Somatosensory evoked magnetic fields to median nerve stimulation: interhemispheric differences in a normal population

• Published in: Electroencephalography and clinical neurophysiology Vol. 104; no. 6; pp. 480 - 487
• Main Authors: Wikström, Heidi, Roine, Risto O, Salonen, Oili, Aronen, Hannu J, Virtanen, Juha, Ilmoniemi, Risto J, Huttunen, Juha
• Format: Journal Article
• Language: English
• Published: Shannon Elsevier Ireland Ltd 01.11.1997; Amsterdam Elsevier
• Subjects: Adult; Aged; Biological and medical sciences; Brain Mapping; Cerebral Cortex - physiology; Electric Stimulation; Evoked Potentials, Somatosensory - physiology; Female; Functional Laterality - physiology; Fundamental and applied biological sciences. Psychology; Hemispheric asymmetry; Humans; Magnetic Resonance Imaging; Magnetoencephalography; Male; Median Nerve - physiology; Middle Aged; Somatosensory evoked magnetic fields; Somesthesis and somesthetic pathways (proprioception, exteroception, nociception); interoception; electrolocation. Sensory receptors; Vertebrates: nervous system and sense organs; Somatosensory evoked magnetic fields; Magnetoencephalography; Hemispheric asymmetry; Human; Hemispheric specialization; Evoked magnetic field; Electrical stimulus; Central nervous system; Median nerve; Laterality; Brain (vertebrata)
• ISSN: 0168-5597; 0013-4694
• DOI: 10.1016/S0168-5597(97)00055-5

The objective of the present study was to evaluate the normal interhemispheric variability of the locations and activation strengths of the somatosensory cortices. Somatosensory evoked magnetic fields (SEFs) were recorded with a 122-channel magnetometer in 23 healthy subjects (mean age 57 years) to stimulation of left and right median nerves. Equivalent current dipole (ECD) strengths and locations were determined for the main SEF deflections at the contralateral primary sensorimotor (SMI) and secondary somatosensory (SIIc) cortices. In a Cartesian co-ordinate system, defined by the preauricular points and the nasion, the SMI sources were slightly but significantly more laterally and anteriorly located in the right than in the left hemisphere. No systematic co-ordinate asymmetries were found for the SIIc sources. In individual subjects, the interhemispheric differences in the ECD co-ordinates averaged less than 6 mm at both SMI and SIIc. The group means of the source strengths did not differ between the hemispheres, but individual differences 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 asymmetries of source locations in the centimetre scale.