Abstract
Although studies have measured sensitivity to visual acceleration and deceleration as a function of motion direction, an unanswered question is whether our perception of acceleration/deceleration is biased depending on motion direction. We tested the hypothesis that objects moving downwards were perceived to accelerate less and/or decelerate more than objects moving upwards with the same acceleration/deceleration, on the grounds that we have adapted to the more commonly experienced downward acceleration. Observers viewed an image of a ball moving from one side of a room to the other in one of four directions - up, down, left, right - in an image of a square room with a floor, ceiling, wallpapered walls, a door, human figures etc. On each trial participants indicated whether they perceived the ball to accelerate or decelerate. The ball moved at one of two average speeds, taking either 1.0s or 1.5s to traverse the room, and there were several values of acceleration and deceleration. Psychometric functions were fitted to the proportion of perceived accelerations as a function of physical acceleration/deceleration. The point-of-perceived-constant-velocity, or PPCV, was estimated as the physical acceleration/deceleration corresponding to the point at which acceleration is perceived 50% of the time. Our hypothesis predicted that for participants to perceive a downwards moving object as moving at constant speed it needed to accelerate more or decelerate less than that of an upward moving object. Data from 11 participants showed a significant bias in PPCVs in the predicted direction. No significant differences in bias were found between rightward and leftward moving objects, and no significant differences were found for any pair of directions in the slopes of the psychometric functions i.e., in sensitivities. We conclude that downwards acceleration is more likely to be identified as constant velocity than upward acceleration.