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Sensitivity of the BOLD fMRI Activation Response to Global Vasodilation

G. Bruce Pike, Bojana Stefanovic, Jan M. Warnking, Karin M. Rylander

McConnell Brain Imaging Centre, Montreal Neurological Institute, Montreal , Canada

To rigorously interpret the BOLD activation response, it is necessary to understand the biophysical basis of the signal and to determine its sensitivity under various physiological conditions. Accordingly, we have explored the hemodynamic and BOLD responses to focal activations under various baseline flow levels. Specifically, we considered normocapnia and three levels of hypercapnia (5-10% CO 2) during a combined motor task and visual stimulation while measuring both CBF and BOLD. We observed a significant decline of activation-induced BOLD response magnitude with increasing basal flow levels, in accordance with the BOLD response models, whereby a pronounced drop in the basal dHb concentration decreases BOLD sensitivity to CBF increases. A very limited effect of the basal vasodilation on the relative CBF response was also observed which testifies to the nature of CBF regulation following functional activation. These findings indicate that the differences in global perfusion can have a profound effect on BOLD sensitivity and must therefore be accounted for before conclusions regarding the activation- or region-specific BOLD response magnitude differences may be drawn. They also suggest hemodynamic scenarios in which functionally active regions may be “silent” on BOLD fMRI maps.