The relationship between motor deficit and primary motor cortex hemispheric activation balance after stroke: longitudinal fMRI study

Background In the chronic stage of stroke, previous work has shown that the worse the hand motor deficit, the greater the shift of primary motor cortex (M 1 ) activation towards the contralesional hemisphere (ie, unphysiological) balance. Whether the same relationship applies at earlier stages of recovery in serially studied patients is not known. Methods fMRI of fixed-rate auditory-cued affected index-thumb tapping was obtained at two time points (mean 36 and 147 days poststroke) in a cohort of nine patients with ischaemic stroke (age: 56±9 years; three women/six men; seven subcortical, one medullary and one cortical). On each fMRI day, the unaffected/affected ratio of maximal index tapping rate (IT-R) was obtained. To assess the M 1 hemispheric activation balance, the authors computed the classic Laterality Index (LI). The correlation between LI and IT-R was computed for each time point separately. Results The expected correlation between LI-M 1 and IT-R, that is, motor performance worse with more unphysiological LI, prevailed at both time points (Kendall p=0.008 and 0.058, respectively), with no statistically significant difference between the two regressions. The same analysis for the dorsal premotor cortex and the supplementary motor area showed no significant correlation at either time-point. Conclusion These results from a small cohort of longitudinally assessed patients suggest that the relationship between M 1 laterality index and hand motor performance appears independent of time since onset of stroke. This in turn may suggest that attempting to restore the hemispheric balance by enhancing ipsilesional M 1 and/or constraining contralesional M 1 activity may have consistent efficacy throughout recovery.