Transcranial Direct Current Stimulation (tDCS)/Transcranial Alternating Current Stimulation (tACS)Original ArticleComparing Cortical Plasticity Induced by Conventional and High-Definition 4 × 1 Ring tDCS: A Neurophysiological Study
Introduction
Neuroplasticity is associated with cognitive functions including learning and memory formation, as well as the recovery process following brain damage. Transcranial direct current stimulation (tDCS) induces cortical plasticity non-invasively via subthreshold neuronal membrane polarization with constant weak direct currents [1], [2], [3]. Pharmacological experiments revealed the NMDA receptor and calcium-dependency of tDCS-induced plasticity [4], [5], [6]. The direction of tDCS-induced plastic changes depends on stimulation polarity – anodal stimulation conventionally results in excitability enhancement, while cathodal tDCS reduces it [1], [2], [3]. tDCS has been applied in the last years in a broad spectrum of experiments, ranging from basic research to clinical application conditions involving physiological and pathological alteration of cortical plasticity, and has been shown to enhance cognitive functions and to improve neurological impairment [4], [5], [6].
Current flow direction and amplitude and electrode montage are the main factors determining the efficacy of tDCS, which is usually applied with an intensity of 260 μA–2 mA and rectangular electrodes sized 16–35 cm2 placed in a bipolar cortical montage (e.g. motor cortex – contralateral supraorbital montage for the stimulation of the primary motor cortex) [4]. Stimulation with these conventional parameters modulates cortical activity in a relatively larger area than that covered by the target electrode (i.e. motor cortex electrode for motor cortex stimulation), as demonstrated in neuroimaging studies [7]. Moreover, modeling studies suggest that the largest cortical current density might not occur directly under the target electrode in these conventional stimulation protocols [8], [9]. However irrespectively of the effects of conventional tDCS on cortical excitability it will always be desirable to enhance or optimize stimulation focality.
Diminished electrode size has been shown to reduce affected cortical area size therefore increase focality [10]. In this study the current intensity was reduced according to size when compared with the standard size of 35 cm2. Using smaller electrodes with gel-based designs, so-called “High-Definition” electrodes (gel-skin contact area: ∼25 ± 2.5 mm2 with a plastic electrode holder), it allows tolerated stimulation of currents of up to 2 mA [11]. Deploying these electrodes in a 4 × 1 ring electrode configuration was predicted to focalize stimulation according to the finite element model analysis (FEM) based on high-resolution magnetic resonance imaging (MRI) [8], [12]. The “4 × 1 HD tDCS” implements a concentric-ring electrode configuration with the active center electrode placed above the target area, which is surrounded by four return electrodes. According to modeling studies, this electrode configuration results in maximal electric field (EF) strength under the target electrode with brain current flow constrained by the 4×1 ring radius (for example 3.5 cm defined by the distance between active and each return electrode), and thus more spatially restricted electric field, as compared to the conventional electrode placement [8], [12].
In these computational studies, EF magnitude was assumed to correlate with modulation of cortical excitability and activity. However, direct neurophysiological evidence for the efficacy of HD-tDCS is still lacking so far. In the present study, we compare motor cortical plasticity induced by both, conventional and HD tDCS, in healthy humans to explore the physiological effects of both stimulation designs. The results might serve as basis for the future application of specific tDCS protocols in both basic and clinical tDCS studies, where different stimulation focality is required.
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Subjects
Fourteen healthy subjects participated in the experiment (6 men, 8 women, age 25.29 ± 3.2 (SD) years). All gave written informed consent. The investigation was approved by the ethics committee of the University of Goettingen and the experiments conform to the Declaration of Helsinki.
Transcranial direct current stimulation of the motor cortex
tDCS was administered with a current strength of 2 mA for 10 min (both for anodal or cathodal tDCS) by a battery-driven constant current stimulator (NeuroConn GmbH, Ilmenau, Germany). Two types of electrode
Results
Some subjects reported a tingling sensation during tDCS with both polarities and electrode configurations. Eight out of 14 subjects mentioned less tingling with ring electrodes compared to the conventional sponge pads. No further adverse effects were reported.
Baseline MEP amplitudes and TMS intensities did not differ between stimulation conditions (see Table 1). The ANOVA revealed significant main effects of electrode configurations (F = 593.646, d.f. = 1, P < 0.001) and time course (F = 8.369,
Discussion
HD tDCS was recently developed on the basis of computational modeling studies to enhance the focality of tDCS [8], [12]. So far, the functional effects of HD tDCS were only tested in one pilot study on pain perception, where HD tDCS over the primary motor cortex appeared to be well tolerated and significantly increased pain threshold [14], similar to the effects of conventional tDCS [15], [16]. Nevertheless, direct physiological evidence for the plasticity-inducing properties of this new
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These authors contributed equally.