Simultaneous monitoring of dynamic changes in cerebral blood flow and oxygenation during sustained activation of the...

Functional neuroimaging was used to investigate the effect of cerebral blood flow (CBF) adjustments on the blood oxygenation level dependent (BOLD) signal during visual stimulation. Temporal responses from both oxygenation- and perfusion-sensitized MRI revealed almost identical features during onset and ongoing activation, i.e. an activation-induced signal rise, and a gradual signal decrease during prolonged activation (overshoot). However, the post-stimulus responses exhibited a pronounced BOLD signal drop below prestimulus baseline (undershoot), but a rather rapid normalisation of the related CBF signal. Thus, an activation-induced initial BOLD signal rise and a gradual signal decrease reflect a coarse upregulation of CBF, which is followed by fine-tuning adjustments of flow. Regulations of other involved physiological parameters, including blood volume and oxidative metabolism give rise to a negative post-stimulus BOLD signal response.