Transcranial electrical stimulation good for all, or not good at all?
In recent years, there has been remarkable progress in the understanding and practical use of transcranial electrical stimulation (tES) techniques. tES affect neuronal states through different current waveforms applied transcranially, the most used forms are direct (tDCS), alternating (tACS) and random noise (tRNS) stimulation. Oversimplifying, the idea is that whereas anodal tDCS and tRNS increase neuronal excitability (excitation) and may consequently enhance behavioural performance, cathodal tDCS decreases neuronal excitability (inhibition) and subsequently worsens behavioural performance, tACS can increase neuronal excitability via entrainment of the desired neuronal firing frequency and consequently modulate performance. Although these effects of tES are well documented, there is evidence that such facilitation\inhibition is not always present at behaviour level and may even be reversed depending on when and how stimulation is applied in respect to task execution, and on the type of task. I will underline that applying this simplistic, sliding-scale reasoning (from excitation to inhibition or vice versa) does not always lead to the desired results at either the neurophysiological or the behavioural level. I will provide a picture of what we know about the theoretical models of tES that have been proposed to date, contextualized in a specific and unitary framework.