Lateralized visuotopic organization in the macaque superior colliculus revealed by fMRI.

The superior colliculus (SC) integrates multisensory inputs from retinal, subcortical, and cortical regions within a map of visual space to support orienting and interactive behaviors. While early models suggested that the SC primarily represents peripheral space for target detection, recent evidence highlights its significant foveal representation, essential for precisely targeting objects. Using ultra-high-resolution phase-encoding fMRI and spatially localized stimuli, we mapped the visuotopic organization of the SC in six macaques up to 40° eccentricity. In addition to confirming previous findings, we identified consistent interhemispheric asymmetries in the fMRI signal. The left SC, unlike the right, displayed a clear eccentricity map with a smooth rostro-caudal progression of responses to stimuli of increasing eccentricity from the fovea to the periphery. Conversely, the right SC showed no evidence of a pronounced eccentricity map and, instead, it exhibited more prominent polar angle maps and spatially broader fMRI responses to peripheral stimuli compared to the left SC. These lateralized responses were consistent across stimulus types and imaging protocols and were mirrored only in the intraparietal sulcus, a major cortical input to the SC. The observed asymmetry may derive from differences in magnification factor, intercollicular or surround inhibition between the left and right SC. Regardless of the underlying mechanism, our results suggest that functional lateralization in nonhuman primates may be more prevalent than previously recognized.