Abstract
Terrestrial gravity restricts human locomotion to surfaces in which turns involve rotations around the body axis. Because observers are usually upright, one might expect the effects of gravity to induce differences in the processing of vertical versus horizontal turns. Subjects observed visual scenes of bending tunnels, either statically or dynamically, as if they were moving passively through the visual scene and were then asked to reproduce the turn deviation of the tunnel with a trackball. In order to disentangle inertia-related (earth-centered) from vision-related (body-centered) factors, the subjects were either upright or lying on their right side during the observations. Furthermore, the availability of continuous optic flow, geometrical cues, and eye movement were manipulated in three experiments. The results allowed us to characterize the factorsamp;amp;lsquo; contributions as follows. Forward turns (pitch down) with all cues were largely overestimated, as compared with backward t
urns
(pi
tch
up). First, eye movements known to be irregular for vertical stimulation were largely responsible for this asymmetry. Second, geometry-based estimations are, to some extent, asymmetrical. Third, a cognitive effect corresponding to the evaluation of navigability for upward and downward turns was found (i.e., top down influences, such as the fear of falling often reported), which tended to increase the estimation of turns in the direction of gravity.
