DISCRIMINATION OF COHERENT MOTION WHEN LOCAL MOTION VARIES IN SPEED AND DIRECTION

Random-dot cinematograms (RDCs) consist of multiple local motion signa ls that can vary in direction and speed. These local motion signals ca n result in coherent motion: the percept of an overall direction and s peed of motion in an RDC. Thresholds were obtained for discriminating differences in the strength of coherent motion. Observers were found t o easily discriminate the strength of coherent motion on the basis of the elements' direction or speed under optimal conditions. However, a nonreciprocal relation was evident when this discrimination was perfor med under nonoptimal conditions. Discrimination of coherent motion tha t was based on the elements' direction was unaffected, but discriminat ion that was based on speed was impaired. Results indicate that humans are sensitive to small differences in coherent motion strength and su ggest that the visual system processes direction and speed information nonreciprocally.