Interhemispheric interactions during the production of unimanual movements

Richard Ivry
Department of Psychology, University of California, Berkeley

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     Last modified: April 28, 2007
     Presentation date: 08/10/2007 2:40 PM in MCC
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Abstract
Interhemispheric interactions during the production of unimanual movements
Timothy Verstynen and Richard Ivry
UC Berkeley

Neuroimaging studies indicate that both hemispheres are frequently activated during unimanual hand movements. This is especially true during left hand movements. We have used a variety of methods to examine the conditions modulating the degree of ipsilateral activation during unimanual actions. Using fMRI, ipsilateral activation was found to increase when the planning and control requirements become more complex; this effect was observed when both sequential and configural requirements for the movements were manipulated. The center of the ipsilateral activity is anterior and lateral to the hand area in primary motor cortex, suggesting a distinct subregion of premotor cortex that is recruited during ipsilateral movements. When executing complex actions, the excitability of the ipsilateral primary motor cortex is lowered as evidenced by increases in TMS-induced motor-evoked potentials elicited in the stationary hand. Indeed, this increase may even lead to the manifestation of overt mirror movements in neurologically healthy adults. We consider two functional hypotheses for this ipsilateral activation. First, it could reflect the recruitment of ipsilateral descending pathways, providing an additional source of control for complex movements. Alternatively, complex movements might reflect bilateral planning processes and the ipsilateral premotor activity may help reduce unwanted activity in the hemisphere ipsilateral to the selected hand. Our preliminary evidence from studies that combine TMS and fMRI are supportive of the latter hypothesis. The degree of coupling between the two motor cortices during unimanual movements increased following repetitive TMS over the ipsilateral premotor region. By inference, we assume that the premotor region helps to minimize correlated activity of the two hemispheres, thus promoting independent control of the two hands during particularly complex actions.