Effects of repetition rate of electric stimulation on MEG and fMRI signals

This study focuses on the physiological responses of somatosensory stimulation using magnetoencephalography (MEG) and functional MRI (fMRI). To investigate the relationship between neural activity and hemodynamic responses, the effect of the stimulus rate of electrical stimulation to the right thumb was investigated. In order to evaluate the blood oxygenation level dependent (BOLD) effects, signal power was calculated. The signal power is the integrated value from the beginning of the stimulation to the end of the sustained response, the point at which the signal recovers to the baseline. The signal power of fMRI and the dipole moment of MEG responses were compared. The MEG signals were measured using a 122-channel whole head SQUID system. When the stimulus frequencies were changed from 0.5 Hz to 10 Hz, the dipole moments of the M20 and M30 components of MEG signals were the largest at 1 Hz and 2 Hz, respectively. A maximum signal power of fMRI occurs at a frequency higher than 2 Hz, approximately at 3 Hz or 5 Hz, and then decreases progressively. These results are in agreement with a previous positron emission tomography (PET) experiment; however, they do not concur with this MEG experiment.