Is motion processing unitary?

Motion discrimination space is conventionally categorized into motion detec tion, speed discrimination, and direction discrimination tasks. But an idea l observer uses a unitary motion mechanism that is affected only by the noi se 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 me chanism. We examined the whole motion discrimination space, using random do ts that underwent a sudden jump or displacement. The discriminability was m easured as a function of the standard and comparison displacements. Both th e 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 expec ted sensitivity for motion detection. This is consistent with a purely temp oral mechanism that uses flicker or a purely spatial mechanism that uses th e 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 s urface again fitted well, although the residuals showed the presence of a s peed energy mechanism. We conclude that a unitary motion mechanism exists ( nonideal observer model), although its performance may be supplemented by o ther mechanisms whose main impact is on discrimination of speeds near zero. (C) 1999 Optical Society of America [S0740-3232(99)00212-4].