L’organizzazione visuotopica del collicolo superiore nel macaco

The superior colliculus (SC) is an ancient midbrain structure containing topographic maps in different sensory and motor modalities to guide (c)overt orienting behavior. While the orderly progression space represented along the collicular surface in primates is well documented, the details regarding the representation of central versus eccentric visual space are controversial. Moreover, anatomical, behavioral and functional data collectively support the hypothesis of a pragmatic rather than a metric representation of the visual field in the primate SC, tuned for the detection of contralateral peripheral stimuli in the upper visual field and for hand-object interactions under foveal control in the lower visual filed. Here, we used sub-mm and conventional resolution fMRI to perform retinotopic mapping in the macaque SC. In six monkeys, we performed phase-encoded retinotopic mapping fMRI experiments at 3T, with stimuli covering either 80° (n = 2) or 25° (n = 4) of the visual field. In two separate passive-viewing fMRI experiments (n = 2), we presented spatially restricted stimuli (80°, 25°, 18° diameter) at different locations (center, and at ~17° eccentricity in the lower left/right quadrants). We obtained clear wide-field retinotopic maps of the macaque SC showing a topology largely consistent with previously reported fMRI maps in humans and most recent electrophysiological findings in macaques. These maps showed an overrepresentation of peripheral and upper visual fields. The same functional bias for the upper visual field was observed in areas FEF and LIP. Surprisingly, however, the representation of peri-foveal fields differed between the left and right SC of all monkeys in all experiments, being largely confined to the left colliculus. None of the other main regions of the oculomotor/attentional system nor early visual cortices showed a similar interhemispheric asymmetry. Overall, the left SC was more sensitive to variations in eccentricities, and the right SC to variations in polar angles. Smaller interhemispheric differences were also found in the intraparietal sulcus and inferior parietal lobule. Our fMRI study presents a detailed picture of macaque SC retinotopic maps aligned with most recent electrophysiological models. These findings support the hypothesis that the primate SC overrepresents relevant sectors of the visual space, and that this pragmatic organization may be a property of the whole oculomotor/attentional system. Finally, the unexpected interhemispheric asymmetries found within the macaque SC and, to a lesser extent, the posterior parietal cortex may suggest larger lateralized functional specializations in the macaque brain than know until now.