Prolonged optokinetic stimulation generates podokinetic after rotation.

Previous studies showed that after prolonged stepping in place on the center of a rotating platform blindfolded subjects could no longer step in place on a firm floor. Instead, they invariably rotated themselves relative to space without perceiving their rotation, a phenomenon termed podokinetic after-rotation (PKAR). We speculated that prolonged optokinetic stimulation (OK) alone may generate similar PKAR. The purpose of this study was to evaluate the effects of prolonged OK on podokinetic (PK) responses. Ten healthy subjects participated in the study. After a control stepping test, they were seated in a circular closed cage and randomly (right or left) exposed to an OK (45 degrees /s) covering the whole visual field for 30 min. After this procedure, blindfolded subjects attempted to step in place on the stationary floor for 30 min. When trying to do so, all subjects turned relative to space without any perception of rotation. The direction of this optokinetically after-rotation (oPKAR) was opposite of that of the direction of OK. Mean peak velocity of oPKAR was 7.8 +/- 4.1 degrees /s, and it was reached after approximately 6 min of stepping. After that, there was a progressive velocity decay, which exhibited a discharging time constant on the order of 42 min toward a final positive asymptote of 3.1 degrees /s. We conclude that OK alone causes oPKAR. Long-term OK probably charges a storage element for podomotor activity with a relatively prolonged time constant. This novel form of neural interaction and adaptive plasticity may have significant implications for the treatment of vestibular and parietal lobe disorders.