Visually-evoked postural responses to small, rapid stimuli in the naïve participant.

Previous work has clearly demonstrated that visual feedback is integrated into the control of postural alignment and orientation, and is suggested to contribute to corrections for unexpected self-motion. Experimentally-generated visual stimuli have typically been oscillating or long duration and analysis has focused on averaged responses. The present study investigated whether a small, rapid unexpected displacement of a visual surround was sufficient to evoke a postural correction and whether the false stimulus was disambiguated on subsequent exposures. In a second group of participants, it was asked whether the provision of light touch feedback disambiguated the visual stimulus. The ramp-and-hold visual stimulus was created by displacing (2.5 cm, 130 mm/s peak velocity) a visual surround either forwards or backwards. Responses were characterized from the anterior-posterior motion of the center of pressure under the feet. All participants in the No Touch cohort responded by swaying in the direction of the visual stimulus on the first trial, preceded by an initial deflection in the opposite direction, with an onset latency of ~ 240 ms. Similar responses, but of markedly smaller amplitude, were observed with the first trial in the Touch cohort. The amplitude of the response decreased on subsequent trials in both cohorts, however the time-varying motion in the center of pressure remained evident in trials 8 to 10. These results indicate that small, rapid visual stimuli are sufficient to induce postural corrections to sensed self-motion. The false-positive self-motion responses are not completely disambiguated by repeated exposure or by additional sensory feedback from touch.