No Evidence for a Vestibular Contribution to Object Motion Prediction

Abstract

Humans can predict an objects motion better if its movements are consistent with gravity. Here we investigate whether this may be due to an internalized strong Earth gravity prior or to vestibular cues reporting instantaneous information about gravity. These two directions can be separated using virtual reality by providing strong visual cues to the direction of up which may or may not be aligned with true gravity. Participants were presented with a ball travelling on a parabola path simulated with either downward acceleration created by simulated Earths gravity (1g) or inverted gravity (-1g) resulting in the ball curving upwards. In both types of trial, the ball disappeared at between 57.5% and 75% of its full trajectory - after it had started its descent in the case of 1g or ascent in the case of -1g. Participants pressed a mouse button when they judged the ball to have got back to the height at which it was launched. Participants were either standing or supine. There were no differences in the estimated time to reach the indicated level between the 1g and -1g simulations nor between lying and sitting.A control experiment showed that reaction times were also not significantly different while lying supine versus while upright. Fiinally, to isolate a vestibular component we compared performance during disruptive galvanic vestibular stimulation (dGVS) or during sham stimulation. As when lying supine, the perceived time for the ball to reach the target height was not significantly different between dGVS and sham stimulation. Overall, participants were - surprisingly - no better at anticipating 1g motion compared to -1g motion, and we also dont find a significantly influence of the vestibular system more generally.