Spatially precise bilateral arm movements are controlled by the contralateral hemisphere

Previous studies have reported that unilateral proximal arm movements are initiated more quickly in response to visual stimuli directed to the ispilateral hemifield than to the contralateral hemifield. This is thought to reflect differences in intrahemispheric and interhemispheric visuomotor integration. When bilateral movements are performed, this difference in reaction time (RT) is abolished owing to the involvement of bilaterally distributed motor pathways. However, these experiments typically use simple motor tasks that do not emphasise spatial precision. We investigated the hemispheric control of precise unilateral and bilateral arm movements in 12 subjects using a lateralized visual stimulus paradigm and found an ipsilateral RT advantage for both unilateral and bilateral movements. We conclude that the requirement to execute spatially precise movements restricts control to the contralateral hemisphere regardless of whether unilateral or bilateral movements are performed.