Effects of anodal transcranial direct current stimulation over the motor cortex on response processing

Anodal transcranial direct current stimulation (tDCS) is the non-invasive application of a stimulating current which has been proposed as a possible intervention technique for a number of pathologies that affect the motor system, including Parkinson’s disease and stroke. When applied over the motor cortices, anodal tDCS has been shown to elicit long lasting changes to motor excitability. Application of anodal tDCS over the motor cortex has also been shown to improve performance on functional motor tasks. However, there is little knowledge of how anodal tDCS produces these changes. This thesis investigates whether anodal tDCS over the motor cortex influences responding, and if so how are response processes changed. This is explored across functional motor performance, as well as behavioural and electrophysiological performance on a cued go/nogo task with informative and uninformative cues. These experimental outcomes are assessed on three different subject groups, healthy young and older adults, as well as chronic stroke patients. The analysis of the results showed that while there was a small improvement of gross motor performance following anodal tDCS in healthy older adults, there was no beneficial effect of stimulation on either behavioural or electrophysiological data. This null effect was consistent across all three subject groups. Bayesian analyses confirmed that null effects models of the data were stronger fits compared to models which included an effect of stimulation. These results indicate that the application of anodal tDCS over the motor cortex does not impact response processes, which calls into question the efficacy of using anodal tDCS as a therapeutic intervention.