Shifts in the population response in the middle temporal visual area parallel perceptual and motor illusions produced by apparent motion

We recorded behavioral, perceptual, and neural responses to targets that pr ovided apparent visual motion consisting of a sequence of stationary flashe s. Increasing the flash separation degrades the quality of motion, but for some separations evoked larger smooth pursuit responses from both humans an d monkeys than did smooth motion. The same flash separations also produced an increase in perceived speed in humans. Recordings from single neurons in the middle temporal visual area (MT) of awake monkeys revealed a potential basis for the illusion in the population response. Apparent motion produce d diminished neural responses relative to smooth motion. However, neurons w ith slow preferred speeds were more affected than were those with fast pref erred speeds. Increasing the flash separation thus caused the population re sponse to become diminished in amplitude and to shift so that the most acti ve neurons had higher preferred speeds. The entire constellation of effects of apparent motion on the magnitude and latency of the initial pursuit res ponse was accounted for if the MT population response was decoded by (1) cr eating an opponent motion signal for each neuron by treating its preferred and opposite direction responses as those of a pair of oppositely tuned neu rons and (2) computing the vector average of these opponent motion signals. Other ways of decoding the population response recorded in MT failed to ac count for one or more aspects of behavior. We conclude that the effects of apparent motion on both pursuit and perception can be accounted for if targ et speed is estimated from the MT population response by a neural computati on that implements a vector average based on opponent motion.