Representation of cortical motor function as revealed by stereotactic transcranial magnetic stimulation
Introduction
In 1985 Barker introduced transcranial magnetic stimulation (TMS) as an electrophysiological tool for investigating human motor cortex and its connectivities (Barker et al., 1985). Several approaches were made to map the human motor cortex non-invasively using this technique by moving a focal stimulating coil over the scalp (Cohen et al., 1990; Levy et al., 1991; Wassermann et al., 1992; Verhagen Metman et al., 1993; Wilson et al., 1993). Those motor output maps were correlated to bony landmarks. Given the large variability of cortical anatomy in respect to bony landmarks (Homann et al., 1987) and the uncertainty in specifying the point of optimal stimulation under the stimulating coil, TMS was unable to identify the cortical location of the site stimulated. TMS could not establish a relationship between the structure and the function of the brain and therefore did not constitute a definitive brain mapping methodology.
Miranda et al. (1997)described the conjunction of TMS with a 3-dimensional (3D) digitizer which allowed precise measurement of the position of the stimulating coil within a coordinate system defined by bony landmarks. Recent advances in image processing allowed us to refine current mapping strategies by combining nuclear magnetic resonance (NMR) imaging modalities with TMS using a 3D digitizer to measure the position of the stimulating coil and map this position onto a magnetic resonance imaging (MRI) data set (Krings et al., 1997a, Krings et al., 1997b). In these studies, a frameless stereotactic system (FSS) was used to correlate scalp stimulation sites to the underlying brain anatomy at the time of stimulation. We thereby combined the anatomical accuracy as provided by MRI with the functional motor specificity as provided by TMS to introduce stereotactic TMS (STMS) as a new brain mapping modality. The accuracy of this method was validated by correlating STMS-maps to cortical motor maps obtained with direct electrical stimulation during neurosurgery (corticography) and to fMRI motor output maps.
In the present study we apply STMS to obtain the cortical representation of 12 muscles along the course of the central sulci in a larger number of healthy volunteers.
Section snippets
Subjects
Eighteen healthy volunteers (10 male, 8 female) were studied. The mean age was 25±6.4 years (mean±S.D.). Exclusion criteria were history of seizures, implanted electronic devices or skull defects. The study was approved by the institutional review board and written informed consent was obtained from all subjects.
Transcranial magnetic stimulation
TMS was performed using a Magstim 200 magnetic stimulator (Magstim Co., Whitland, UK), which induced a largely monophasic pulse with a rise time of 100 μs, a maximum duration of 1 ms
Results
Cortical motor output maps were readily obtained from the muscles contralateral to the stimulated hemisphere in all 18 subjects. No undesired side effects were experienced by the subjects. The combination of transcranial magnetic stimulation (TMS) and a frameless stereotactic system (FSS) allowed us to stimulate in relation to the underlying cortical anatomy rather than to bony landmarks.
The somatotopic order of the cortical representation of the motor homunculus was observed within all
Discussion
Stereotactic transcranial magnetic stimulation STMS was able to discriminate the localization of the epicenter of different muscles along the course of the central sulcus and to correlate their representation to the individual anatomy. The sites of optimal stimulation for different muscles are in agreement with the concept of somatotopic motor representation in the human motor cortex, except the results from subjects 1 and 10 where a hand muscle is represented more medially than expected.
Acknowledgements
We are grateful to Dr. K.H. Chiappa for his critical reading of the manuscript. The work was funded by a grant of the BMBF MEDWIS: A036.
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