DYNAMIC MRI SENSITIZED TO CEREBRAL BLOOD OXYGENATION AND FLOW DURING SUSTAINED ACTIVATION OF HUMAN VISUAL-CORTEX

Changes in cerebral blood oxygenation and flow during prolonged activa tion of human visual cortex (6-min video projection) were monitored us ing high-resolution T-2- and T-1-weighted gradient-echo MRI in identi cal sessions. Oxygenation-sensitive recordings displayed an initial si gnal increase (oxygenation ''overshoot''), a subsequent signal decreas e extending over 4-5 min (relative deoxygenation), and a signal drop a fter the end of stimulation that mirrored the initial response (oxygen ation ''undershoot''). Flow-sensitive MRI demonstrated that the inflow effect remained elevated during the entire period of stimulation, The observation of gradually decreasing cerebral blood oxygenation, despi te persisting elevation of blood flow, may be understood to be an accu mulation of deoxyhemoglobin due to the progressive up-regulation of ox idative phosphorylation, The present findings support a concept in whi ch transitions between functional states lead to an uncoupling of perf usion (oxygen delivery) from oxidative metabolism (oxygen consumption) whereas steady-state activity achieves their recoupling.