Visuospatial orientation: Differential effects of head and body positions.

To orientate in space, the brain must integrate sensory information that encodes the position of the body with the visual cues from the surrounding environment. In this process, the extent of reliance on visual information is known as the visual dependence. Here, we asked whether the relative positions of the head and body can modulate such visual dependence (VD). We used the effect of optokinetic stimulation (30°/s) on subjective visual vertical (SVV) to quantify VD as the average optokinetic-induced SVV bias in clockwise and counter-clockwise directions. The VD bias was measured in eight subjects with a head-on-body tilt (HBT) where only the head was tilted on the body, and also with a whole-body tilt (WBT) where the head and body were tilted together. The VD bias with HBT of 20° was in the same direction of the head tilt position (left tilt VD -1.35 ± 0.1.2°; right VD 1.60 ± 0.9°), whereas the VD bias with WBT of 20° was in a direction away from the body tilt position (left tilt VD 2.5 ± 1.1°; right tilt VD -2.1 ± 0.9°). These findings show differential effects of relative head and body positions on visual cue integration, a process which could facilitate optimal interaction with the surrounding environment for spatial orientation.