Influence of the supplementary motor area on primary motor cortex excitability during movements triggered by neutral or emotionally unpleasant visual cues

The stronger anatomo-functional connections of the supplementary motor area (SMA), as compared with premotor area (PM), with regions of the limbic system, suggest that SMA could play a role in the control of movements triggered by visual stimuli with emotional content. We addressed this issue by analysing the modifications of the excitability of the primary motor area (M1) in a group of seven healthy subjects, studied with transcranial magnetic stimulation (TMS), after conditioning TMS of SMA, during emotional and nonemotional visually cued movements. Conditioning TMS of the PM or of contralateral primary motor cortex (cM1) were tested as control conditions. Single-pulse TMS over the left M1 was randomly intermingled with paired TMS, in which a conditioning stimulation of the left SMA, left PM or right M1 preceded test stimulation over the left M1. The subjects carried out movements in response to computerised visual cues (neutral pictures and pictures with negative emotional content). The amplitudes of motor-evoked potentials (MEPs) recorded from the right first dorsal interosseous muscle after paired TMS were measured and compared with those obtained after single-pulse TMS of the left M1 under the various experimental conditions. Conditioning TMS of the SMA in the paired-pulse paradigm selectively enhanced MEP amplitudes in the visual-emotional triggered movement condition, compared with single-pulse TMS of M1 alone or with paired TMS during presentation of neutral visual cues. On the other hand, conditioning TMS of the PM or cM1 did not differentially influence MEP amplitudes under visual-emotional triggered movement conditions. This pattern of effects was related to the intensity of the conditioning TMS over the SMA, being most evident with intensities ranging from 110% to 80% of motor threshold. These results suggest that the SMA in humans could interface the limbic and the motor systems in the transformation of emotional experiences into motor actions.