Neuroenhancement via transcranial direct current stimulation: cognitive and neurophysiological aspects.
Since time immemorial, human beings strive for self-improvement, an endeavour that is not restricted to physical attributes but encroaches on mental abilities. For decades, neuroenhancers were the means of choice. However, the (re-)emergence of non-invasive brain stimulation techniques, including transcranial direct current stimulation (tDCS), promised well in basic as well as translational research. But more recently, due to increasing numbers of non-reproducible results, the reliability of the stimulation outcomes has been called into question.
On closer examination, the heterogeneity of tDCS outcomes follows a distinct pattern with low performers in a given task generally showing higher stimulation-induced gains. This holds true across distinct populations, insofar as greater tDCS effect sizes were demonstrated in healthy elderly and cognitively impaired individuals in contrast to healthy young adults. In a first study, we demonstrated that even within a relatively uniform population the same pattern of a differential benefit in a verbal learning task substantiates.
Behaviour and the respective impact of tDCS are based on neurophysiological principles. Thus, in search of valid biomarkers for stimulation-induced benefits the combination of tDCS and multimodal imaging methods has become evermore common. Following this line of reasoning, we have moved the previous experimental setting into the MR scanner with the aim to assess neurotransmitter levels in addition to task performance. This enables us to explore potential correlations between the measures as well as to investigate the mode of action of tDCS. These insights are particularly crucial when establishing tDCS as a valid treatment option.