In HVs following SMA disruption and in CMM patients, interhemispheric connectivity was modified during the delay period and the interhemispheric inhibition was decreased. In HVs, interhemispheric connectivity during the delay period was modulated according to whether or not hand coordination was required for the forthcoming movement. We used functional MRI and transcranial magnetic stimulation in 22 healthy volunteers (HVs), and then in two models of SMA dysfunction: (a) in the same group of HVs after transient disruption of the right SMA proper by continuous transcranial magnetic theta‐burst stimulation (b) in a group of 22 patients with congenital mirror movements (CMM), whose inability to produce asymmetric hand movements is associated with SMA dysfunction. With a delayed‐response task, we investigated interhemispheric interactions underlying normal movement preparation and the role of the SMA in these interactions during the delay period of unimanual or bimanual hand movements. How the SMA controls bimanual coordination remains unclear, although there is evidence suggesting that the SMA could modulate interhemispheric interactions. The supplementary motor area (SMA) is involved in movement preparation and bimanual coordination. The execution of coordinated hand movements requires complex interactions between premotor and primary motor areas in the two hemispheres. CP = conditioning pulse HV = healthy volunteers ISI = interstimulus interval TP = test pulse.

Individual data are presented as dot plots alongside the mean and SEM. (e, f) Interhemispheric interactions involving the right PMd at rest (left panel) and during motor preparation (right panel) in HVs before and after inhibition of the SMA proper (gray and blue plots) and in CMM patients and HVs (gray and orange plots). (c, d) Interhemispheric interactions involving the right SMA at rest (left panel) and during motor preparation (right panel) in HVs before and after inhibition of the SMA proper (gray and blue plots) and in CMM patients and HVs (gray and orange plots). In all groups, there was a significant increase of the TP amplitude during the task as compared to the rest condition. (b) Amplitude of the TP alone in each group at rest and during the preparation of right unimanual movement. A CP delivered on the right PMd resulted in smaller MEP amplitude only in HVs before SMA proper inhibition. A CP delivered on the right SMA did not affect the MEP amplitude in any group. A CP on the right M1 significantly reduced the MEP amplitude in HVs before and after SMA proper inhibition but not in congenital mirror movement (CMM) patients.

(a) Effect of a CP delivered on the right M1 or right supplementary motor area (SMA) or right dorsal premotor cortex (PMd) on the amplitude of the motor‐evoked potential (MEP) evoked by stimulation of the left M1 at rest. Figure S4 Additional transcranial magnetic stimulation results.

Impulse by Vanessa Garden