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. 1998 Oct 1;512 ( Pt 1)(Pt 1):163-79.
doi: 10.1111/j.1469-7793.1998.163bf.x.

Multimodal output mapping of human central motor representation on different spatial scales

J Classen  1 , U Knorr, K J Werhahn, G Schlaug, E Kunesch, L G Cohen, R J Seitz, R Benecke
Affiliations

Multimodal output mapping of human central motor representation on different spatial scales

J Classen et al. J Physiol. .

Abstract

1. Non-invasive mapping by focal transcranial magnetic stimulation (TMS) is frequently used to investigate cortical motor function in the intact and injured human brain. We examined how TMS-derived maps relate to the underlying cortical anatomy and to cortical maps generated by functional imaging studies. 2. The centres of gravity (COGs) of TMS maps of the first dorsal intersosseus muscle (FDI) were integrated into 3-D magnetic resonance imaging (MRI) data sets in eleven subjects. In seven of these subjects the TMS-derived COGs were compared with the COG of regional cerebral blood flow increases using positron emission tomography (PET) in an index finger flexion protocol. 3. Mean TMS-derived COG projections were located on the posterior lip of the precentral gyrus and TMS-derived COG projections were in close proximity to the mean PET-derived COG, suggesting that the two methods reflect activity of similar cortical elements. 4. Criteria for a reliable assessment of the COG and the number of positions with a minimum amplitude of two-thirds of the maximum motor-evoked potential (T3Ps) were determined as a function of the number of stimuli and extension of the stimulation field. COGs and T3Ps were compared with an estimate of the size of the human motor cortex targeting alpha-motoneurons of forearm muscles. This comparison suggests that TMS can retrieve spatial information on cortical organization below the macroanatomic scale of cortical regions. 5. Finally, we studied the cortical representation of hand muscles in relation to facial and foot muscle representations and investigated hemispherical asymmetries. We did not find any evidence for a different ipsi- or contralateral representation of the mentalis muscle. Also, no difference was found between FDI representations on the dominant versus the non-dominant hemisphere.

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Figures

Figure 1
Figure 1. First step of the method of transformation of 2-D TMS co-ordinates into 3-D skull co-ordinates (schematic representation)
See Methods for an explanation of the lettering used.
Figure 2
Figure 2. Example of MEP maps derived from stimulating 49 scalp positions overlying the left hemisphere in one subject
A and B, average of MEPs evoked in the relaxed (20 stimuli at each position) or slightly contracted (10 stimuli at each position) right FDI. Co-ordinates refer to positions (in cm) on the medio-lateral axis (from right to left) and on the fronto-occipital axis (from top to bottom). C and D, data normalized to the maximum mean amplitude of each map. Positions with a mean amplitude of at least 67 % of the maximum in black, positions with mean amplitudes between 33 and 67 % of the maximum amplitude in grey.
Figure 3
Figure 3. Dependence of accuracy of COG estimates on the number of stimulations applied at each position, and the extension of the stimulation area centred by the provisional point of optimal excitability
COGs are referenced to the COG computed using 20 stimulations per position and the most extended stimulation area. A, mean difference of COG calculated at an increasing number of trials for 3 extensions of stimulation area (5 cm × 5 cm, 7 cm × 7 cm, maximal area) from the optimal COG estimate. B, 95 % confidence intervals for the mean difference from the optimal COG estimate for 3 numbers of trials (5, 10, 20) per position. Error bars represent s.e.m.
Figure 4
Figure 4. Effect of contraction of FDI on the COG
COG positions were determined for three subjects at rest (▪) and at slight (15 % of maximal force production) contraction (♦) of the target muscle.
Figure 5
Figure 5. Dependence of accuracy of number of excitable scalp positions on the number of stimuli per stimulation site
Top-third positions (T3Ps) were defined as positions with a mean amplitude of at least 67 % of the maximum mean amplitude. A, increase of the number of T3Ps with increasing number (F = 1.75; P < 0.05; ANOVA). Error bars indicate s.e.m.B, lack of order effect of number of T3Ps. Number of T3Ps remained essentially constant when analysed on successive sets of single trials.
Figure 6
Figure 6. Topographical scalp distribution of 95 % confidence regions of COGs
The shaded ellipses represent the distribution of MENT, FDI and AH regions in 11 subjects.
Figure 7
Figure 7. Response maps of MENT on contralateral (upper panels) and ipsilateral (lower panels) side
Results from 3 subjects. Continuous lines, representing the mid-sagittal (vertical line) and interaural (horizontal line) axes intersect at Cz. Amplitudes are given as a percentage of the maximal mean response amplitude following contralateral or ipsilateral stimulation, respectively.
Figure 8
Figure 8. Integration of TMS and MR imaging
A, identification of co-ordinates of projections of COG onto the MR anatomy. The example is of FDI mapping in 1 subject. COG projections were viewed in 3 orthogonal planes of the MR images. The arrow indicates location of the central sulcus. The templates corresponding best to the MR planes were identified in the atlas of Talairach & Tournoux (1988) and the 3-D co-ordinates of the COG projection were determined where it entered the brain. R, right; L, left. B, regions of COG projections of right (first and second column of panels) and left (third and fourth column) AH, FDI and MENT (mean ± 2 s.d.) in sections of the standard brain (Roland & Zilles, 1994) oriented in parallel with the axial and coronal planes in the atlas of Talairach & Tournoux (1988).
Figure 9
Figure 9. Integration of TMS and PET
A, Talairach co-ordinates (Talairach & Tournoux, 1988) of COG projections of TMS maps (right FDI; ○) and COG of the PET region metabolically activated by right index finger flexion (•) projected individually onto the mid-sagittal plane. Lines connect data from the same subject (n = 7). B, mean Talairach co-ordinates of COG projections of TMS maps (right FDI) and COG of regional activation (right index finger flexion; PET) projected onto the horizontal plane nearest to the z-co-ordinate in the atlas of Talairach & Tournoux (1988) (n = 7). Locations of COGs are indicated by the red dot. The dark area marks the posterior lip of the precentral gyrus.
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