Poor speed discrimination suggests that there is no specialized speed mechanism for cyclopean motion

Luminance-defined and stereo-defined (cyclopean) motion share some common properties, suggesting that the two forms of motion may be detected by similar mechanisms. For luminance-defined motion there are at least two levels of processing: direction is detected and then speed is thought to be extracted by a specialized processing mechanism at a higher level. Here, we tested whether there is also a specialized speed processing mechanism for stereo-defined motion. Speed discrimination thresholds were compared for stimuli containing only stereo-defined motion, and stimuli that contained both stereo-defined and luminance-defined motion. When the stimulus contained luminance-defined motion, increment thresholds were around 0.05-0.1. For stereo-defined motion, increment thresholds were never better than 0.3. By careful analysis, it was possible to test what cues were being used to solve the speed discrimination task. Results were consistent with observers responding to distance cues rather than to speed for stereo-defined motion, suggesting that there is no specialized mechanism for processing the speed of stereo-defined motion.