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The projection of frontal cortical oculomotor areas to separate laminae of the superior colliculus was examined in adult domestic cats. Following the placement of WGA-HRP/HRP injections within the superficial, intermediate, and deep layers of the superior colliculus, several observations may be made regarding the frontal corticotectal projection in the cat. Large injections including all of the collicular layers result in the retrograde labeling of neurons in frontal cortical areas that have been suggested previously to be analogous to the frontal eye fields in monkeys. These areas are a portion of the ventral bank and lip of the cruciate sulcus, including a small portion of the medial wall of the hemisphere, and the medial and lateral banks and fundus of the presylvian sulcus. The same pattern of retrograde labeling in frontal cortical areas is seen when the injection sites are restricted to either the intermediate or deep collicular layers. That is, different frontal cortical areas of the cat project separately but not differentially upon different collicular laminae. The greatest numbers of labeled neurons within frontal cortical oculomotor areas are observed following injections placed laterally within the caudal portion of the superior colliculus, regardless of whether the injection sites are located within the intermediate or deep collicular layers. Injections restricted to the superficial gray layer do not result in retrograde labeling in any of the frontal cortical oculomotor areas of the cat. These results are discussed in terms of the patterns of visuomotor integration specific to the cat, as well as in relation to previous evidence showing similarities between frontal cortical areas in cats and monkeys.

The projection of the perihypoglossal (PH) complex to the superior colliculus (SC) in the rhesus monkey was investigated using the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Following physiological identification by electrical stimulation and multiunit recording, small injections of the tracer were placed within the SC of three monkeys. The largest numbers of retrogradely labeled neurons within the PH complex were found in the contralateral nucleus prepositus hypoglossi (NPH), in the laterally adjacent medial vestibular nucleus, and in the ventrally adjacent reticular formation (the nucleus reticularis supragigantocellularis). These labeled neurons are strikingly heterogeneous in size and morphology. The nuclei supragenualis and intercalatus also contain numerous labeled neurons in the 2 cases in which the injections involve the caudal SC. Large numbers of retrogradely labeled neurons as well as anterogradely transported WGA-HRP are observed alo throughout the pontine and medullary reticular formation, including the midline raphe. The PH complex, particularly the NPH, is known to be involved in the coding of eye position and has been hypothesized to be a critical component of the “neural integrator.” Our data demonstrate the existence of a robust projection from the PH complex to the contralateral SC in the rhesus monkey. This projection may serve as the anatomical substrate by which a corollary of eye position could reach the SC. Such a signal is a prerequisite for the computation, at the collicular level, of saccadic motor error signals observed in the SC of rhesus monkeys.