Interhemispheric motor cortex influence during bimanual unloading

Using the transcranial magnetic stimulation (TMS) of motor cortex we examined changes in the motor evoked potential (MEP) during natural bimanual unloading, during lifting of an equivalent weight by the contralateral arm while the ipsilateral forearm was held stationary (CONTRA) and during practice of unnatural unloading. During natural unloading, MEP amplitude decreased proportionally to the muscle activity. In CONTRA task MEP amplitude decreased, but the muscle activity was not changed. It suggests that the motor cortex activity related to the "postural" arm was inhibited by the contralateral motor cortex related to the "lifting" arm. This inhibition was diminished during the unloading task. When learning the unnatural unloading, the muscle activity decreased significantly with insignificant changes of MEP amplitude. Active role of the motor cortex during learning of the new task might be related to the reduction of the contralateral inhibition. This suggestion is supported by the observation that MEP amplitude decreased stronger than muscle activity in the first learning session similar to that in CONTRA task. MEP amplitude and background activity of the muscle proportionally decreased in the last learning trial. The results show that motor cortex activity in natural and unnatural unloading task might be related to the reduction of the interhemispheric inhibition.