Interhemispheric inhibition (IHI) is an important mechanism to maximize the independent functioning of each hemisphere and is most likely mediated by transcallosal fibres. IHI can be investigated by paired pulse transcranial magnetic stimulation (TMS) whereby, in half of the trials, a test stimulus (TS) over one hemisphere is preceded by a conditioning stimulus (CS) over the other hemisphere. Whereas various studies have investigated IHI in rest, less is known about interhemispheric interactions during voluntary muscle activation. Here, we investigated the influence of tonic muscle activity (5% of the maximal voluntary contraction) in either the right wrist flexor or extensor versus rest on IHI from the active (left) to the resting (right) hemisphere. Our main finding was that tonic activation of the right wrist flexor, led to an increase in IHI from the active (dominant left) to the resting (non-dominant right) hemisphere as compared to rest. A control experiment employed the same design but CS intensity was lowered to match MEP amplitudes of the conditioning hand between active and rest conditions. This resulted in a relative decrease of IHI. It is hypothesized that functional regulation of IHI might prevent the occurrence of mirror activity in the primary motor cortex (M1) of the resting hemisphere and, thus, might play an important role in the execution of unimanual actions.

Language and certain aspects of motor control are typically served by the left hemisphere, whereas visuospatial and attentional control are lateralized to the right. Here a (visuo)motor tracing task was used to identify hemispheric lateralization beyond the general, contralateral organization of the motor system. Functional magnetic resonance imaging (fMRI) was applied in 40 male right-handers (19-30 yrs) during line tracing with dominant and nondominant hand, with and without visual guidance. Results revealed a network of areas activating more in the right than left hemisphere, irrespective of the effector. Inferior portions of frontal gyrus and parietal lobe overlapped largely with a previously described ventral attention network responding to unexpected or behaviourally relevant stimuli. This demonstrates a hitherto unreported functionality of this circuit that also seems to activate when spatial information is continuously exploited to adapt motor behaviour. Second, activation of left dorsal premotor and postcentral regions during tracing with the nondominant left hand was more pronounced than that in their right hemisphere homologues during tracing with the dominant right hand. These activation asymmetries of motor areas ipsilateral to the moving hand could not be explained by asymmetries in skill performance, the degree of handedness, or interhemispheric interactions. The latter was measured by a double-pulse transcranial magnetic stimulation paradigm, whereby a conditioning stimulus was applied over one hemisphere and a test stimulus over the other. We propose that the left premotor areas contain action representations strongly related to movement implementation which are also accessed during movements performed with the left body side.