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1 Department of Psychology, Princeton University, Princeton, NJ, USA; Center for the Study of Brain, Mind and Behavior, Princeton University, Princeton, NJ, USA
* To whom correspondence should be addressed. E-mail: kas{at}princeton.edu.
The superior colliculus (SC) is a multimodal laminar structure located on the roof of the brainstem. The SC is a key structure in a distributed network of areas that mediate saccadic eye movements and shifts of attention across the visual field and has been extensively investigated in non-human primates. In humans, it has proven difficult to study the SC with functional magnetic resonance imaging (fMRI) because of its small size, deep location and proximity to pulsating vascular structures. Here, we performed a series of high-resolution fMRI studies at 3 T to investigate basic visual response properties of the SC. The retinotopic organization of the SC was determined using the traveling wave method with flickering checkerboard stimuli presented at different polar angles and eccentricities. SC activations were confined to stimulation of the contralateral hemifield. Although a detailed retinotopic map was not observed, across subjects, the upper and lower visual fields were represented medially and laterally, respectively. Responses were dominantly evoked by stimuli presented along the horizontal meridian of the visual field. We also measured the sensitivity of the SC to luminance contrast, which has not been previously reported in primates. SC responses were nearly saturated by low contrast stimuli and showed only small response modulation with higher contrast stimuli indicating high sensitivity to stimulus contrast. Responsiveness to stimulus motion in the SC was demonstrated by robust activations evoked by moving versus static dot stimuli that could not be attributed to eye movements. The responses to contrast and motion stimuli were compared to those in the human lateral geniculate nucleus. Our results provide first insights into basic visual responses of the human SC and demonstrate the feasibility of studying subcortical structures using high-resolution fMRI.
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