Figure 1.
Definition of coil orientation angles for stimulation of the right foot.
(a) As standard reference for stimulation of the right foot a lateral right-to-left coil orientation was used. We indicated this coil orientation with 0° and used it in session 1 as reference; (b) The opposite coil orientation (indicated with 180°) to the standard coil orientation. The arrows show the rotational direction.
Figure 2.
Coronal view of a structural MRI at hot-spot level.
From the hot-spot (red dot) at the skull the normal to the skalp surface is extended into the brain. The intersection of this line with the precentral gyrus is used for later evaluation of the angle of the precentral gyrus with respect to the interhemispheric cleft (cf. Figure 8).
Figure 3.
Recordings of threshold vs. coil orientation for each subject (labeled with acronyms).
The reference coil orientation for each session is indicated with a dotted vertical line. The minimum values for each subject are highlighted with open circles. In the left panel the recordings for session 1 with the standard coil orientation as reference are shown. The right plot shows the recordings for session 2. Note that the individual curves are vertically shifted for best presentability. Subject ‘La’ was excluded from further analysis as no clear minimum could be estimated in two sessions. For the sake of completeness subject ‘La’ is still shown in this figure.
Figure 4.
Polar plots of the motor thresholds averaged throughout subjects.
(a) shows the average of the real MTs (cf. Table 1); (b) shows the average of the standardized values with error bars (cf. Table 2). For both subplots: Recordings on the right side represent the findings for the standard coil orientation. Recordings in the left part were obtained with the coil handle towards the left hemisphere. Threshold minima occur in opposite positions as expected for a biphasic pulse.
Table 1.
Motor thresholds averaged throughout subjects.
Table 2.
Standardized motor thresholds averaged throughout subjects.
Figure 5.
The motor threshold recordings for both sessions of subject ‘ChO’.
A sinusoidal curve (dotted line) was fitted to the recordings. The global minimum at the optimal coil orientation is slightly smaller than the local minimum at the opposite coil orientation. The standard coil orientation (at 0°), however, is clearly not optimal.
Figure 6.
Correlation between optimal coil orienation and angle of the precentral gyrus.
The individual values are also listed in Table 3.
Table 3.
Optimal coil orientation and angle of the precentral gyrus.
Figure 7.
Schematic view on the precentral gyrus at the medial wall.
The standard current direction is in lateral direction which is in alignment with the orientation of pyramidal neurons at the medial wall. The optimal current direction in our results is approximately 30° rotated. This would be in alignment with pyramidal neurons at the posterior (or anterior) wall.
Figure 8.
Hot-spot (dot with surrounding circle) for two subjects projected in the MRI images in a transversal view.
An area of the precentral gyrus at the edge to the central sulcus and close to the interhemispheric cleft is in focus for stimulation. The white arrows denote the found optimal coil orientation angle for the individual subject.