| ReferencesReferences
Background
It is widely accepted that medial frontal regions are involved in voluntary action control. Indeed, have recently suggested that one
of the mechanisms through which the supplementary motor area (SMA) contri
butes to voluntary control is automatic and unconscious motor inhibition. In this study, they administered a visuo-motor subliminal masked prime task to two patients with micro-lesions
of the SMA and demonstrated an absence
of automatic and unconscious inhibition as evoked by masked prime stimuli. This finding has been supported by neuroimaging data . Here, the aim
of our research was to corroborate this result by means
of a ¡°virtual lesion¡± approach.
Methods
For this purpose, we examined the effects of 1 Hz rTMS (train of 20 min; stimulus intensity 120 % of resting motor threshold) over the SMA of 10 healthy volunteers, previously localized by functional magnetic resonance imaging (fMRI), on reaction time (RT) performance in the subliminal masked prime task. The functional localizer experiment consisted of four blocks of sequential finger tapping and 15 s of rest after each block. Imaging data were analyzed with SPM 8 and then were imported into the Brainsight software version 2.1.5. With such system, we were able to navigate across the subjects¡¯ brain. The peak voxel in the SMA for each subject (at a statistical threshold of p < 0.05 uncorrected) was used as a target point for the rTMS session.
Results
The mean motor threshold was 50.9 % of maximal stimulator output (SD: ¡À4.86 % ). Wilcoxon tests showed a significant effect of compatibility on RTs (sham: Z = 2.7, p = 0.007; rTMS: Z = 2.8, p = 0.005) and accuracy rate (sham: Z = 2.5, p = 0.01; rTMS: Z = 2.1, p = 0.03), subjects being slower and making more errors in compatible trials (sham: 391.64 ¡À 52 ms, 87.3 % of accuracy; rTMS: 396.66 ¡À 37 ms, 86.3 % of accuracy) in comparison to incompatible trials (sham: 357.45 ¡À 36 ms, 92.5 % of accuracy; rTMS: 356.25 ¡À 28 ms, 92.7 % of accuracy), suggesting motor inhibition. However, this NCE was preserved after rTMS over the SMA (RTs: Z = 0.87, p = 0.39; accuracy rate: Z = 0.71, p = 0.47).
Conclusions
We conclude that long trains of low intensity 1 Hz rTMS did not affect the modulation of RT by subliminal stimuli, suggesting that the SMA might not be mandatory for the implementation of this automatic process. The limitation of this study is relative to the neural efficacy argument because we are not sure that TMS was strong enough to disturb the redundant organizational processing in the SMA or that other regions were not able to compensate for the virtually lesioned area.
