Translation and rotation are detected by different patterns of optic flow and by different divisions of the vestibular system. A given movement (eg. yaw rotation or up/down translation) involves different sensors depending on the orientation of the movement with respect to gravity. Here we assess the contribution of these different sense systems to the "whole system" response to self motion. Our subjects' task was to distinguish self produced from external visual motion during rotation around the yaw, pitch and roll axes and during translation in the x (naso-occipital), y (sideways) and z (up and down) directions. The axis or direction of motion was parallel or orthogonal to the direction of gravity.

Subjects wore a helmet-mounted display whose position was monitorred by a mechanical head tracker with minimal lag. The visual display was modified in response to head movement. The ratio between head and image motion was varied randomly using the method of constant stimuli. Subjects indicated whether the display appeared earth-stationary or not.

For both rotation and translation there was a large range of ratios that was tolerated as perceptually stable. The ratio most likely to be accepted as stable corresponded to visual motion being faster than head motion. For rotation there were no consistent differences between yaw, pitch or roll axes and the orientation of the axis relative to gravity also had no effect. For translation motion in the x direction was on average matched with less visual motion than y or z motion. Although there was no consistent effect of whether motion was parallel or orthogonal to gravity, posture, relative to gravity, did have an effect.

Sponsorred by CRESTech and NSERC.