Non-invasive brain stimulation to assess and modulate neuroplasticity in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative and progressive disease related to a gradual decline in cognitive functions such as memory, attention, perceptual-spatial abilities, language, and executive functions. Recent evidence has suggested that interventions promoting neural plasticity can induce significant cognitive gains especially in subjects at risk of or with mild AD. Transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) are non-invasive techniques that can induce significant and long-lasting changes in focal and non-focal neuroplasticity. In this review, we present initial preliminary evidence that TMS and tDCS can enhance performance in cognitive functions typically impaired in AD. Also, we reviewed the initial six studies on AD that presented early findings showing cognitive gains such as in recognition memory and language associated with TMS and tDCS treatment. In addition, we showed that TMS has also been used to assess neuroplasticity changes in AD supporting the notion that cortical excitability is changed in AD due to the neurodegenerative process. Due to the safe profile, cost of these tools, and initial clinical trials results, further studies are warranted in order to replicate and extend the initial findings of rTMS and tDCS as cognitive enhancers in AD. Further trials should explore different targets of stimulation along with different paradigms of stimulation including combination with behavioural interventions.