Independent speed-tuned global-motion systems

Several experiments were conducted to investigate the role of speed in global-motion processing; the extraction of the direction of motion of a small subset of coherently-moving (signal) dots in a stimulus in which the other (noise) dots move in random directions. The specific aim of the experiments was to determine whether multiple speed-tuned global-motion systems exist. The results of these experiments are: (1) when the signal dots were chosen from a group of dots moving at 1.2 degrees s(-1), the speed of additional-noise dots had to be below 4.8 degrees s(-1) for them to affect global-motion extraction; (2) the addition of static dots did not impair the extraction of a global-motion signal carried by dots moving at 1.2 degrees s(-1) (3) noise dots moving at 1.2 degrees s(-1) impaired the extraction of a global-motion signal from dots moving at 10.8 degrees s(-1), though not to the same extent as dots moving at a higher speed; and (4) these results were dependent upon speed, not spatial-step size or luminance contrast. These results are interpreted as indicating that global-motion extraction occurs within at least two independent speed tuned systems. One of these systems is sensitive to high speeds and the other to low speeds. (C) 1998 Elsevier Science Ltd. All rights reserved.