CORTICOCORTICAL INPUT TO THE SMOOTH AND SACCADIC EYE-MOVEMENT SUBREGIONS OF THE FRONTAL EYE FIELD IN CEBUS MONKEYS

1. The locations and connections of the smooth and saccadic eye moveme nt subregions of the frontal eye field (FEFsem and FEFsac, respectivel y) were investigated in seven hemispheres of five Cebus monkeys. The s upplementary eye field was also mapped in seven hemispheres and the ha nd/arm regions of the dorsal and ventral premotor areas were localized in five hemispheres. Monkeys were immobilized during experiments with Telazol, a dissociative anesthetic agent that has no significant effe ct on microstimulation-induced eye movement parameters (threshold, vel ocity, and duration). The functional subregions were defined with the use of low threshold intracortical microstimulation (current less than or equal to 50 mu A). Then different retrogradely transported fluores cent tracers were placed into these functionally defined regions. 2. T he FEFsac in Cebus monkey is in the same location as the one in macaqu e monkeys, which is in Walker's areas 8a and 45. The FEFsem is located in the posterior shoulder of the superior arcuate sulcus near its med ial tip and is therefore more accessible for tracer injections than th e one in macaque monkeys. This subregion is within cytoarchitectural a rea 6a beta, which is distinct from the adjacent area 6a alpha (dorsal premotor area). This smooth eye movement subregion may be comparable with the one in macaque monkeys. 3. Cortical connection patterns of th e FEFsac and FEFsem are similar and parallel to each other. The predom inant neural input to these two subregions originates in other cortica l eye fields, including the supplementary eye field, the parietal eye field, the middle superior temporal area, and the principal sulcus reg ion. These cortical eye fields each contain two separate, almost nonov erlapping, distributions of labeled neurons that project to the corres ponding frontal eye field (FEF) subregions. These results suggest that there may be similar, but relatively independent, parallel corticocor tical networks to control pursuit and saccadic eye movements. The weak connections between the middle temporal area (MT) and FEF suggest tha t the MT may not provide the major source of visuomotion inputs to the FEF, but that it rather plays a role in mediating visual information that is relayed from the striate and extrastriate cortices via MT to t he parietal cortex and then to the FEF. In addition to the well-known neural connections between the lateral intraparietal area and the FEF, additional parietal projections have been demonstrated from the dorso medial visual area area specifically to the FEFsac and from area Im sp ecifically to the FEFsem.