Cerebellar Transcranial Direct Current Stimulation to Influence Motor Learning
Stroke is a widespread health-care problem that causes impairments of neural structure and function, and often limits the ability to move. Despite some spontaneous recovery and rehabilitative efforts, recovery is often incomplete and ongoing disability poses a significant burden both to the person who has experienced a stroke and to their family and caregivers. Recovery relies heavily on neural plasticity mechanisms that are essential for learning lost motor functions. Therefore, interventions that can modulate neural plasticity have potential to promote recovery following stroke; these interventions can be applied as adjuncts to standard rehabilitation to augment the gains achieved.
Cerebellar transcranial direct current stimulation (ctDCS) is a non-invasive brain stimulation technique that alters neural plasticity through weak, continuous, direct currents delivered to the cerebellum. In healthy individuals and people with stroke, the cerebellum has a central role in motor learning; that is, the learning or re-learning of a motor task that results in permanent improvement in performance. The cerebellum is particularly active during error-based motor learning via its inhibitory connections with the cortex. Thus, the cerebellum is an ideal target for stimulation when the goal is to promote motor learning. However, whilst ctDCS can modulate the excitability of the cerebellum, it is not known whether it can enhance motor learning. Therefore, the overall aim of this thesis was to evaluate the effect of ctDCS on motor learning.
Study A, a systematic review, investigated the effects of ctDCS on motor learning in healthy individuals. This review revealed that a single session of ctDCS had no effect on motor performance during or immediately following stimulation but appeared to improve motor learning for up to 48 hours after stimulation. Improvements were seen with anodal stimulation, using a positively charged electrode, but not cathodal stimulation. The findings shed new light on the ability of ctDCS to produce gains that outlast the stimulation period. However, it was not clear whether repeated sessions of ctDCS could produce improvements that last longer than 48 hours.
Study B, a double-blinded, parallel-group, randomised controlled trial (RCT) in healthy individuals, evaluated the effects of repeated sessions of anodal ctDCS on learning a split belt treadmill task. The study demonstrated that three consecutive sessions of anodal ctDCS did not affect the ability to adapt to split-belt treadmill walking but significantly prolonged the maintenance of adapted walking patterns at one-week follow-up. For the first time, this study established the ability of repeated anodal ctDCS to influence longer-term motor learning in a complex functional task. This finding provided support for applying anodal ctDCS in combination with split-belt treadmill training (SBTT) to improve walking function following stroke.
Study C, a pilot parallel-group RCT in people with chronic stroke evaluated the feasibility of a research protocol in which repeated sessions of anodal ctDCS combined with SBTT were delivered. The planned RCT was not feasible due to limitations related to the criteria for inclusion and challenges maintaining the fidelity of the SBTT intervention. Future research should focus on either optimising the methods for SBTT delivery or utilising an alternative motor adaptation task to determine the effects of ctDCS on motor learning in people with stroke.