Role of the Contralesional vs. Ipsilesional Hemisphere in Stroke Recovery

Abstract

Following a stroke, the resulting lesion creates contralateral motor impairment and an interhemispheric imbalance involving hyperexcitability of the contralesional hemisphere. Neuronal reorganization may occur on both the ipsilesional and contralesional hemispheres during recovery to regain motor functionality and therefore bilateral activation for the hemiparetic side is often observed. Although ipsilesional hemispheric reorganization is traditionally thought to be most important for successful recovery, definitive conclusions into the role and importance of the contralesional motor cortex remain under debate. Through examining recent research in functional neuroimaging investigating motor cortex changes post-stroke, as well as brain-computer interface (BCI) and transcranial magnetic stimulation (TMS) therapies, this review attempts to clarify the contributions of each hemisphere toward recovery. Several functional magnetic resonance imaging studies suggest that continuation of contralesional hemisphere hyperexcitability correlates with lesser recovery, however a subset of well-recovered patients demonstrate contralesional motor activity and show decreased functional capability when the contralesional hemisphere is inhibited. BCI therapy may beneficially activate either the contralesional or ipsilesional hemisphere, depending on the study design, for chronic stroke patients who are otherwise at a functional plateau. Repetitive TMS used to excite the ipsilesional motor cortex or inhibit the contralesional hemisphere has shown promise in enhancing stroke patients' recovery.

Keywords: brain-computer interface; contralesional; ipsilesional; motor recovery; stroke; transcranial magnetic stimulation.

Figure 1

Interhemispheric Imbalance Post Stroke: Normally during unilateral motor performance tasks, activation of the contralateral hemisphere inhibits activation of the ipsilateral hemisphere (black X is showing inhibition of the ipsilateral side). However, when the contralateral M1 is impacted by stroke, this inhibition is lost (black X on the blue inhibitory arrow) and therefore the contralesional (analogous to ipsilateral for healthy controls) becomes more activated. Some studies suggest that this increased contralesional activation may also contribute to the interhemispheric imbalance by imposing increased inhibition on the ipsilesional (contralateral for healthy patients) hemisphere designated by the blue arrow and black X on the ipsilesional hemisphere (Murase et al., 2004).