The gradient-echo BOLD fMRI signal demonstrates greatest changes with activation near large draining veins which results in cortical-depth activation profiles that increase linearly with depth from the white to pial surface. However, due to the sparsity of pial vessels these profiles are likely to vary across cortical locations depending on proximity to vessels. We investigated the temporal evolution and spatial variability of the cortical-depth profiles of the BOLD response within human visual cortex and demonstrated that the response is initially strongest in the mid-cortical depths and in time becomes dominated by the large veins at the pial-surface.
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