Alternate motor pathways for upper limb control: Implications for noninvasive brain stimulation after stroke

Abstract

Descending pathways other than the lateral corticospinal tract may play a role in the recovery of upper limb function after stroke. Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that has shown the potential to improve motor function of the upper limb. This thesis investigated the neurophysiological and behavioural effects of motor cortex (M1) tDCS on control of the upper limb in healthy adults and chronic stroke patients. The effects of various tDCS electrode montages on corticomotor excitability were examined, with an emphasis on the proximal upper limb. Transcranial magnetic stimulation (TMS) was used to infer corticomotor excitability of crossed, uncrossed, and transcallosal motor pathways, cervical propriospinal neurons, and intracortical inhibition in M1. The first three experiments examined the effects of dual-hemisphere tDCS on the excitability of propriospinal neurons, separate motor pathways, and upper limb coordination. Dual-hemisphere tDCS modulated subcortical excitability to cervical propriospinal neurons, and transcallosal inhibition. The electrode montage anodal tDCS (a-tDCS) was examined in two experiments with healthy adults and in the final thesis experiment with chronic stroke patients. The effects of a-tDCS varied in healthy adults. The variability of tDCS after-effects on motor evoked potentials were correlated with an individual’s preferential recruitment of early vs late indirect (I)-waves. In another experiment, it was revealed that short-interval paired-pulse TMS can be used to examine intracortical inhibition on uncrossed motor projections to the proximal upper limb. This novel TMS paradigm provides new opportunities to investigate the role of the ipsilateral M1 during movement. In the final experiment, the interhemispheric imbalance model for chronic stroke was challenged. Anodal tDCS of the contralesional M1 was hypothesised to improve motor function of chronic stroke patients that may rely on alternate motor pathways for upper limb control. Motor function of the paretic arm was positively affected by contralesional a-tDCS for patients who had greater spasticity. By identifying novel biomarkers this may help individualise tDCS protocols for future clinical trials.