Targeting interhemispheric inhibition with neuromodulation to enhance stroke rehabilitation

- This review focuses on interhemispheric inhibition and its role in the healthy and stroke lesioned brain.
- Measurement method and movement phase should be considered when comparing studies associating interhemispheric inhibition with functional recovery.
- Neuromodulation of interhemispheric inhibition to augment stroke recovery requires the targeting of specific neural circuitry.We discuss the effectiveness of current and novel neurostimulation techniques at targeting interhemispheric inhibition and enhancing stroke rehabilitation.

Background/ObjectivesInterhemispheric inhibition in the brain plays a dynamic role in the production of voluntary unimanual actions. In stroke, the interhemispheric imbalance model predicts the presence of asymmetry in interhemispheric inhibition, with excessive inhibition from the contralesional hemisphere limiting maximal recovery. Stimulation methods to reduce this asymmetry in the brain may be promising as a stroke therapy, however determining how to best measure and modulate interhemispheric inhibition and who is likely to benefit, remain important questions.MethodsThis review addresses current understanding of interhemispheric inhibition in the healthy and stroke lesioned brain. We present a review of studies that have measured interhemispheric inhibition using different paradigms in the clinic, as well as results from recent animal studies investigating stimulation methods to target abnormal inhibition after stroke.Main findings/DiscussionThe degree to which asymmetric interhemispheric inhibition impacts on stroke recovery is controversial, and we consider sources of variation between studies which may contribute to this debate. We suggest that interhemispheric inhibition is not static following stroke in terms of the movement phase in which it is aberrantly engaged. Instead it may be dynamically increased onto perilesional areas during early movement, thus impairing motor initiation. Hence, its effect on stroke recovery may differ between studies depending on the technique and movement phase of eliciting the measurement. Finally, we propose how modulating excitability in the brain through more specific targeting of neural elements underlying interhemispheric inhibition via stimulation type, location and intensity may raise the ceiling of recovery following stroke and enhance functional return.