As noted in an earlier section, the superior colliculus influences activity of cerebral cortex by means of its projections to several thalamic nuclei that innervate the cortex. The cortex, in turn, sends projections back to the col-liculus. In most mammals that have been studied, the densest projections to the colliculus arise from visual cortex and other sensory areas of the posterior cerebrum; projections from frontal cortex are present, but are relatively scant (e.g., opossums: Martinich et al., 2000; insectivores: Kunzle, 1995; cats: Tortelly et al., 1980; Harting et al., 1992; rabbits: Buchanan et al., 1994; rats: Beckstead, 1979; Leichnetz and Gonzalo-Ruiz, 1987; Reep et al., 1987; tree shrews: Casseday et al., 1979). In primates, by contrast, dense corticotectal projections originate from frontal cortex and from higher-order parietal and temporal areas (Figure 8), as well as from visual areas (Fries, 1984). In Old World and New World monkeys, frontotectal projections arise mainly from portions of dorsolateral prefrontal cortex, including the frontal eyefield (FEF), principalis cortex, the cortex dorsal to the principal sulcus, and motor areas (the supplementary motor area, PMD, and M1) (Fries, 1984; Leichnetz et al., 1981). The frontal territories from which the densest projections arise represent eye and head movements (FEF, PMDr), forelimb movements (the forelimb representations of PMDc and M1), and territories involved in spatial representation (principalis cortex). The density of corticotectal projection neurons in FEF and PMD equals or exceeds that of visual cortex (Fries, 1984). In addition, the frontal projections of anthropoid primates provide substantial innervation of the superficial collicular layers, which are the major targets of retinotectal projections and the major source of projections to the LGN, whereas in the nonprimates that have been examined, frontal projections appear to innervate the middle and deep layers of the SC almost exclusively (Segal et al., 1983). The corticocollicular projections of strepsirhine primates have not been examined in detail, although existing evidence suggests they are similar to those of anthropoids. Kaas and Huerta (1988) review published evidence of projections arising from visual areas in Galago. In Otolemur, Preuss and Goldman-Rakic (unpublished observations) noted dense collicular labeling resulting from the tracer injections of frontal, parietal, and temporal cortex and described in their study of intracortical connectivity (Preuss and Goldman-Rakic, 1991c); injections that involved the FEF produced strong labeling of the superficial layers as well as deeper layers.
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