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February 16, 2011
With functional MRI scanners researchers can measure, with great spatial precision, changes in blood oxygenation level in the brain. The idea behind fMRI scans is that if a brain area is active, energy consumption goes up, and more oxygen is transported through the blood to that area. In this way, one can very precisely localise those brain areas that become active during a certain task.
However, the fMRI signal (also called the BOLD signal - Blood Oxygenation Level Dependent signal) is not a 'direct' reflection of brain activity, Bastiaansen explains. 'Our study aims at better understanding the relationship between the BOLD signal and neural activity as recorded with EEG. EEG directly reflects brain activity, but has the disadvantage that it doesn't give precise spatial information.'
'We did this by simultaneously measuring EEG and fMRI - a technical challenge that only a few labs worldwide can handle. The results are twofold. First, we show that the BOLD signal is positively correlated with high-frequency neuronal activity (around 70 Hz, the so-called gamma rhythm). This was already demonstrated before, but only for animals. We extend this to the human case. Second, and most important, is that the BOLD signal correlates 'negatively' with low-frequency neuronal activity (around 20 Hz, the so-called beta rhythm). Moreover, we show that the positive BOLD-gamma correlations, and the negative BOLD-beta
correlations are independent of each other.'
'This implies that the relation between the BOLD signal and brain activity is not as straightforward as has been previously assumed', Bastiaansen concludes. 'An increase in BOLD may either reflect an increase in high-frequency brain activity, or a decrease in low-frequency activity. Therefore, our study deeply impacts on the interpretation of the results of fMRI experiments more generally.'