A study of the human primary and secondary somatosensory cortices with FMRI and MEG

We studied MEG and fMRI responses to electric median and tibial nerve stimulation in five healthy volunteers. The aim was to compare the fMRI activation and MEG equivalent current dipole (ECD) localisations in the primary (SI) and secondary (SII) somatosensory cortices. EPI functional images were acquired with a SIEMENS 1.5 T scanner according to a block paradigm. MEG data were acquired with a 165-channel whole head neuromagnetometer. Matching of MEG and fMRI subject reference frames was performed with current carrying coils whose signal was recorded in MEG and whose position was identified in structural MRI with oil capsules. Stimuli were rectangular pulses with duration 200 to 400 microseconds and frequency 0.3 Hz for MEG and 1.9 Hz for fMRI. A higher stimulus rate in fMRI was used to increase regional cerebral blood flow. The stimulus intensity was settled at a level producing a painless, clearly visible thumb opposition or foot flexion. A somatotopic organization of SII was seen both with MEG and fMRI, with the upper limb areas located more anteriorly and more inferiorly than the lower limb areas. ECD locations and fMRI activation were substantially consistent, with an average mismatch of about 1 cm. Similar differences were observed in SI.