Pattern of striate cortical projections to the pretectal complex in the guinea pig

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Abstract
The primary goal of this study was a determine whether the striate cortex (Oc 1) of the guinea pig projects to the pretactal nucleus of the optic tract (NOT), the first postretinal station of the horizontal optokinetic 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 organization in this pathway. Axonal tracers (biocytin or 3H‐leucine) were injected into the striate cortex (Oc 1), and the tissue processed with histochemical or light autoradiographic techniques. All subcortical terminal labeling is ipsilateral in the basal ganglia and thalamic nuclei. Furthermore, projections are traced to the ipsilateral brainstem, including two areas of the pretectal complex: (1) one in the NOT, extending in some cases to the adjacent lateral portion of the posterior pretectal nucleus (PPN), and (2) one in the pars compacta of the anterior pretectal nucleus (APNc). The terminal fields in the APN are consistently located rostrally in the dorsolateral portion of the nucleus, independently of the injection site in Oc 1, whereas in the NOT the 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 organization in the cortico‐NOT projection; accordingly, the rostrocaudal axis of the striate cortex projects to the lateromedial axis of the NOT, with a 90° rotation, whereas lateral parts of the striate cortex project diffusely throghout the rostrocaudal extent of the NOT. These data show for the first that the NOT in the guinea pig receives a substantial projection form the visual cortex. Given the fact that in the guinea pig the optokinetic nystagmus shares some of the characteristics found in cat and monkey (i.e., consistent intial fast rise in the slow phase velocity and reduced asymmetry in monocular stimulation), the present findings lend support to the hypothesis that a cortical input to the NOT is a necessary condition for these oculomotor properties to be present.