PATTERN OF STRIATE CORTICAL PROJECTIONS TO THE PRETECTAL COMPLEX IN THE GUINEA-PIG

The primary goal of this study was to determine whether the striate co rtex (Oc 1) of the guinea pig projects to the pretectal nucleus of the optic tract (NOT), the first postretinal station of the horizontal op tokinetic pathway, and, if so, to analyze the anatomical organization of this cortico-NOT projection. Other goals of this investigation are to identify other pretectal nuclear projections from the visual cortex in the guinea pig, and to determine whether there is any visuotopic o rganization in this pathway. Axonal tracers (biocytin or H-3-leucine) were injected into the striate cortex (Oc 1), and the tissue processed with histochemical or light autoradiographic techniques. All subcorti cal terminal labeling is ipsilateral in the basal ganglia and thalamic nuclei. Furthermore, projections are traced to the ipsilateral brains tem, including two areas of the pretectal complex: (1) one in the NOT, extending in some cases to the adjacent lateral portion of the poster ior pretectal nucleus (PPN), and (2) one in the pars compacta of the a nterior pretectal nucleus (APNc). The terminal fields in the APN are c onsistently located rostrally in the dorsolateral portion of the nucle us, independently of the injection site in Oc 1, whereas in the NOT th e terminal fields shift slightly after injections placed in different locations in the striate cortex. A correlation of the injection sites in Oc 1 and terminal fields in the NOT reveals a loose topographic org anization in the cortico-NOT projection; accordingly, the rostrocaudal axis of the striate cortex projects to the lateromedial axis of the N OT, with a 90 degrees rotation, whereas lateral parts of the striate c ortex project diffusely throughout the rostrocaudal extent of the NOT. These data show for the first time that the NOT in the guinea pig rec eives a substantial projection from the visual cortex. Given the fact that in the guinea pig the optokinetic nystagmus shares some of the ch aracteristics found in cat and monkey (i.e., consistent initial fast r ise in the slow phase velocity and reduced asymmetry in monocular stim ulation), the present findings lend support to the hypothesis that a c ortical input to the NOT is a necessary condition for these oculomotor properties to be present. (C) 1994 Wiley-Liss, Inc.