Motion discrimination space is conventionally categorized into motion detection, speed discrimination, and direction discrimination tasks. But an ideal observer uses a unitary motion mechanism that is affected only by the noise level and the difference in speed (or displacement) between two stimuli. We tested whether human performance in the various motion tasks showed the working of a unitary mechanism or the combined outputs of more than one mechanism. We examined the whole motion discrimination space, using random dots that underwent a sudden jump or displacement. The discriminability was measured as a function of the standard and comparison displacements. Both the ideal observer model and a nonideal observer model that contains additive internal noise predict a planar response surface. When the dot motion was noiseless, the planar surface fitted well except for much higher than expected sensitivity for motion detection. This is consistent with a purely temporal mechanism that uses flicker or a purely spatial mechanism that uses the length of time-averaged streaks. It is also consistent with a Weber’s law device. When motion noise was added to the displays, the planar response surface again fitted well, although the residuals showed the presence of a speed energy mechanism. We conclude that a unitary motion mechanism exists (nonideal observer model), although its performance may be supplemented by other mechanisms whose main impact is on discrimination of speeds near zero.
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