Editorial: Non-invasive Brain Stimulation and Plasticity Changes in Aging

Our conceptualization of brain changes across the lifespan is evolving (Pascual-Leone et al., 2011). There appears to be no period when the brain and its functions are static. Instead, changes are continuous throughout the lifespan, some resulting in benefits, others in functional loss and decline Therefore, the most suitable framework appears to be that of lifelong , continued " developmental " processes that influence each other, and there is a growing need for deeper understanding of brain changes (plasticity) from prenatal states and infancy through childhood into adult and old age. In humans, non-invasive brain stimulation (NIBS) techniques like transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) are well-suited to explore brain plasticity across the lifespan (Bashir et al.). First, they can provide valuable translatable physiologic biomarkers of the state of cortical reactivity, brain network connectivity and dynamics, and mechanisms of brain plasticity (Diester et al., 2015). Further, used alone or in combination with other approaches, like cognitive training, they can induce or modulate neuroplastic responses, which can reveal causal relations between brain and behavior, translate into behavioral modification, and even lead to new therapeutic interventions (Bartrés-Faz and Vidal-Piñeiro, 2016). The present research topic offers novel insights into this growing field of NIBS in the study of aging. A first set of studies focus on motor cortex plasticity. Young-Bernier et al. hypothesize that measures of short-latency afferent inhibition (SAI), reflecting cortical cholinergic activity, mediate age-related individual differences to the plasticity-inducing effects of theta burst stimulation (TBS). While their results confirm high inter-individual variability to TBS, no association could be observed with responsiveness to SAI. Particular sample characteristics or unexplored specific responses to the stimulation protocols on aged individuals could account for these results, and further research is needed. Puri et al. investigated the role of the Val66Met BDNF polymorphism (encoding for a synaptic potentiation related neurotrophine) in motor cortical plasticity following tDCS. After 20, but not 10 min of stimulation, they observed greater motor plasticity responses to tDCS in the Met allele carriers, along with marked reduction in inter-individual variability. Hence, BDNF may modulate the time-course of motor plastic responses in elders and may be important to stratify sample selection where more homogeneous responses to NIBS are required (Shpektor et al.). Finally, Dickins et al. showed that the intermanual transfer of training simple motor skills is preserved in elders. However, for complex motor tasks, cortical excitability of the …