Feasibility of home-based, self-applied transcranial direct current stimulation to enhance motor learning in middle-aged and older adults

We read with great interest the recent letter sent to you by McConnell and colleagues [1], in which they discuss one of the first applications of truly independent home-based electrical brain stimulation, applied following safety recommendations. Transcranial direct-current stimulation (tDCS) is a non-invasive brain stimulation technique capable of modulating cortical excitability beyond the stimulation period [2,3]. tDCS presents interesting options as a therapeutic intervention in multiple neurological disorders, such as stroke, depression, chronic pain, schizophrenia, and Alzheimer's and Parkinson's diseases [4,5]. The effects of this technique can accumulate over multiple stimulation sessions [6], but visiting specialized centers to regularly receive such an intervention presents practical issues (e.g., travel time and costs) that limit access to these benefits. Portable, inexpensive devices for tDCS are available on the market, making them an attractive option for home-based stimulation, an approach that has been gaining momentum in the past decade [7]. Nevertheless, readily-availability of this technology allows its irresponsible use [8], which poses safety hazards for users and threatens to demerit the potential of this tool for therapeutic applications. Even though there have been first studies concerning tDCS applied at home [7], few of them have required users to apply stimulation by themselves, as there are multiple parameters to control for stimulation to be effective. Nevertheless, seeking to provide home-based tDCS therapeutically calls for the users’ independent application, such that it might be unconstrained by availability of specialized personnel. We are currently conducting a study involving the remotely-controlled, unsupervised self-application of home-based tDCS during the execution of a motor training. Participants train two different tasks with their left hand (i.e. a sequencetapping task (Fig.1B) and a visuo-motor task (Fig. 1C)) for two weeks each, while receiving anodal electrical stimulation over the hand representation in the right motor cortex according to safety recommendations [9]. In total, participants receive 400 minutes of tDCS over the course of 20 days (five consecutive days per week). So far, five healthy participants (2 women, 64± 7.3 yr., range 51e68 yr.) have completed training. Participants gave their informed consent under protocol guidelines approved by the cantonal ethics committee Vaud, Switzerland (project No.2017-00301), according to the Declaration of Helsinki. We do the first session in the lab to teach the participants how to use the setup, and we give them a detailed manual including illustrations with every step of the