Box plot of the temporal HRF properties of the GE HRF across the cortical depth and SE HRF across all subjects.

<p>A) Onset time (s), B) TTP (s) C) FWHM (s), and D) PSC (%). Comparing the SE HRF with surface gray matter GE HRF (0–1 mm cortical depth) shows that all temporal properties are significantly increased (P<0.01, Wilcoxon signed–rank test). This indicates that the surface gray matter GE contrast probes a different part of the vascular system upon activation than the SE contrast. Surface gray matter in V1 contains mainly postcapillary veins (diameters ∼70 μm) and much less capillaries (diameters ∼5 μm) than deep gray matter regions <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0054560#pone.0054560-Duvernoy1" target="_blank">[20]</a>. Looking at the deep gray matter GE HRFs (>1 mm cortical depth) and the SE HRF we found that the TTP, FWHM and PSC were all significantly increased for the GE HRF. The onset times, however, did not differ significantly. These findings suggest that the earlier part of the deep gray matter GE HRF is in close correspondence to the SE HRF and hence is weighted towards the microvasculature. ^*** denotes significant difference between the two compartments (Wilcoxon signed–rank test for P<0.01, ** for P<0.05, * for P<0.1).