Localization of the motor hand area using transcranial magnetic stimulation and functional magnetic resonance imaging

Objective: The anatomical location of the motor area of the hand may be rev ealed using functional magnetic resonance imaging (fMRI). The motor cortex representation of the intrinsic hand muscles consists of a knob-like struct ure. This is omega- or epsilon-shaped in the axial plane and hook-shaped in the sagittal plane. As this knob lies on the surface of the brain, it can be stimulated non-invasively by transcranial magnetic stimulation (TMS). It was the aim of our study to identify the hand knob using fMRI and to revea l if the anatomical hand knob corresponds to the hand area of the motor cor tex, as identified by TMS, by means of a frameless MRI-based neuronavigatio n system. Methods: Suprathreshold transcranial magnetic stimuli were applied over a g rid on the left side of the scalp of 4 healthy volunteers. The motor evoked potentials (MEPs) were recorded from the contralateral small hand muscles, and the centers of gravity (CoG) of the MEPs were calculated. The exact an atomical localization of each point on the grid was determined using a fram eless MRI-based neuronavigation system. In each subject, the hand area of t he motor cortex was visualized using fMRI during sensorimotor activation ac hieved by clenching the right hand. Results: In all 4 subjects, the activated precentral site in the fMRI and t he CoG of the MEP of all investigated muscles lay within the predicted anat omical area, the so-called hand knob. This knob had the form of an omega in two subjects and an epsilon in the other two subjects. Conclusions: TMS is a reliable method for mapping the motor cortex. The CoG calculated from the motor output maps may be used as an accurate estimatio n of the location of the represented muscle in the motor cortex. (C) 1999 E lsevier Science Ireland Ltd. All rights reserved.