TEMPORAL PATTERNS OF UNCOUPLING BETWEEN OXIDATIVE-METABOLISM AND REGIONAL CEREBRAL BLOOD-FLOW DEMONSTRATED BY FUNCTIONAL MAGNETIC-RESONANCE-IMAGING

RATIONALE AND OBJECTIVES Brain activation can be detected by functiona l magnetic resonance imaging, Termination of stimulation can cause dro p of postactivation signal below preactivation baseline; the nature of this ''undershoot'' remains controversial, The authors investigated p ostactivation signal after stimulation of the visual cortex with photi c stimuli of different duration, METHODS. Activation of visual cortex in 11 healthy subjects was studied. The authors underwent short and lo ng stimulation, The relation between activation and postactivation sig nal was investigated after both durations of stimulation.RESULTS. Aver age postactivation signal after short stimulation was -1.71 +/- 2.66% and after long stimulation 0.82 +/- 1.59% (P less than or equal to 0.0 1). 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 ca used by decreased oxygen content in venous blood compared with the pre activation level and may reflect increased extraction of oxygen as a r esult of glycolytic metabolism, The absence of ''undershoot'' after lo nger stimulation suggests a gradual shift from uncoupling between regi onal cerebral blood flow and oxygen consumption toward a steady state.