Podokinetic after-rotation does not depend on sensory conflict

Humans who have been stepping for 10 min or more about their vertical axis on a counterrotating platform while fixating on a stationary visual scene continue to circle in the same direction when they attempt, thereafter, to step on firm ground in darkness without turning (”podokinetic after-rotation”: PKAR). In the present report, we investigate whether PKAR is due to: (1) a sensory reinterpretation triggered by the conflict between the visual signal of stationarity and the somatosensory message of feet-on-platform rotation, or (2) an adaptation of the somatosensory afferents to prolonged unilateral stimulation irrespective of visual stimulation. Subjects (Ss) circled for 10 min about their vertical axis on an either stationary or counterrotating platform while they were either in darkness, or exposed to an optokinetic stimulus, or to a ”head-fixed” stationary pattern. Thereafter, Ss first stood motionless in darkness for 30 s, allowing vestibular after-effects to decay, and then tried (still without vision) to step in place on the stationary platform without turning while their body rotation was recorded by a potentiometer coupled to the head. All conditions involving podomotor activity without concomitant optokinetic stimulation evoked similar PKAR. With optokinetic stimulation, PKAR became larger, apparently because it was summed with an optokinetically induced after-rotation (oPKAR). This oPKAR could be demonstrated in isolation when Ss were passively rotated in front of the OKN-pattern instead of actively circling. PKAR could not be ”dumped”; it reappeared after 30 s of straight stepping under visual control. We suggest that PKAR is caused by adaptation of the somatosensory channel and not by a sensory conflict. Received: 16 April 1999 / Accepted: 30 June 1999