Shaping the Effects of Transcranial Direct Current Stimulation of the Human Motor Cortex

Abstract

Transcranial DC stimulation (tDCS) induces stimulation polarity-dependent neuroplastic excitability shifts in the human brain. Because it accomplishes long-lasting effects and its application is simple, it is used increasingly. However, one drawback is its low focality, caused by 1) the large stimulation electrode and 2) the functionally effective reference electrode, which is also situated on the scalp. We aimed to increase the focality of tDCS, which might improve the interpretation of the functional effects of stimulation because it will restrict its effects to more clearly defined cortical areas. Moreover, it will avoid unwanted reversed effects of tDCS under the reference electrode, which is of special importance in clinical settings, when a homogeneous shift of cortical excitability is needed. Because current density (current strength/electrode size) determines the efficacy of tDCS, increased focality should be accomplished by 1) reducing stimulation electrode size, but keeping current density constant; or 2) increasing reference electrode size under constant current strength. We tested these hypotheses for motor cortex tDCS. The results show that reducing the size of the motor cortex DC-stimulation electrode focalized the respective tDCS-induced excitability changes. Increasing the size of the frontopolar reference electrode rendered stimulation over this cortex functionally inefficient, but did not compromise the tDCS-generated motor cortical excitability shifts. Thus tDCS-generated modulations of cortical excitability can be focused by reducing the size of the stimulation electrode and by increasing the size of the reference electrode. For future applications of tDCS, such paradigms may help to achieve more selective tDCS effects.

Exploring prefrontal cortex functions in healthy humans by transcranial electrical stimulation

“…Also, advanced stimulation protocols, e.g. using smaller stimulation electrodes, large return electrodes, or multiple electrode approaches, might be helpful [79,80] . Moreover, combining measures of task performance with physiological outcome parameters via simultaneous EEG, ERP, or functional imaging approaches, which is now technically possible, will further enhance our understanding of psychological-physiological interactions.…”

Imaging human brain networks to improve the clinical efficacy of non-invasive brain stimulation

“…As such, any effects of tDCS and tRNS manifest at both the anode and cathode, as well as between the electrodes, thus modifying excitability over a much larger area of cortex than rTMS. It is A c c e p t e d M a n u s c r i p t Sale et al,16 noteworthy, though, that changing the relative size of the electrodes [large return electrode and small active electrode (Nitsche et al, 2007)] can improve the spatial selectivity of the stimulation. Recently, a high-definition tDCS montage has been developed, whereby a small inner active electrode is surrounded by four small outer 'ring' return electrodes.…”

Noninvasive Brain Stimulation Protocols in the Treatment of Epilepsy: Current State and Perspectives

“…54 Similarly, no seizure induction has been reported to date for tDCS in humans or animal seizure models. 50,52 However, because excitability-reducing cathodal tDCS includes anodal excitability-enhancing tDCS of another cortical area in the standard stimulation protocols, such a risk cannot be ruled out completely, but might be circumvented by the use of either large, ineffective reference electrodes 55 or an extracephalic reference.…”