Oscillopathic features and neuromodulation with tACS in people with Parkinson's disease: A semi-systematic review

Parkinson's disease (PD) is associated with prominent changes in neural oscillatory activity. These occur at cortical and subcortical sites, strongly predict clinical deficit, and are normalised by therapy. Interventions targeting oscillatory activity therefore represent an avenue for novel clinical approaches in people with PD. Transcranial alternating current stimulation (tACS) holds substantial appeal in this endeavour. This non-invasive brain stimulation (NIBS) technique can induce functionally and clinically relevant changes in oscillatory activity. Furthermore, it has not been associated with any serious adverse effects, is portable and relatively cheap. A growing number of studies have therefore investigated applications of tACS in PD and the aim of this review was to provide an overview of this literature. Of particular focus was identifying how recent advances within the neuromodulation space - across conceptual, physiological and methodological domains - might inform next generation applications of tACS. Review of the existing literature reveals several studies that have reported beneficial effects of tACS on symptomatology and neurophysiological deficit in people with PD. Furthermore, preliminary trials using multi-session dosing report long-lasting effects, supporting the clinical utility of a tACS based therapy. In addition, work in animal models provides important characterisation of neurobiological mechanisms contributing to beneficial effects of tACS, highlighting neuroprotective processes involving neurotrophins. Together, this literature demonstrates the potential benefits of tACS in people with PD. However, further development in future work stands to increase the impact and application of this technique, with examination of novel neurophysiological features, greater individualisation and optimised targeting being prominent areas on interest.