Cortical projections to the superior colliculus in gray squirrels (Sciurus Carolinensis)

Abstract

The superior colliculus is an important midbrain structure involved with integrating information from varying sensory modalities and sending motor signals to produce orienting movements toward environmental stimuli. Because of this role, the superior colliculus receives a multitude of sensory inputs from a wide variety of subcortical and cortical structures. Proportionately, the superior colliculus of gray squirrels is among the largest in size of all studied mammals, suggesting the importance of this structure in the behavioral characteristics of gray squirrels. Yet, our understanding of the connections of the superior colliculus in gray squirrels is lacking, especially with respect to possible cortical influences. In the present study, we placed anatomical tracer injections within the medial aspect of the superior colliculus of five gray squirrels (Sciurus carolinensis) and analyzed the areal distribution of corticotectal projecting cells in flattened cortex. V1 projections to the superior colliculus were studied in two additional animals. Our results indicate that the superior colliculus receives cortical projections from visual, higher order somatosensory, and higher order auditory regions, as well as limbic, retrosplenial, and anterior cingulate cortex. Few, if any, corticotectal projections originate from primary motor, primary somatosensory, or parietal cortical regions. This distribution of inputs is similar to the distribution of inputs described in other rodents such as rats and mice, yet the lack of inputs from primary somatosensory and motor cortex are features of corticotectal inputs more similar to those observed in tree shrews and primates, possibly reflecting a behavioral shift from somatosensory (vibrissae) to visual navigation.

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