Multiple frequency steady-state evoked magnetic field mapping of digit representation in primary somatosensory cortex

Magnetic source imaging of multiple frequency steady-state somatosensory ev oked responses was examined using a 151-channel magnetoencephalography (MEG ) system and a dual-channel electrical stimulator. Somatotopy of digit. rep resentation was studied in healthy subjects and effects of injury-related c ortical plasticity in patients with unilateral transections of the median o r the ulnar nerve. Dipole source locations exhibited somatotopic order with overlap between neighboring digits. In two of three nerve injury patients evidence for cortical reorganization was found. The location of sources rel ated to digits neighboring deafferented digits was changed and their dipole moments were enlarged by comparsion with the sources related to contralate ral homologue control digits. As a basis for magnetic source imaging, the r ecording of multiple frequency somatosensory steady-state evoked responses may be a viable and time saving alternative to the recording of transient e voked responses.