshold was lower for rolling th an for pitching. Interestingly, in these postural conditions, an unconsciou s compensation in the direction opposite to the platform rotation was obser ved on most trials. The threshold for movement perception was much higher w hen the head was the only segment free to oscillate about the neck joint. T hese results suggest that, in static conditions, the otoliths are poor dete ctors of the direction of gravity forces. They also suggest that accurate p erception of body orientation is improved when proprioceptive information c an be dynamically integrated.