In-phase bilateral arm movements are not a summation of unilateral movements, but a product of integrated motion of both arms: an fMRI study in acute stroke

Introduction: Bilateral upper limb training (BT) has therapeutic potential in stroke rehabilitation. Neuroimaging allows understanding of cortical mechanisms of action underlying BT. This study aimed to determine regions of brain activation during bilateral in-phase arm movement in acute stroke using functional magnetic resonance imaging (fMRI).
Method: 13 acute stroke individuals underwent fMRI scanning while performing self-paced continuous unilateral and bilateral in-phase wrist flexion-extension. Regions of brain activation during bilateral in-phase mode were compared to unilateral mode.
Results: During bilateral in-phase wrist flexion-extension, regions of brain activation were neither identical (i.e. mirror-symmetric) between the 2 hemispheres nor identical with combined unilateral paretic and non-paretic wrist flexion-extension. In particular, 1 stroke participant who did not show significant activation in M1 during paretic and non-paretic wrist flexion-extension, significantly activated M1 in the lesioned hemisphere during bilateral in-phase wrist flexion-extension. Bilateral in-phase wrist flexion-extension did not activate additional brain regions to those already activated during unilateral wrist flexion-extension. Instead, bilateral in-phase wrist flexion-extension did not significantly activate the frontal subcallosal gyrus, claustrum, superior occipital gyrus and lentiform nucleus when compared to unilateral wrist flexion-extension.
Conclusion: Bilateral in-phase arm movements were not a summation of independent unilateral movements, but a product of integrated motion of both arms. The production of bilateral in-phase arm movements involved regions of brain distinct from and not a mirror of unilateral movements. The fact that bilateral in-phase activated fewer brain regions than unilateral movements combined provides evidence of differential neural networks and suggests a coupling interaction during bilateral in-phase not present during unilateral movements.