Motion interference in speed discrimination

Human speed discrimination can be degraded by additional stimuli in close spatial and temporal proximity to the designated test target. In these experiments, observers judged the relative asynchrony between a pair of briefly flashed dots: speed discrimination for two-dot apparent motion. The addition of two irrelevant (interfering) flashed dots to the stimulus, which produces accelerating apparent motion, impaired speed discrimination. We call this impairment motion interference; adjacent stimuli are not processed independently by the motion system. Motion interference is time selective; interfering dots simultaneous with the target dots do not impair speed discrimination, nor do interfering dots that precede or follow the target by 200 msec or more. Motion interference was observed even when the interfering dots were as far away as 1 deg from the test pair. Similar effects were observed with a smoothly moving test target and with interfering stimuli composed only of high spatial frequencies. A multiple-independent-channel model containing several parallel motion-energy detectors with different receptive-field sizes is considered and rejected. We conclude that speed discrimination depends on a time-selective combination of local motion signals from many detectors. These aggregate detectors combine information from local subunits, degrading information about acceleration.