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Sucharit Katyal, David Ress; Attentional base response in intermediate layers of human superior colliculus measured using high-resolution fMRI. Journal of Vision 2013;13(9):224. doi: 10.1167/13.9.224.
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Purpose: Human SC has a retinotopic response when covert attention is deployed upon a visual stimulus. Here we tested if SC also exhibits a "base response," an attentional response in the absence of high-contrast visual stimulation, which has been observed in visual cortex during threshold contrast detection. Methods: Subjects (N = 3) fixated and, cued by a small arrow, alternated their attention between two black-outlined sectors located to the left and right of fixation, in 12-second blocks, attempting to detect briefly presented threshold-contrast Gabors. In separate localizer sessions, high-contrast stimuli were used within the same sectors to delineate retinotopic ROIs. Control experiments (N = 2) were performed to test if Gabors of the same contrast and duration evoked a measurable response within SC while subjects performed a demanding fixation-point task. For all experiments, high-resolution fMRI (1.2 mm voxels, 3-shot spiral) was acquired (3 sec/volume) in 8 slices covering SC (24-sec period, 9 cycles/run). Results: There was a significant base response in all six individual (p <0.01) and combined colliculi (p ~ 0) within localized ROIs. Responses between left and right colliculi were counter-phase, as expected. In the control experiment there was no significant stimulus-evoked response in all four individual (p > 0.15) and combined colliculi (p > 0.68). Laminar depth-profiles showed that the peak of the base response occurred significantly (p <0.005) deeper (~1.4 mm) than the superficial response of attention toward high-contrast stimulation (~0.4 mm) observed in our previous study (Katyal et al, J Neurophysiol 104:3074, 2010). Conclusion: Our results are consistent with the laminar dissociation of responses by neuronal subtypes within SC observed in monkeys. Intermediate layer visuomotor neurons receive projections from FEF and exhibit non-stimulus-evoked attentional enhancement. Our results here demonstrate a similar top-down attentional effect during threshold contrast detection in the intermediate layers of human SC using fMRI.
Meeting abstract presented at VSS 2013
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