Investigating the effects of transcranial direct current stimulation on upper-limb sensorimotor adaptation

Weightman, Matthew Christopher (2022). Investigating the effects of transcranial direct current stimulation on upper-limb sensorimotor adaptation. University of Birmingham. Ph.D.

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Abstract

Due to its capacity to modulate neural excitability and induce plasticity in the motor system, transcranial direct current stimulation (TDCS) has become a popular research tool, with hope for clinical translation. As a result, over the last two decades there has been a rapid rise in studies utilising the technique, with many positive findings in basic research and rehabilitation settings. However, of late there have been a growing number of studies reporting null or conflicting findings, casting doubt over the potential use of TDCS for real-world therapeutic application. This thesis aimed to draw on mechanistic theories of TDCS action to investigate ways in which stimulation can be applied more effectively to modulate behaviour.

Results show that anodal TDCS targeted at the cerebellum or primary motor cortex (M1) can selectively improve motor adaptation of the proximal and distal upper limb respectively, which is dependent on the level of control exerted by the stimulated regions over fractionated hand and finger or whole arm reaching movements. This double-dissociated finding was extended using cathodal stimulation, where cerebellar stimulation inhibited adaptive performance of reach behaviour. Paradoxically, adaptation dexterous hand movements were also enhanced by cathodal cerebellar TDCS, highlighting how the connectivity of brain regions can affect stimulation results. Further results suggest that the timing of stimulation can also be important. When short epochs of TDCS were applied coincidentally with movements during an adaptation task, only movements performed during simultaneous stimulation were enhanced, with similar interleaved movements unaffected. Finally, and after a shift in focus due to circumstances relating to the COVID-19 pandemic, findings suggest that prolonged motor preparation periods are insufficient to reliably achieve complete compensation during motor adaptation tasks, with residual errors present at different magnitudes of cursor rotation.

Collectively, these results highlight how different stimulation protocols can affect behavioural outcomes. They particularly underscore the importance of considering functional and anatomical factors associated with the stimulated regions when targeting different neural substrates, as well as canonical theories associated with the behaviour under investigation. Finally, the results presented here provide support for the continued development of TDCS research, with the hope it can help contribute to meaningful advances in the field and increase overall knowledge of the brain function.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):