Representation of cortical motor function as revealed by stereotactic transcranial magnetic stimulation

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Abstract

Cortical motor representation of 12 muscles of the trunk and the upper and lower extremity was investigated in 18 healthy subjects using focal transcranial magnetic stimulation (TMS) in conjunction with a frameless stereotactic system (FSS). This combination allowed us to orientate stimulation sites to the individual central sulcus rather than to bony landmarks. Distinct but overlapping areas of muscle representation were identified and the 3-dimensional representation of those 12 muscles along the course of the central sulcus was obtained. With increasing stimulus intensity, the cortical output maps changed in that more muscles became excitable, motor evoked potential (MEP) amplitude and size of the responsive area increased and latency of the MEP decreased. These effects were more pronounced for proximal than for distal muscles, indicating a more widespread organization of corticospinal motor projection related to proximal muscles. The combination of TMS and FSS represents a method with which functional information can be directly related to underlying cortical anatomy. This correlation will be useful in the assessment of higher brain functions with TMS.

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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