Shift in smooth pursuit initiation and MT and MST neuronal activity under different stimulus conditions

The activity of neurons in extrastriate middle temporal (MT) and medial superior temporal (MST) areas were studied during the initiation of pursuit eye movements in macaque monkeys. The intersecting motion of two stimuli was used to test hypotheses about how these direction- and speed-sensitive neurons contribute to the generation of pursuit. The amplitude and direction of the initial saccade to the target and the initial speed and direction of pursuit were best predicted by a vector-average model of the underlying neuronal activity with relatively short time and spatial separation before a visual pursuit target and a distracter stimulus crossed in the visual field. The resulting eye movements were best described by a winner-take-all model when the time and spatial separation between the two stimuli was increased before the stimuli crossed. Neurons in MT and MST also shifted their activity from that best described by a vector average to a winner-take-all model under the same stimulus conditions. The changes in activity of neurons in both areas were generally similar to each other during these changes in pursuit initiation. Thus a slight alteration in the target motion produced a concurrent shift in both the neuronal processing and the movement execution. We propose that the differences in the oculomotor behavior can be accounted for by shifts in the overlap of active neuronal populations within MT and MST.