Functional imaging and anatomical connections in squirrel monkeys reveal parietal-frontal circuits underlying eye movements.

The posterior parietal cortex (PPC) of squirrel monkeys contains subregions where long trains of intracortical microstimulation evoke complex, behaviorally meaningful movements. Recently, we showed that such stimulation of a part of the PPC in the caudal lateral sulcus (LS) elicits eye movements in these monkeys. Here, we studied the functional and anatomical connections of this oculomotor region we call parietal eye field (PEF) with frontal eye field (FEF) and other cortical regions in 2 squirrel monkeys. We demonstrated these connections with intrinsic optical imaging and injections of anatomical tracers. Optical imaging of frontal cortex during stimulation of the PEF evoked focal functional activation within FEF. Tracing studies confirmed the functional PEF-FEF connections. Moreover, tracer injections revealed PEF connections with other PPC regions on the dorsolateral and medial brain surface, cortex in the caudal LS, and visual and auditory cortical association areas. Subcortical projections of PEF were primarily with superior colliculus, and pontine nuclei as well as nuclei of the dorsal posterior thalamus and caudate. These findings suggest that PEF in squirrel monkey is homologous to lateral intraparietal (LIP) area of macaque, supporting the notion that these brain circuits are organized similarly to mediate ethologically relevant oculomotor behaviors.