Commentary: Preconditioning tDCS facilitates subsequent tDCS effect on skill acquisition in older adults

Citation: Opie GM and Cirillo J (2017) Commentary: Preconditioning tDCS facilitates subsequent tDCS effect on skill acquisition in older adults. Long-term potentiation and depression (LTP and LTD) represent powerful means of altering synaptic communication. However, they have an inherent potential to excessively modulate neuronal excitability, possibly resulting in unfavorable destabilization of neural function. It has been suggested that homeostatic regulation (i.e., metaplasticity) of neuronal networks avoids such destabilization. This construct proposes that the threshold for modification of neuronal excitability is based on previous post-synaptic activity, with an elevated threshold following high neuronal activity, but a reduced threshold following low neuronal activity (Bienenstock et al., 1982). Studies using non-invasive brain stimulation (NIBS) over human primary motor cortex (M1) support the utilization of such a mechanism (Karabanov et al., 2015). Furthermore, NIBS plasticity interventions have demonstrated an ability to interact homeostatically with motor learning, a process that is at least partially LTP-dependent (Ziemann et al., 2004; Jung and Ziemann, 2009). This has led to the exciting possibility of using " priming " NIBS to enhance the response to motor training following injury, or compensate for deficits in neuroplastic capacity. While relevant to many different populations, such an approach is of particular interest in older adults, where deficits in neuroplastic capacity (Barnes, 2003) and motor learning (Voelcker-Rehage et al., 2006) are well defined. However, it is currently unknown whether priming stimulation remains effective for potentiating the response to training in the elderly. In a recent paper within Neurobiology of Aging, Fujiyama and colleagues (Fujiyama et al., 2017) attempted to address this gap in knowledge by utilizing transcranial direct current stimulation (tDCS; a NIBS technique able to reliably induce LTP-and LTD-like plasticity in M1) to prime skill acquisition in young and older adults. The study utilized a visuomotor training task that involved moving a cursor on a computer screen through a range of targets by abducting the non-dominant index finger against a force transducer. Each target required a different level of force and target order was consistent across trials. During training, all subjects received 20 mins of excitatory anodal tDCS (atDCS). However, prior to training, half received 10 min of inhibitory cathodal tDCS (ctDCS), whereas the other half received 10 min of sham stimulation. Single-and paired-pulse TMS over the right M1 was used to quantify corticomotor excitability (motor evoked potential (MEP) area), and intracortical function (short-interval intracortical inhibition, SICI; intracortical facilitation, ICF), respectively. Changes …