PERCEPTION OF DIRECTION OF VISUAL-MOTION .1. INFLUENCE OF ANGULAR BODY ACCELERATION AND TILT

We investigated, psychophysically, the influence of body rotation on v isual motion direction thresholds for both upright sitting and tilted observers. Four angular accelerations (0, 20, 40 and 60 degrees/s(2)) were combined with 3 concurrent backward-tilt positions (0, 45 and 90 degrees). This led to combined stimulation of the semicircular canals and otoliths. Vestibular stimulation was combined with a visual motion stimulus. Random-dot kinematograms in which varying percentages of pi xels coherently moving to the left were presented upon a background of otherwise randomly moving pixels (random walk). The smallest percenta ge of coherently moving pixels leading to a clear perception of motion direction represented as the perceptual threshold. Angular accelerati ons about the longitudinal body axis significantly increased motion-di rection thresholds. Concurrent backward tilt did not influence thresho lds. These results differ from those of studies in which translational linear acceleration was employed. Our results support the view that i t is necessary to distinguish between linear acceleration caused by gr avitational forces and that caused by additional linear accelerations about the x-, y-, and z-axes.