Hemodynamic and metabolic responses to neuronal inhibition

Functional magnetic resonance imaging (fMRI) was used to investigate the changes in blood oxygenation level dependent (BOLD) signal, cerebral blood flow (CBF) and cerebral metabolic rate of oxygen consumption (CMRO2) accompanying neuronal inhibition. Eight healthy volunteers performed a periodic right-hand pinch grip every second using 5% of their maximum voluntary contraction (MVC), a paradigm previously shown to produce robust ipsilateral neuronal inhibition. To simultaneously quantify CBF and BOLD signals, an interleaved multislice pulsed arterial spin labeling (PASL) and T-2*-weighted gradient echo sequence was employed. The CMRO2 was calculated using the deoxyhemoglobin dilution model, calibrated by data measured during graded hypercapnia. In all subjects, BOLD, CBF and CMRO2 signals increased in the contralateral and decreased in the ipsilateral primary motor (M1) cortex. The relative changes in CMRO2 and CBF were linearly related, with a slope of similar to0.4. The coupling ratio thus established for both positive and negative CMRO2 and CBF changes is in close agreement with the ones observed by earlier studies investigating All perfusion and oxygen consumption increases. These findings characterize the hemodynamic and metabolic downregulation accompanying neuronal inhibition and thereby establish the sustained negative BOLD response as a marker of neuronal deactivation. (C) 2004 Elsevier Inc. All rights reserved.