ISOLATING THE EFFECT OF ONE-DIMENSIONAL MOTION SIGNALS ON THE PERCEIVED DIRECTION OF MOVING 2-DIMENSIONAL OBJECTS

A considerable body of evidence suggests the existence of a two-stage mechanism for the detection of global motion. In the first stage the m otion of elongated contours is extracted and then at the second stage these one-dimensional (1D) motion signals are combined. What is the na ture of the computation carried out in combining the 1D motion signals towards forming a global motion percept? We devised a set of stimuli that differentiate between different possible computations. In particu lar, they distinguish between a velocity-space construction (such as i ntersection of constraints) and a linear computation such as vector av eraging. In addition, these stimuli do not contain two-dimensional (2D ) motion signals such as line intersections, that allow unambiguous de termination of global velocity. Stimuli were presented in uncrossed di sparity relative to the aperture through which they were presented, to reduce the effect of line terminator motion. We found that subjects a re unable to detect the veridical global direction of motion for these stimuli. Instead, they perceive the stimulus pattern to be moving in a direction which reflects the average of its 1D motion components. Ou r results suggest that the visual system is not equipped with a mechan ism implementing a velocity-space computation of global motion.