The present study investigated the effect of optic flow periodicity on postural sway. Head and center-of-pressure (COP) displacements in response to an oscillating full-field bullseye-and-checkerboard pattern were recorded in six healthy adults. Scene movement was driven by one of five signals: (1) 0.1 Hz sinusoid, (2) 0.3 Hz sinusoid, (3) 0.5 Hz sinusoid, (4) the periodic sum of these three sinusoids (PSUM), or (5) a non-periodic counterpart (NPSUM = 0.1+ pi/10 + 0.5 Hz). Sway response power at the various stimulus frequencies were compared: (1) among the three pure sinusoidal groups; and (2) between the two sum-of-sinusoid groups. Head and COP responses displayed similar spectral content, though sway magnitude was larger for the head. Sway responses to the moving scenes were significantly larger than those observed during quiet stance. Each sinusoidal moving scene evoked a strong response at the stimulus frequency, as well as increased sway at non-stimulus frequencies, primarily below 0.2 Hz. For thesum-of-sinusoids stimuli, both PSUM and NPSUM signals elicited sway responses at each of their component frequencies. The amplitudes of these responses were similar to one another at 0.1 and 0.3 Hz, but significantly different at 0.5 Hz, with PSUM responses on average four times larger than those for NPSUM. These findings indicate that spectrally similar periodic and non-periodic stimuli elicit quantitatively different sway responses. The observed behaviors may be due to postural sensitivity to the predictability of visual motion, or due to other nonlinear and/or time-varying mechanisms in the postural control system.
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