Antal A, Boros K, Poreisz C, Chaieb L, Terney D, Paulus W. Comparatively weak after-effects of transcranial alternating current stimulation (tACS) on cortical excitability in humans. Brain Stimul. 2008;1(2):97-105

Department of Clinical Neurophysiology, Georg-August University, Göttingen, Germany.
Brain Stimulation (Impact Factor: 4.4). 04/2008; 1(2):97-105. DOI: 10.1016/j.brs.2007.10.001
Source: PubMed


Interference with brain rhythms by noninvasive transcranial stimulation that uses weak transcranial alternating current may reveal itself to be a new tool for investigating cortical mechanisms currently unresolved. Here, we aim to extend transcranial direct current stimulation (tDCS) techniques to transcranial alternating current stimulation (tACS).
Parameters such as electrode size and position were taken from those used in previous tDCS studies.
Motor evoked potentials (MEPs) revealed by transcranial magnetic stimulation (TMS), electroencephalogram (EEG)-power, and reaction times measured in a motor implicit learning task, were analyzed to detect changes in cortical excitability after 2-10 minutes of AC stimulation and sinusoidal DC stimulation (tSDCS) by using 1, 10, 15, 30, and 45 Hz and sham stimulation over the primary motor cortex in 50 healthy subjects (eight-16 subjects in each study).
A significantly improved implicit motor learning was observed after 10 Hz AC stimulation only. No significant changes were observed in any of the analyzed frequency bands of EEG and with regard to the MEP amplitudes after AC or tSDCS stimulation. Similarly, if the anodal or cathodal DC stimulation was superimposed on 5, 10, and 15 Hz AC stimulation, the MEP amplitudes did not change significantly.
Transcranial application of weak AC current may appear to be a tool for basic and clinical research in diseases with altered EEG activity. However, its effect seems to be weaker than tDCS stimulation, at least in the present context of stimulus intensity and duration. Further studies are required to extend cautiously the safety range and uncover its influence on neuronal circuitries.

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    • "Although a subjective measure, the threshold for evoking phosphenes can be reliably measured and changes in the threshold for evoking phosphenes provides an indication of changes in the excitability (plasticity) of the visual cortex (Cowey & Walsh, 2000; Pascual-Leone & Walsh, 2001). Modulations in phosphene threshold have been observed following application of rTMS to the occipital cortex (likely V1/V2), and modulations in the direction of moving phosphenes evoked by TMS to the motion-selective area V5 have been observed following visual motion adaptation (Antal et al., 2002; Boroojerdi, Prager, Muellbacher, & Cohen, 2000; Cattaneo & Silvanto, 2008; Guzman-Lopez, Silvanto, & Seemungal, 2011; Ray, Meador, Epstein, Loring, & Day, 1998). Secondly, it is possible to investigate plasticity induction by quantifying changes in evoked potentials generated in the target cortical region. "
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    ABSTRACT: The last couple of decades have seen the development of a number of non-invasive brain stimulation (NIBS) techniques that are capable of inducing short-lasting plasticity in the human cortex. Importantly, the induction of lasting plastic changes can, under some conditions, reversibly modify behaviour and interact with learning. These techniques have provided novel opportunities to study human cortical plasticity and examine the role of cortical regions in behaviour. In this review we briefly summarise current NIBS techniques, outline approaches to characterise and quantify cortical plastic change, and describe mechanisms that are implicated in the induced plastic changes. We then outline the areas in which these techniques might be useful, namely, investigating the mechanisms of human cortical plasticity, the characterisation of influences on plasticity, and the investigation of the role of cortical regions in behaviour. Finally, we conclude by highlighting some current limitations of the techniques and suggest that further development of the current NIBS paradigms and more focussed targeting should further enhance the utility of these powerful non-invasive techniques for the investigation of the cortical plasticity and pathophysiology.
    Cortex 09/2014; DOI:10.1016/j.cortex.2013.12.006 · 5.13 Impact Factor
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    • "Transcranial alternating current stimulation (tACS), which usually employs a single-frequency sinusoidal current, can modulate and even synchronize with an ongoing brain activity. tACS has been shown to modulate behavioral performance [27]–[30], generate phosphenes [31], and modulate MEPs [27], [32] and EEGs [29], [30], [33]–[36]. tACS variants including phase variations [30], [34] and DC offsets [27], [29] have been proposed. "
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    ABSTRACT: Electric brain stimulations such as transcranial direct current stimulation (tDCS), transcranial random noise stimulation (tRNS), and transcranial alternating current stimulation (tACS) electrophysiologically modulate brain activity and as a result sometimes modulate behavioral performances. These stimulations can be viewed from an engineering standpoint as involving an artificial electric source (DC, noise, or AC) attached to an impedance branch of a distributed parameter circuit. The distributed parameter circuit is an approximation of the brain and includes electric sources (neurons) and impedances (volume conductors). Such a brain model is linear, as is often the case with the electroencephalogram (EEG) forward model. Thus, the above-mentioned current stimulations change the current distribution in the brain depending on the locations of the electric sources in the brain. Now, if the attached artificial electric source were to be replaced with a resistor, or even a negative resistor, the resistor would also change the current distribution in the brain. In light of the superposition theorem, which holds for any linear electric circuit, attaching an electric source is different from attaching a resistor; the resistor affects each active electric source in the brain so as to increase (or decrease in some cases of a negative resistor) the current flowing out from each source. From an electrophysiological standpoint, the attached resistor can only control the extracellular impedance and never causes forced stimulation; we call this technique transcranial extracellular impedance control (tEIC). We conducted a behavioral experiment to evaluate tEIC and found evidence that it had real-time enhancement and depression effects on EEGs and a real-time facilitation effect on reaction times. Thus, tEIC could be another technique to modulate behavioral performance.
    PLoS ONE 07/2014; 9(7):e102834. DOI:10.1371/journal.pone.0102834 · 3.23 Impact Factor
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    • "In tRNS the areas underneath both electrodes are stimulated with a current whose amplitude varies randomly in time within the frequency range of 100–640 Hz (Terney et al., 2008; Ruffini et al., 2013). In tACS, an alternating current (AC) with a pre-determined frequency passes from anodal to cathodal and the frequency is usually set within the EEG frequency spectrum (1–100 Hz) (Antal et al., 2008; Kanai et al., 2010). The protocol for tCS stimulation, especially the anodal and cathodal electrodes location, is usually determined based on neuroimaging findings (e.g., EEG, fMRI) evidencing that a certain region is involved in the target brain function which the researcher wants to modulate. "
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    ABSTRACT: Transcranial current brain stimulation (tCS) is becoming increasingly popular as a non-pharmacological non-invasive neuromodulatory method that alters cortical excitability by applying weak electrical currents to the scalp via a pair of electrodes. Most applications of this technique have focused on enhancing motor and learning skills, as well as a therapeutic agent in neurological and psychiatric disorders. In these applications, similarly to lesion studies, tCS was used to provide a causal link between a function or behaviour and a specific brain region (e.g., primary motor cortex). Nonetheless, complex cognitive functions are known to rely on functionally connected multitude of brain regions with dynamically changing patterns of information flow rather than on isolated areas, which are most commonly targeted in typical tCS experiments. In this review article, we argue in favour of combining tCS method with other neuroimaging techniques (e.g. fMRI, EEG) and by employing state-of-the-art connectivity data analysis techniques (e.g. graph theory) to obtain a deeper understanding of the underlying spatiotemporal dynamics of functional connectivity patterns and cognitive performance. Finally, we discuss the possibilities of these combined techniques to investigate the neural correlates of human creativity and to enhance creativity.
    Frontiers in Systems Neuroscience 07/2014; 8. DOI:10.3389/fnsys.2014.00132
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