Pronounced overestimation of support surface tilt during stance

A veridical internal notion of the kinematic state of the foot support is essential for postural control. The means by which this is obtained is still a matter of debate. We therefore measured the conscious perception of support tilt during transient anterior-posterior rotations of a motion platform in six healthy subjects, using a psychophysical matching procedure. Furthermore, we evaluated subjects' postural responses (in terms of displacement of subjects' center of mass, COM, and their ankle torque, as represented by the center of foot pressure, COP). The platform tilts were applied in absence of visual and auditory orientation cues. The platform rotations consisted of smoothed position ramps with different dominant frequencies (0.025, 0.05, 0.1, 0.2, 0.4, and 0.8 Hz) and different amplitudes (0.125 degrees, 0.25 degrees, 0.5 degrees, 1 degree, 2 degrees, 4 degrees, and 8 degrees) for the forward and backward directions, which yielded a 6x14 stimulus matrix. The stimuli were repeated five times in a random order. For the matching procedure, subjects tried to maintain an upright body orientation, while trying to orient a light-weight rod, which was attached to a belt around their waists, parallel to the perceived platform surface. We measured the stimulus-evoked angular excursions of the rod and of the subjects' COM as well as the COP shift. We found that the subjects' rod indications overestimated the platform tilts, particularly with small stimulus amplitudes. To characterize the overestimation, we compared the rod indications obtained while subjects stood on the tilting platform, to rod indications in a situation in which they stood next to the platform and tried to match the rod angle to the now visually perceived platform angle. From this comparison, we inferred that the subjects' kinesthetically derived notion of platform tilt overestimates the actual tilt by a factor of approximately 4. The estimates were linearly related to the angle between body (COM) and platform, i.e., to approximately the angle of the ankle joint, a finding which suggests a proprioceptive source of the overestimation. Further analyses supported this view; they showed that the onset latencies of the rod indications could be approximated by a theoretical indication mechanism with a reaction time of about 0.31 s, a velocity threshold of 0.099 degrees/s, and a displacement threshold of 0.12 degrees. These threshold values are well in line with previous work on the leg proprioceptive detection threshold of conscious perception of body sway. We therefore assume that the phenomenon of support tilt overestimation reflects a still unknown mechanism of leg proprioception in postural control.