SOMATOSENSORY-EVOKED MAGNETIC-FIELDS TO MEDIAN NERVE-STIMULATION - INTERHEMISPHERIC DIFFERENCES IN A NORMAL POPULATION

The objective of the present study was to evaluate the normal interhem ispheric variability of the locations and activation strengths of the somatosensory cortices. Somatosensory evoked magnetic fields (SEFs) we re recorded with a 122-channel magnetometer in 23 healthy subjects (me an age 57 years) to stimulation of left and right median nerves, Equiv alent current dipole (ECD) strengths and locations were determined for the main SEF deflections at the contralateral primary sensorimotor (S MI) and secondary somatosensory (SIIc) cortices. In a Cartesian co-ord inate system, defined by the preauricular points and the nasion, the S MI sources were slightly but significantly more laterally and anterior ly located in the right than in the left hemisphere. No systematic co- ordinate asymmetries were found for the SIIc sources. In individual su bjects, the interhemisphetic differences in the ECD co-ordinates avera ged less than 6 mm at both SMI and SIlc. The group means of the source strengths did not differ. between the hemispheres, but individual dif ferences were on average 20% for the SMI and 65% for the SIIc sources. We conclude that at the individual level, the median nerve SEFs from SMI can be used to detect abnormally large interhemispheric asymmetrie s of source locations in the centimetre scale. (C) 1997 Elsevier Scien ce Ireland Ltd.