Overlap of saccadic and pursuit eye movement systems in the brain stem reticular formation.

Recent physiological studies have suggested that there are several sites of interaction between the neural pathways that control saccadic eye movements and those that control visual pursuit movements. To address the question of saccade/pursuit interaction from a neuroanatomical point of view, we have studied the connections from the smooth and saccadic eye movement subregions of the frontal eye field (FEFsem and FEFsac, respectively) to the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) in four Cebus apella monkeys. The riMLF has traditionally been considered to be a premotor center for vertical saccadic eye movements on the basis of single neuron recording experiments, microstimulation experiments, and surgical or chemical lesion experiments. We localized the functional subregions of the FEF with the use of low-threshold (≤50 μA) intracortical microstimulation. Biotinylated dextran amine or lectin from triticum vulgaris (wheat germ), peroxidase labeled, was placed into these functionally defined subregions to label anterogradely the terminals of axons that originated in the FEF. Our results demonstrate that both the FEFsem and FEFsac send direct projections to the ipsilateral riMLF. The distribution and density of labeling from the FEFsem are comparable to those from the FEFsac. The direct FEFsem-to-riMLF projection suggests a possible role of the riMLF in smooth pursuit eye movements and supports the hypothesis that there is interaction between the saccadic and pursuit subsystems at the brain stem level.