REORGANIZATION OF THE EXECUTIVE MOTOR SYSTEM AFTER STROKE

We wished to characterize changes in function of the executive motor s ystem individually and to correlate these with the degree of recovery from motor disability following stroke. Six male stroke patients under went perfusion positron emission tomography scanning while undertaking a repetitive, graded force exertion task with the hand affected by st roke, and at rest. We correlated regional cerebral blood flow (rCBF) a nd force exerted, and compared the distribution of blood flow in all a ctive states with that at rest. The statistical parametric maps of tas k-related flow changes that were generated were coregistered with anat omical magnetic resonance imaging scans. A comparison of force exertio n with rest (categorical comparison) in the group as a whole showed fe wer activations than found in normal subjects in both the infarcted an d unaffected hemispheres, indicating disruption of the entire motor ne twork. We found a polynomial correlation between synaptic activity (as indexed by blood flow change) and force exerted in 3 patients in the contralateral primary sensorimotor cortex, distinctly different from t he logarithmic curve in normal volunteers. For the group there was an initial steep rise of relative rCBF at lower exerted forces, a plateau , and then a second steep rCBF increase at a force of approximately 50 % of maximal voluntary contraction (MVC). The ipsilateral ventral post erior supplementary motor area (pSMA) and parietal areas showed correl ated activity with force exerted that was not found in normal subjects . Coregistered functional and anatomical images in individuals indicat ed considerable intersubject variability in the patterns of activation of the contralateral primary motor cortex, pSMA, anterior cingulate c ortex, dorsolateral prefrontal cortex, anterior opercular cortex, and parietal cortex. Compensatory activation of cortical areas that compri se components of the sensorimotor system not normally activated by the task used favours the concept that recruitment of preexisting cortico cortical and possibly parallel corticospinal pathways plays a prominen t role in functional reorganization. Ipsilateral force-correlated rCBF changes were sometimes present in dorsolateral premotor, insular and parietal cortex consistent with bilateral representation of movements in these motor-associated areas. Quantitative analysis of the rCBF-for ce relationship provides clearer evidence for large-scale functional r eorganization than simple task-control comparisons. The binominal rela tionship between rCBF and force suggests increased synaptic activity a t approximately 50% MVC, a level at which many stroke patients experie nce an increased sense of effort.