Temporal patterns of uncoupling between oxidative metabolism and regional cerebral blood row demonstrated by functional magnetic resonance imaging

RATIONALE AND OBJECTIVES. Brain activation can be detected by functional magnetic resonance imaging. Termination of stimulation can cause drop of postactivation signal below preactivation baseline; the nature of this “undershoot” remains controversial. The authors investigated postactivation signal after stimulation of the visual cortex with photic stimuli of different duration. METHODS. Activation of visual cortex in 11 healthy subjects was studied. The authors underwent short and long stimulation. The relation between activation and postactivation signal was investigated after both durations of stimulation. RESULTS. Average postactivation signal after short stimulation was -1.71 ± 2.66% and after long stimulation 0.82 ± 1.59% (P ≤ 0.01). Significant “undershoot” was detected after short stimulation in the majority of cases, although this was typically absent after long stimulation. CONCLUSIONS. The authors propose that “undershoot” is caused by decreased oxygen content in venous blood compared with the preactivation level and may reflect increased extraction of oxygen as a result of glycolytic metabolism. The absence of “undershoot” after longer stimulation suggests a gradual shift from uncoupling between regional cerebral blood flow and oxygen consumption toward a steady state.