Responses of neurons in the medial superior temporal visual area to apparent motion stimuli in macaque monkeys.

Monkeys fixated a stationary spot during presentation of dot textures that moved in apparent motion defined by the spatial and temporal separations, Deltax and Deltat, between successive flashes of each dot. For each neuron, we assessed the speed tuning for smooth motion (Deltat = 2 or 4 ms) at speeds < or =128 degrees /s and the effect of varying the value of Deltat at speeds of 16 and 32 degrees /s. Many medial superior temporal (MST) neurons, like middle temporal (MT) neurons, were tuned for the speed of smooth motion and showed decreases in firing rate as the value of Deltat increased at a constant speed. A subset of MST neurons, however, showed monotonically increasing firing rates as a function of smooth stimulus speed and responses to apparent motion that paralleled a previously discovered illusion where estimates of target speed increase with the value of Deltat. Opponent firing rate, defined as the difference between responses for motion in the preferred and opposite directions, peaked at values of Deltat that were consistent with the behavioral illusion. Comparison with a new sample of MT neurons recorded with the same stimuli failed to reveal comparable effects. Attempts to map the population responses in MT and MST onto the behavioral illusion of increased speed succeeded by averaging the opponent response across MST neurons, but only by applying vector averaging to determine the preferred speed of the most active MT neurons. We suggest that a vector-averaging computation transforms MT's place code for target speed into the rate code of some MST neurons.