How does the brain coordinate the choice of target between two different motor systems like saccadic and smooth pursuit eye movements? In principle this could be done in parallel – sending a command to choose a target to both systems at once. Or it could be in serial – first choosing a target with the saccadic movement and then sending that command in serial to the pursuit system. In a series of behavioral and physiological studies we have found that the choice of target is sent in serial from the saccadic to the pursuit motor system. The demo above steps you through a series of microstimulation studies that we used to show this.

Gardner, J. L. , and Lisberger, S. G. (2002) Serial linkage of target selection for orienting and tracking eye movements. Nature Neuroscience 5:892-899 DOI <News and Views by Michael N. Shadlen> Abstract

Many natural actions require the coordination of two different kinds of movements. How are targets chosen under these circumstances: do central commands instruct different movement systems in parallel, or does the execution of one movement activate a serial chain that automatically chooses targets for the other movement? We examined a natural eye tracking action that consists of orienting saccades and tracking smooth pursuit eye movements, and found strong physiological evidence for a serial strategy. Monkeys chose freely between two identical spots that appeared at different sites in the visual field and moved in orthogonal directions. If a saccade was evoked to one of the moving targets by microstimulation in either the frontal eye field (FEF) or the superior colliculus (SC), then the same target was automatically chosen for pursuit. Our results imply that the neural signals responsible for saccade execution can also act as an internal command of target choice for other movement systems. pdf

Gardner, J. L. , and Lisberger, S. G. (2001) Linked target selection for saccadic and smooth pursuit eye movements. Journal of Neuroscience 21(6):2075-2084 link Abstract

In natural situations, motor activity must often choose a single target when multiple distractors are present. The present paper asks how primate smooth pursuit eye movements choose targets, by analysis of a natural target-selection task. Monkeys tracked two targets that started 1.5 degrees eccentric and moved in different directions (up, right, down, and left) toward the position of fixation. As expected from previous results, the smooth pursuit before the first saccade reflected a vector average of the responses to the two target motions individually. However, post-saccadic smooth eye velocity showed enhancement that was spatially selective for the motion at the endpoint of the saccade. If the saccade endpoint was close to one of the two targets, creating a targeting saccade, then pursuit was selectively enhanced for the visual motion of that target and suppressed for the other target. If the endpoint landed between the two targets, creating an averaging saccade, then post-saccadic smooth eye velocity also reflected a vector average of the two target motions. Saccades with latencies >200 msec were almost always targeting saccades. However, pursuit did not transition from vector-averaging to target-selecting until the occurrence of a saccade, even when saccade latencies were >300 msec. Thus, our data demonstrate that post-saccadic enhancement of pursuit is spatially selective and that noncued target selection for pursuit is time-locked to the occurrence of a saccade. This raises the possibility that the motor commands for saccades play a causal role, not only in enhancing visuomotor transmission for pursuit but also in choosing a target for pursuit. pdf