  Vision is known to improve human postural responses to external
perturbations. This study investigates the role of vision for the responses to
continuous pseudorandom support surface translations in the body sagittal plane
in three visual conditions: with the eyes closed (EC), in stroboscopic
illumination (EO/SI; only visual position information) and with eyes open in
continuous illumination (EO/CI; position and velocity information) with the
room as static visual scene (or the interior of a moving cabin, in some of the
trials). In the frequency spectrum of the translation stimulus we distinguished
on the basis of the response patterns between a low-frequency, mid-frequency,
and high-frequency range (LFR: 0.0165-0.14 Hz; MFR: 0.15-0.57 Hz; HFR:
0.58-2.46 Hz). With EC, subjects' mean sway response gain was very low in the
LFR. On average it increased with EO/SI (although not to a significant degree p
= 0.078) and more so with EO/CI (p < 10<sup>-6</sup>). In contrast, the average
gain in the MFR decreased from EC to EO/SI (although not to a significant
degree, p = 0.548) and further to EO/CI (p = 0.0002). In the HFR, all three
visual conditions produced, similarly, high gain levels. A single inverted
pendulum (SIP) model controlling center of mass (COM) balancing about the ankle
joints formally described the EC response as being strongly shaped by a
resonance phenomenon arising primarily from the control's proprioceptive
feedback loop. The effect of adding visual information in these simulations
lies in a reduction of the resonance, similar as in the experiments. Extending
the model to a double inverted pendulum (DIP) suggested in addition a
biomechanical damping effective from trunk sway in the hip joints on the
resonance.
