Visual Motion Processing in The Tree Shrew Superior Colliculus.

Recent research has established that the superior colliculus (SC) plays a key role in visual motion processing and visually guided behaviors. However, differences across species have made it difficult to integrate findings from various animal models to form a general understanding of the SC. Here we use the tree shrew-a species evolutionarily intermediate between rodents and primates-to help bridge our understanding of this ancient brain structure. We recorded visual responses from the tree shrew (of either sex) SC neurons in vivo using a battery of motion stimuli, including drifting gratings, random dot kinematograms, and plaid patterns of superimposed gratings. Tree shrew SC neurons overall preferred low spatial and high temporal frequencies, as well as high speed of motion. They showed a mixed selectivity for motion components and integrated pattern, with integration consistent with a vector sum rule. Compared to mice, tree shrew SC showed similar tuning properties to basic visual features but exhibited a lower degree of motion integration reminiscent of visual cortices in other species. Finally, tree shrews displayed optokinetic eye movements, a visual-motion-induced reflexive behavior, and the response induced by plaids largely followed the vector sum rule. Together, our study provides fundamental insights into visual motion representation in the tree shrew SC and establishes a foundation for future comparative studies on visual processing in the SC.