Corticotectal and corticostriatal projections from the frontal eye fields of the cat: an anatomical examination using WGA-HRP

Corticofugal projections from the frontal eye fields (FEF) are believed to access the superior colliculus (SC) directly (i.e., monosynaptically) and i ndirectly (i.e., multisynaptically) through the basal ganglia. The present results suggest that these two pathways are derived from largely segregated populations of corticofugal neurons. Furthermore, while the different subr egions of the FEF from which these pathways originate have different termin ation patterns in the basal ganglia (i.e., striatum, ST), they share a comm on termination pattern in the SC. Injections of wheat germ agglutinin-horse radish peroxidase (WGA-HRP) into the two major subdivisions of the FEF (pre sylvian and cruciate sulci) resulted in dense label in both the ST (bilater ally) and the SC (ipsilaterally). Corticostriatal labeling was found in the caudal part of the head of the caudate nucleus (heaviest ipsilaterally), w ith labeling from cruciate injections located ventromedial to that produced by presylvian injections. Only presylvian injections resulted in labeling in the putamen. Retrograde tracing experiments demonstrated that both presy lvian and cruciate corticostriatal projections originated from neurons in l amina III and the upper aspects of lamina V. An additional but small group of presylvian corticostriatal projections was found in lamina VI. Corticote ctal terminal labeling was restricted to the deep laminae of the SC and was derived exclusively from lamina V neurons in cortex. They differed from th eir corticostriatal counterparts in laminar/sub-laminar location and in som a sizes.