Linear coupling between functional magnetic resonance imaging and evoked potential amplitude in human somatosensory cortex

The interpretation of task-induced functional imaging of the brain is criti cally dependent on understanding the relationship between observed blood fl ow responses and the underlying neuronal changes. However, the exact nature of this neurovascular coupling relationship remains unknown. In particular , it is unclear whether blood oxygen level-dependent functional magnetic re sonance imaging (BOLD fMRI) responses principally reflect neuronal synaptic activity. In order to address this issue directly in humans, we measured t he increase in somatosensory evoked potential amplitude and fMRI BOLD chang es to increases in intensity of median nerve electrical stimulation in five healthy non-anaesthetized subjects. We found that mean N20-P22 amplitudes increased significantly with stimulus intensity in all subjects, as did fMR I BOLD percentage signal intensity change. Moreover, the intensity of the B OLD signal was found to correlate linearly with evoked potential amplitude in four of the five subjects studied. This suggests that the BOLD response correlates with synchronized synaptic activity, which is the major energy c onsuming process of the cortex. (C) 2000 IBRO, Published by Elsevier Scienc e Ltd. All rights reserved.