Negative covariation between task-related responses in alpha/beta-band activity and BOLD in human sensorimotor cortex: An EEG and fMRI study of motor imagery and movements
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Abstract
Similar to the occipital alpha rhythm, electroencephalographic (EEG) signals in the
alpha- and beta-frequency bands can be suppressed by movement or motor imagery and
have thus been thought to represent the "idling state" of the sensorimotor cortex.
A negative correlation between spontaneous alpha EEG and blood-oxygen-level-dependent
(BOLD) signals has been reported in combined EEG and fMRI (functional Magnetic Resonance
Imaging) experiments when subjects stayed at the resting state or alternated between
the resting state and a task. However, the precise nature of the task-induced alpha
modulation remains elusive. It was not clear whether alpha/beta rhythm suppressions
may co-vary with BOLD when conducting tasks involving varying activations of the cortex.
Here, we quantified the task-evoked responses of BOLD and alpha/beta-band power of
EEG directly in the cortical source domain, by using source imaging technology, and
examined their covariation across task conditions in a mixed block and event-related
design. In this study, 13 subjects performed tasks of right-hand, right-foot or left-hand
movement and motor imagery when EEG and fMRI data were separately collected. Task-induced
increase of BOLD signal and decrease of EEG amplitudes in alpha and beta bands were
shown to be co-localized at the somatotopic sensorimotor cortex. At the corresponding
regions, the reciprocal changes of the two signals co-varied in the magnitudes across
imagination and movement conditions. The spatial correspondence and negative covariation
between the two measurements were further shown to exist at somatotopic brain regions
associated with different body parts. These results suggest an inverse functional
coupling relationship between task-induced changes of BOLD and low-frequency EEG signals.
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