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Cortical inhibition and excitation by bilateral transcranial alternating current stimulation
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Restorative Neurology and Neuroscience xx (20xx) x–xx DOI 10.3233/RNN-140411 IOS Press
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Andrea Cancellia,b , Carlo Cottonea,c , Giancarlo Zitoa,d , Marina Di Giorgiod , Patrizio Pasqualettie,f and Franca Tecchioa,f,∗
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Abstract. Purpose: Transcranial electric stimulations (tES) with amplitude-modulated currents are promising tools to enhance neuromodulation effects. It is essential to select the correct cortical targets and inhibitory/excitatory protocols to reverse changes in specific networks. We aimed at assessing the dependence of cortical excitability changes on the current amplitude of 20 Hz transcranial alternating current stimulation (tACS) over the bilateral primary motor cortex. Methods: We chose two amplitude ranges of the stimulations, around 25 A/cm2 and 63 A/cm2 from peak to peak, with three values (at steps of about 2.5%) around each, to generate, respectively, inhibitory and excitatory effects of the primary motor cortex. We checked such changes online through transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs). Results: Cortical excitability changes depended upon current density (p = 0.001). Low current densities decreased MEP amplitudes (inhibition) while high current densities increased them (excitation). Conclusions: tACS targeting bilateral homologous cortical areas can induce online inhibition or excitation as a function of the current density.
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of Electrophysiology for Translational neuroScience (LET’S) – ISTC – CNR, at Department of Neuroscience, Fatebenefratelli Hospital, Rome, Italy b Institute of Neurology, Department of Geriatrics, Neurosciences & Orthopaedics, Catholic University of Sacred Heart, Rome, Italy c Department of Neuroscience and Imaging, G. d’Annunzio University of Chieti – Pescara, Italy d Department of Clinical Neuroscience, Fatebenefratelli Hospital, Rome, Italy e Medical Statistics and Information Technology, Fatebenefratelli Foundation for Health Research and Education, AFaR Division, Rome, Italy f Unit of Neuroimaging, IRCCS San Raffaele Pisana, Rome, Italy
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Keywords: Neuromodulation, transcranial alternating current stimulation (tACS), neuronavigation, motor cortex (M1), superficial current density, personalized electrode
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1. Introduction
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Among neuromodulation techniques, transcranial alternating current stimulation (tACS) induces cor∗ Corresponding
author: Dr. Franca Tecchio, LET’S Laboratory of Electrophysiology for Translational neuroScience, ISTCCNR, Dipartimento di Neuroscienze, Osp. Fatebenefratelli, Isola Tiberina, 00186 Roma, Italy. Tel./Fax: +39 06 6837382; E-mail: [email protected].
tical excitability changes, depending on the current frequency and the target location (Guleyupoglu et al., 2013; Reato et al., 2013; Zaghi et al., 2010). tACS interacts with ongoing brain oscillations, causing facilitator responses of cortical neurons, provided that the frequency is properly delivered with dependence on the actual experimental circumstances, e.g. in idle state (Feurra et al., 2011, 20 Hz) or while executing motor tasks when stimulating M1 (Feurra et al., 2013, 5Hz)
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induced by tACS depending on the current density. In a realistic conductive model, Salvador and colleagues observed an inhomogeneity of the induced currents, in such a way that the magnitude of the current density measured in the depth of the cortical sulci beneath the central region was about twice that measured at the borders of the electrode. In our aforementioned study (Tecchio et al., 2013), to test both S1 and M1 personalized electrodes with the online protocol, we had positioned them slightly apart from the central sulcus to separate the otherwise contiguous electrodes. Thus, although other factors, such as the bilateral nature of the stimulation, cannot be excluded from the explanations for the discrepant results, in our opinion a smaller current reached the M1’s pyramidal neurons in our experiment than in Feurra’s. In the present work, we pursued the double aim of inducing an increased M1 excitability via our bilateral electrode and testing whether or not a bilateral tACS is also able to achieve both inhibition and excitation by applying appropriate current intensities.
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or in lit or darkened environments while stimulating visual areas (Kanai et al., 2008). Several studies confirm that the modulating effects are also topographically dependent because they are observed selectively when the oscillating currents are applied on the scalp overlying the cortical area involved in the task. The personalization of therapeutic interventions is considered worldwide the most promising frontier in overcoming the physiological variability of responses typically displayed by several people subjected to the same treatment. In transcranial electric stimulation (tES) procedures, the electrode focusing on a predetermined target is a relevant parameter susceptible to personalization. In particular, the personalization of specific cortical targeting by an ad hoc MRI-guided neuronavigation procedure has shown promising results (Tecchio et al., 2013). The aim of the present study was to modulate neuronal excitability patterns by targeting a bilateral cortical patch of homologous areas. This procedure consists of shaping and positioning the stimulating electrode according to the three-dimensional reconstruction of each subject’s structural MRI acquired at high spatial resolution. We engineered the stimulating electrodes to modulate the excitability of bilateral primary somatosensory cortices (S1) covering the whole body representation area ((Tecchio et al., 2014), Fig. 1). When searching for an enhancing effect on cortical excitability, an inhibitory (instead of excitatory) response was produced, in contradiction with our expectations (Tecchio et al., 2013). In particular, we had applied the same stimulation parameters (current intensity, frequency and surface area of the electrodes) as in the protocol introduced by Feurra and colleagues (Feurra et al., 2011); and as these authors did, we also measured the online effects induced on the amplitude of motor potentials evoked (MEPs) by the transcranial magnetic stimulation (TMS) through coil positioned over the tACS electrode positioned to cover the bilateral primary motor areas (M1). While Feurra and colleagues found an augmented excitability induced by transcranial alternating current stimulation (tACS) at 20 Hz, we observed a reduction (Tecchio et al., 2013). Among the possible explanations for such discordance, we hypothesize that a reduction in excitability might be due to a much lower current volume reaching the targeted area, thus integrating the results of two previous works (Moliadze et al., 2012, Salvador et al., 2010). Moliadze and colleagues documented that inhibitory vs. excitatory effects can be
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2. Material and methods
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We employed the inhibitory and excitatory current density values of Moliadze and colleagues (Moliadze et al., 2012) to test, by the protocol of (Feurra et al., 2011), whether or not analogous inhibition and excitation effects can be obtained via our bilateral electrode (with tACS at 20 Hz instead of 140 Hz to apply to the online protocol). From (Moliadze et al., 2012), we considered the tACS intensities able to inhibit (0.4 mA) or to facilitate (1.0 mA) MEPs. More precisely, we derived from Fig. 3B that the MEP amplitude ratio (vs. baseline) was 0.84 mA with SD = 0.13 after 0.4 mA stimulation and 1.37 mA with SD = 0.13 after 1.0 mA. Assuming these effect sizes as true, a sample size of eight subjects allows reaching powers of 0.85 mA and 0.99 mA (respectively) to be recognized as significant with a two-sided alpha level set at 0.05. Furthermore, we also performed a power analysis for the changes with respect to sham stimulation. Using once again the protocol of Moliadze et al., we computed that the 0.4 mA intensity induced a relative decrease vs. sham of 0.19 mA with SD = 0.19 and that 1.0 mA intensity induced a relative increase of 0.34 mA with SD = 0.15. Power analysis indicated that a sample of eight subjects
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Eight healthy, right-handed volunteers (five females, age range 25–47 years) with normal neurological examinations and medical history were included in the study, after signing the informed consent approved by the Ethics Committee of ‘San Giovanni Calibita’ Fatebenefratelli Hospital. None of the volunteers had been taking psychoactive drugs for the six months prior to the study.
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Best, Netherlands; MPRAGE contiguous sagittal slices with full brain coverage). The MRI data were elaborated by the SofTaxic Neuronavigation System ver. 2.0 (www.softaxic.com, E.M.S., Bologna, Italy) in order to guide the stereotaxic procedure for the electrodes’ personalization. We projected the central sulcus of each subject on the scalp surface (Tecchio et al., 2013). Parallelograms of 2 cm widths were then constructed on the obtained trace, starting from Cz, and maintained at the same length in the left and right hemispheres to set the electrode area to 35 cm2 . Finally, an individual electrode was obtained by cutting two strips from a sponge and sewing them together into a tube into which a copper wire was inserted for the current transmission. Electrode positioning was obtained by SofTaxic navigation in line with the central sulcus, 1.5 cm anteriorly and 0.5 cm posteriorly. The closing loop (‘reference’) electrode was a rectangle (7 × 10 cm2 ) positioned above Oz (Bikson et al., 2010).
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has a power of 0.68 to detect the inhibition and of 0.95 to detect the excitation. To further test the consistency of excitatory and inhibitory effects, we collected data in small ranges of current (about 2% steps) around the two levels (17.7 uA/cm2 inhibiting and 44.5 uA/cm2 enhancing). In this way, the consistency of the effects around inhibiting and enhancing values will increase the power reducing the standard error.
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Fig. 1. Experimental setup. A) For the 8 subjects, the tACS regional personalized electrodes shaped to target the bilateral individual M1 (see methods). B) The optical neuronavigation system (left) used to position the personalized electrode (right, reference landmarks are visible frontally). C) Experimental setup for the TMS session probing the online effects of tACS. The TMS focal coil overlies the tACS M1 personalized electrode, with its position monitored throughout the session once the OP hot-spot is identified. D) Representation of the tACS/TMS session: 20 Hz tACS stimulation (tACS on) lasted 1.5 min and was interspersed by 3–4 min (tACS off). For each tACS, the TMS was carried out during the 1.5 min of tACS on, and in the previous 1.5 min of tACS off providing the baseline MEPs’ values for the following stimulation. E) The seven tACS conditions, i.e. three levels of low/high current density: Low CD1, CD2, CD3 and High CD1, CD2, CD3 (corresponding to 850, 875, 900, 2175, 2200, 2225 uA respectively) and the sham, were delivered in random order across subjects.
A few days before the experimental session, each subject underwent a structural brain MRI exam with a 1.5 T scanner (Achieva, Philips Medical Systems,
2.4. transcranial Alternating Current Stimulation (tACS) The ‘active’ and ‘reference’ sponge electrodes were previously soaked in a saline solution and the contact
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areas in this frequency band (Engel et al., 2013; Polania et al., 2011, 2012). To probe differential tACS effects depending on current intensity, a single-pulse TMS was performed through a standard focal coil (diameter of each wing = 70 mm) connected to a SuperRapid module (The Magstim Company Ltd, Whitland, UK). Subjects were asked to stay relaxed while sitting comfortably in a reclining chair and looking at a fixed point in front of them with their arms fully relaxed in a natural position and their hands pronated. TMS-MEPs were recorded on the opponens pollicis (OP) of the right hand by surface electrodes in a belly-tendon montage (2.5 cm apart). The hot spot of the right OP muscle was identified while the TMS coil was positioned above the tACS electrode. Thereafter, the coil position was digitized and monitored throughout the session with the SofTaxic neuronavigator (Fig. 1B). For each subject, the TMS was applied at an intensity adjusted to produce OP MEP amplitudes of about 500 V in basal conditions (i.e., TMS applied through the tACS electrode, with tACS off). TMS stimuli were elicited with an inter-stimulus interval randomly changing between 5 s and 7 s (about 15 repetitions for each session), both while each tACS (real or sham) was active and during the last 1.5 min of tACS off, to constitute the baseline of the subsequent stimulation (Fig. 1C).
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surfaces were covered by a conductive gel (Aquasonic 100, Parker, Fairfield, NJ 07004 USA) and affixed to the subject’s head with an elastic net (Fig. 1). Impedances were below 10 k throughout the stimulations. We selected the stimulating current densities according to the results of Moliadze and colleagues (Moliadze et al., 2012). They observed the inhibitory effect for 0.4 mA and the excitatory from 1 mA across an electrode sized 4 × 4 cm2 (with a reference electrode of 6 × 14 cm2 ). This corresponded to a peak current superficial density of 25 A/cm2 and 63 A/cm2 for inhibitory and excitatory effects, respectively. Since our electrode area was 35 cm2 , we selected current intensities around 875 A and about 2200 A respectively. To be more confident about the stability of the effects, we delivered currents at three intensities around each of these two values: 850, 875, 900 A (called Low intensity — CD1, CD2, CD3) and 2175, 2200, 2225 A (High intensity — CD1, CD2, CD3). Stimulations with a sinusoidal modulated current at 20 Hz at the six levels of current amplitudes or a sham were delivered through a current stimulator charged with a battery (Eldith Stimulator by NeuroConn, Ilmenau, Germany). The effective currents (RMS = peak √ amplitude/ 2) were thus 603, 621, 638, 1543, 1560, 1578 A, corresponding to effective superficial current densities around 17.7 and 44.5 A/cm2 . The selected frequency of the alternating current, 20 Hz, was chosen to induce online effects (Feurra et al., 2011; Tecchio et al., 2013). The tACS stimulations lasted 1.5 min and were intermingled by about 3–4 min of tACS off (Fig. 1C). A sham stimulation was also provided (4 s of active stimulation at the beginning and end of each 1.5 min block). To circumvent the intra-session time variable, the 7 tACS blocks (the sham and six current intensities) were randomly ordered across subjects. At debriefing, no subject reported feeling any difference or discomfort across stimulations.
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2.5. Transcranial Magnetic Stimulation (TMS) probing the tACS current intensity-dependent effects We exploited an online protocol (Feurra et al., 2011, Tecchio et al., 2013) using the 20 Hz tACS, since it is efficacious in modulating M1 excitability; it is especially suitable for a bilateral M1 alternating current stimulation, as it takes advantage of the naturally phase-coupled activity of left and right sensorimotor
2.6. Statistical analysis After a logarithmic transformation and performing the Shapiro-Wilk test to check the fit of the Gaussian distribution, the mean transformed MEP amplitudes averaged across all repetitions for each tACS off and on were computed (Tecchio et al., 2008). The ratios of on and off mean values were calculated and submitted to a repeated measure Analysis of Variance (ANOVA) with Stimulation Intensity (Low, High) and Level of Stimulation Intensity (CD1, CD2, CD3) as within-subjects factors. In this design we assessed the direction of the induced changes on the individual small changes around the current densities expected to inhibit and enhance M1 excitability. For this reason we did not include sham at this analysis stage. In this case the comparison among each current intensity and sham was evaluated by a paired two-tailed t-test. To further test the changes induced by the stimulation, a single sample t-test against value 1 was calculated for the 7 tACS conditions.
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The mean TMS intensity was 68 ± 6% of the maximal stimulator output. Shapiro-Wilk tests indicated that no transformed MEP amplitude distributions differed from a Gaussian distribution (p > 0.200 consistently for the 7 stimulation values). Mean MEP amplitudes for all tACS intensities and sham are presented in Table 1, distinguishing the baseline (tACS off) and neuromodulated (tACS on) values for each. The ANOVA showed a clear Stimulation Intensity effect [F(1, 5) = 60.331, p = 0.001]. Post-hoc comparisons indicated that MEP amplitude was reduced with respect to the sham for all the lowest current intensities (Table 2, Fig. 2) while augmented for all the high current intensities (Table 2, Fig. 2). The lack of an interaction effect indicated that the inhibition and excitation were comparable for the three current densities in the low and high level ranges. A single sample t-test comparing the ratios during stimulation with respect to no-effect showed a clear difference with respect to value 1 for all current intensities, except for the sham (Table 2). ANOVA design applied to MEPs at baseline did not show any effect (Stimulation Intensity p > 0.200), denoting the absence of after-effects at any stimulation.
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4.1. Mechanisms enabling tACS-induced cortical inhibition and excitation Evidence shows that sinusoidal tACS modulates the neuronal membrane potential and can entrain neuronal oscillations, for which network interactions seem more crucial than single cell effects. In fact, AC fields can modulate rate and timing of spiking neurons and thus modulate recurrent interaction between neurons (Frohlich and McCormick, 2010). The effects we observed are probably mainly generated by pyramidal cells. A recent theoretical model (Ali et al., 2013) showed that the human brain includes synaptic inhibition randomly connected with pyramidal neurons, inhibitory interneurons and feedback inhibition circuits, and that the transcranial current is mainly injected into the pyramidal neurons, since these cells have elongated somato-dendritic axes that make them more susceptible to applied electric fields than the inhibitory interneurons (Lopez et al., 1991; Radman et al., 2009). Nevertheless, different pyramidal cell subpopulations, such as regular spiking and intrinsic bursting pyramidal cells, can show orthogonal forms of response to modulator events with facilitation or inhibition induced by the same intervention (Jacob et al., 2012). If we consider that these neuronal pools
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The main result of our study is the possibility to induce a reduction or an increase in cortical excitability by a bilateral tES, depending on the stimulating current density. Specifically, we documented the ability to inhibit or enhance M1 excitability during a bilateral tACS at 20Hz.
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Table 1 MEP amplitudes in baseline and during tACS. Mean and standard deviation of the MEP amplitude (V, transformed by natural logarithm) for each delivered current intensity and sham (tACS on) and the corresponding baseline condition (tACS off).
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The same ANOVA design was performed on baseline values to investigate possible current intensity after effects. tACS off MEP amplitudes were logarithmically transformed and submitted to ANOVA instead of the TACS on vs. tACS off ratios, i.e. with the Stimulation Intensity and Level of Stimulation Intensity within-subjects factors.
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The means are also inversely retransformed (Exp(mean)) for comparisons with typical MEP amplitude values.
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Table 2 MEP amplitude stimulation vs. baseline ratios. Ratios of the means of logarithm transformed MEP amplitudes during stimulation (Stim) and in baseline (Bl) with standard deviation in brackets, for the 7 tACS current intensities tACS intensity (V) 850 875 900 2175 2200 2225 Sham
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0.000 0.012 0.000 0.007 0.033 0.011 0.826
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The third column shows the significance p in the single sample twotailed t-test (ss t-test) with respect to value 1. In the fourth column the significance p in the paired sample two-tailed t-test (ps t-test) with respect to sham are reported.
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tioning reduced M1 intracortical facilitation and did not change intracortical inhibition as assessed by a paired-pulse TMS protocol. This aftereffect seems to support changes to the global cortical network via modulation of local circuit neurons. With smaller current densities (peak current density range [1, 6–2,5] A/cm2 ), no changes were induced by tACS at frequencies similar to those of our study (1, 10, 15, 30, 45 Hz, (Antal et al., 2008)). Nevertheless, in that study, a 20 Hz frequency was not applied so we cannot exclude the possibility that inhibitory effects could also be observed by their current densities. Further efforts should be spent integrating non-invasive experimental data in humans with simulations, as well as in-vitro and ex-vivo findings (Schmidt et al., 2014) to better understand physiological mechanisms beneath tACS-induced neuronal excitability inhibition and enhancement. 4.2. Bilateral stimulation We observed that the bilateral M1 stimulation did induce effects, both inhibitory and excitatory, similar to those of monolateral M1 stimulation for similar current densities over the tested areas (present findings compared with (Moliadze et al., 2012)). Although it would be very interesting to test bilateral cortical
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have diverse activation thresholds, we can hypothesize that the inhibitory vs. excitatory effects are accounted for by different pyramidal cells’ sub-populations. To the best of our knowledge, while interventions typically induced cortical excitability enhancement (Chaieb et al., 2011, Feurra et al., 2011, 2013, Kanai et al., 2008), only (Zaghi et al., 2010) in addition to (Moliadze et al., 2012) observed inhibitory tACS effects. Authors (Zaghi et al., 2010) found that 15 Hz tACS (peak current density of 8 A/cm2 ) precondi-
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Fig. 2. Cortical excitability modulation induced by tACS dependent upon current intensity. For each peak current intensity, the single subject values (Left) and mean and the standard deviation (Right) of the two-tailed non-paired t values calculated on the amplitudes (natural log transformed) of the MEP repetitions with the tACS stimulation on and off.
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As opposed to the protocol used by Moliadze and colleagues, which investigated neuromodulation offline, we tested the tACS effects on the ongoing brain activity. In their study the stimulation at each intensity was delivered for 10 minutes and the neuromodulation effects tested immediately following stimulation, and at approximately 5, 10, 20, 30, 40, 50, 60 and 90 min post stimulation. The entire protocol was repeated for each tACS and tRNS intensities (Moliadze et al., 2012). We collected data during the 1.5 min of (20 Hz) tACS stimulation. The consistency of the results of these two studies implies three key concepts. 1. The bilateral transcranial current stimulation induces comparable inhibitory/excitatory effects to the monolateral stimulation in the same left M1 area, as long as the same superficial current density is delivered (we adjusted the current intensity in proportion to our 35 cm2 vs. their 16 cm2 electrode). 2. Different electrode configurations and spectral properties of the modulating current (tACS at 20 or 140Hz or tRNS) induce similar increases/decreases in the excitability of cortical pyramidal neurons for similar current superficial densities over the tested M1 area. In particular, (Moliadze et al., 2012) employed a 4 × 4 cm stimulation electrode over the left M1 and the reference electrode was 6 × 14 cm placed over the contralateral orbit. As these authors did, we also opted for a cephalic reference to ensure efficacy independent of the inter-electrode distance (Moliadze et al., 2010) and to be uniform with the conditions of the online protocol (Feurra et al., 2011, 2013; Tecchio et al., 2013). For this same reason, we preferred an occipital positioning instead of the frontal reference of (Moliadze et al., 2012). 3. The protocol introduced by Feurra et al. for assessing online effects is also suitable for testing the stimulation parameters for studies aimed at offline effects, as required in almost all clinical applications in order to obtain long-lasting effects. Notably, although similar effects in terms of inhibition and excitation were found, further attention to the timing of offline effects is required for offline protocols. For example, while amplitude enhancement induced by both tACS and tRNS was observed immediately after the stimulation, the low-density inhibition appeared 20 minutes after
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producing, respectively, amelioration and deterioration of behavioral execution quality in healthy people (Polania et al., 2011, 2012).
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excitability, as well as to further investigate local and inter-hemispheric inhibitory effects, our result discerns and rejects the hypothesis that a bilateral stimulation of a homologous area cancels out any net effect. This hypothesis of no-effect in bilateral M1 stimulation derives from the well-known motor system organization, which also relies on transcallosal inhibition of homologous M1 areas. Thus, one can conceive that trying to simultaneously enhance the excitability of both left and right M1 areas, the excitability increase of left (right) M1 is made negligible by the enhancement of inhibition acted by the right (left) M1, whose excitability is augmented as well. Overall, no such effects were observed. The opposite observation, of left M1 excitability modulated in similar ways by bilateral and monolateral stimulations, further strengthens the notion that the effects are mainly due to direct influences on pyramidal neurons (Artola et al., 1990; Bar-Ilan et al., 2012; Le Roux et al., 2006). In fact, the right (or left) M1 inhibits the left (or right) M1 through projections on local interneurons, so that if we had changed the excitability of these local networks the net effect measured by pyramidal neurons excitability would differ between bilateral and monolateral interventions. In favor of tACS effects induced directly on pyramidal neurons, we can also interpret the consistency of the directions of the neuromodulator effects induced by spectrally diverse interventions (tACS at 20 Hz, tACS 140 Hz or transcranial random noise stimulation — tRNS). This suggests that these interventions mainly act on the same neurons, since interneurons in the cerebral cortex are heterogeneous in biophysical properties (Gupta et al., 2000; Markram et al., 2004), so that they are modulated by different frequencies of stimulation as observed both in animal models (Lu et al., 2007; Sjostrom et al., 2001) and in humans (Benali et al., 2011). A mechanism that can promote the bilateral excitability enhancement that we found is an induced beta band in-phase stimulation of bilateral homologous motor areas, which are naturally coupled in this frequency range (Engel et al., 2001, 2013, Siegel et al., 2012). On the opposite side, the anti-phase intervention able to desynchronize the inter-hemispheric oscillations impaired behavioral performance, conceivably associated with a reduced excitability in homologous areas (Struber et al., 2014). Accordingly, tACS can be successfully used to artificially induce inter-areas coupling and decoupling of brain rhythms, concurrently
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4.5. Neuromodulation aimed at neuronal activity entrainment
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The tDCS study performed by Nitsche and Paulus (Nitsche and Paulus, 2000) documented less efficacious excitability enhancements than with the 20 Hz tACS for similar efficacious current superficial densities (see Fig. 3). Furthermore, 20 Hz tACS induced a greater enhancement than 140 Hz tACS, which was stronger than that induced by tRNS (Moliadze et al.,
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In the direct current stimulations (tDCS), it was observed that the neuromodulation effects decrease with the current densities, so a reference much wider than the stimulation electrode is suggested to reduce the induced effects under the return electrode. In the tACS interventions, the size of the reference electrode should be chosen so that the current density is in the no-effect range, i.e. should not be too large since this will produce inhibitory effects.
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2012). In fact, while the inhibition was quite similar, the enhancement we observed for the higher current was stronger than that found by Moliadze (Fig. 3). Since the efficacious current superficial density was the same, we can conceive that the greater enhancement was due to the better entrainment ability of the 20 Hz than the 140 Hz tACS for M1 neurons. In light of the results found by Feurra and colleagues (Feurra et al., 2011), where the regional excitability modulation displayed a definite dependence on the frequency content (Gal and Marom, 2013), we argue that the neuromodulation intervention can be built to properly entrain the neuronal pool activity of the target area. Once the neuronal ensemble of interest is entrained, the modulation effects can be inverted depending on the current intensity. While networking properties revealed functional connectivity in a state of rest expressed by the correlation of power (Intrinsic Coupling Modes, ICM, (Engel et al., 2013), which was strongest in the gamma frequency range (8–32 Hz) for the sensorimotor structures (Hipp et al., 2012; Tecchio et al., 2008), beta band (in agreement with our results) appeared typical of local activity in motor areas (Pogosyan et al., 2009; Wach et al., 2013) and inter-hemispheric connectivity between homologous areas (Engel et al., 2013). Concurrently, the consistency of the direction of the effects induced by tDCS, tRNS, tACS at 140 Hz, tACS at 20 Hz and monolateral and bilateral electrode configurations seems to suggest that the effects are induced mainly on pyramidal neurons whose excitability is tested through MEPs evoked by supra-motor-threshold TMS protocols.
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stimulation ended in both tACS and tRNS (Moliadze et al., 2012).
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Fig. 3. Literature-based cortical excitability modulation dependent upon current density. A summary of the three works that studied the relation between the current intensity and the cortical excitability was tested by TMS-induced MEP amplitudes. Our data (diamonds) are reported alongside Nitsche & Paulus (2000, triangles) and Moliadze et al. (2012, squares), after the currents were re-calculated in terms of efficacious current superficial densities. The ratios of mean MEPs after/during stimulation and in baseline are plotted. Asterisks indicate significant changes.
4.6. Conclusion We documented that by properly defining the current density it is possible to induce inhibitory or excitatory effects through transcranial alternate current stimulation targeting bilateral M1 areas. Our results indicate that the bilateral stimulation of homologous areas induces common effects with monolateral interventions. The present finding enhances the usefulness of tACS interventions. The ability to invert the neuromodulation effects depending on current properties appears to be especially useful for all tACS and tRNS stimulations, where electrode polarity does not exist since the two electrodes deliver the same stimulation cancelling the distinction between anode and cathode.
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Jacob, V., Petreanu, L., Wright, N., Svoboda, K., & Fox, K. (2012). Regular spiking and intrinsic bursting pyramidal cells show orthogonal forms of experience-dependent plasticity in layer V of barrel cortex. Neuron, 73(2), 391-404.
609
Kanai, R., Chaieb, L., Antal, A., Walsh, V., & Paulus, W. (2008). Frequency-dependent electrical stimulation of the visual cortex. Current Biology : CB, 18(23), 1839-1843.
613
Le Roux, N., Amar, M., Baux, G., & Fossier, P. (2006). Homeostatic control of the excitation-inhibition balance in cortical layer 5 pyramidal neurons. Eur J Neurosci, 24(12), 3507-3518.
616
Lopez, L., Chan, C.Y., Okada, Y.C., & Nicholson, C. (1991). Multimodal characterization of population responses evoked by applied electric field in vitro: Extracellular potential, magnetic evoked field, transmembrane potential, and current-source density analysis. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 11(7), 1998-2010.
619
Lu, J.T., Li, C.Y., Zhao, J.P., Poo, M.M., & Zhang, X.H. (2007). Spike-timing-dependent plasticity of neocortical excitatory synapses on inhibitory interneurons depends on target cell type. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 27(36), 9711-9720.
625
Markram, H., Toledo-Rodriguez, M., Wang, Y., Gupta, A., Silberberg, G., & Wu, C. (2004). Interneurons of the neocortical inhibitory system. Nature Reviews Neuroscience, 5(10), 793-807.
630
Moliadze, V., Antal, A., & Paulus, W. (2010). Electrode-distance dependent after-effects of transcranial direct and random noise stimulation with extracephalic reference electrodes. Clinical Neurophysiology: Official Journal of the International Federation of Clinical Neurophysiology, 121(12), 2165-2171.
634
Moliadze, V., Atalay, D., Antal, A., & Paulus, W. (2012). Close to threshold transcranial electrical stimulation preferentially activates inhibitory networks before switching to excitation with higher intensities. Brain Stimulation, 5(4), 505-511.
640
Nitsche, M.A., & Paulus, W. (2000). Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation. J Physiol, 527(Pt 3), 633-639.
644
Frohlich, F., & McCormick, D.A. (2010). Endogenous electric fields may guide neocortical network activity. Neuron, 67(1), 129-143.
Pogosyan, A., Gaynor, L.D., Eusebio, A., & Brown, P. (2009). Boosting cortical activity at Beta-band frequencies slows movement in humans. Current Biology : CB, 19(19), 1637-1641.
647
Gal, A., & Marom, S. (2013). Entrainment of the intrinsic dynamics of single isolated neurons by natural-like input. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 33(18), 7912-7918.
Polania, R., Nitsche, M.A., Korman, C., Batsikadze, G., & Paulus, W. (2012). The importance of timing in segregated theta phasecoupling for cognitive performance. Current Biology: CB, 22(14), 1314-1318.
650
Guleyupoglu, B., Schestatsky, P., Edwards, D., Fregni, F., & Bikson, M. (2013). Classification of methods in transcranial electri-
Polania, R., Nitsche, M.A., & Paulus, W. (2011). Modulating functional connectivity patterns and topological functional
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Artola, A., Brocher, S., & Singer, W. (1990). Different voltagedependent thresholds for inducing long-term depression and long-term potentiation in slices of rat visual cortex. Nature, 347(6288), 69-72. Bar-Ilan, L., Gidon, A., & Segev, I. (2012). The role of dendritic inhibition in shaping the plasticity of excitatory synapses. Front Neural Circuits, 6, 118. Benali, A., Trippe, J., Weiler, E., Mix, A., Petrasch-Parwez, E., Girzalsky, W., et al. (2011). Theta-burst transcranial magnetic stimulation alters cortical inhibition. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 31(4), 1193-1203. Bikson, M., Datta, A., Rahman, A., & Scaturro, J. (2010). Electrode montages for tDCS and weak transcranial electrical stimulation: Role of “return” electrode’s position and size. Clinical Neurophysiology: Official Journal of the International Federation of Clinical Neurophysiology, 121(12), 1976-1978.
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Hipp, J.F., Hawellek, D.J., Corbetta, M., Siegel, M., & Engel, A.K. (2012). Large-scale cortical correlation structure of spontaneous oscillatory activity. Nature Neuroscience, 15(6), 884-890.
Antal, A., Boros, K., Poreisz, C., Chaieb, L., Terney, D., & Paulus, W. (2008). Comparatively weak after-effects of transcranial alternating current stimulation (tACS) on cortical excitability in humans. Brain Stimulation, 1(2), 97-105.
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548
602
dA
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Gupta, A., Wang, Y., & Markram, H. (2000). Organizing principles for a diversity of GABAergic interneurons and synapses in the neocortex. Science, 287(5451), 273-278.
Ali, M.M., Sellers, K.K., & Frohlich, F. (2013). Transcranial alternating current stimulation modulates large-scale cortical network activity by network resonance. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 33(27), 11262-11275.
cte
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Chaieb, L., Antal, A., & Paulus, W. (2011). Transcranial alternating current stimulation in the low kHz range increases motor cortex excitability. Restorative Neurology and Neuroscience, 29(3), 167-175. Engel, A.K., Fries, P., & Singer, W. (2001). Dynamic predictions: Oscillations and synchrony in top-down processing. Nature Reviews Neuroscience, 2(10), 704-716.
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cal stimulation (tES) and evolving strategy from historical approaches to contemporary innovations. Journal of Neuroscience Methods, 219(2), 297-311.
Engel, A.K., Gerloff, C., Hilgetag, C.C., & Nolte, G. (2013). Intrinsic coupling modes: Multiscale interactions in ongoing brain activity. Neuron, 80(4), 867-886.
co
544
References
Feurra, M., Bianco, G., Santarnecchi, E., Del Testa, M., Rossi, A., & Rossi, S. (2011). Frequency-dependent tuning of the human motor system induced by transcranial oscillatory potentials. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 31(34), 12165-12170.
Un
543
9
Feurra, M., Pasqualetti, P., Bianco, G., Santarnecchi, E., Rossi, A., & Rossi, S. (2013). State-dependent effects of transcranial oscillatory currents on the motor system: What you think matters. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 33(44), 17483-17489.
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Reato, D., Rahman, A., Bikson, M., & Parra, L.C. (2013). Effects of weak transcranial alternating current stimulation on brain activity-a review of known mechanisms from animal studies. Frontiers in Human Neuroscience, 7, 687. Salvador, R., Mekonnen, A., Ruffini, G., & Miranda, P.C. (2010). Modeling the electric field induced in a high resolution realistic head model during transcranial current stimulation. Conf Proc IEEE Eng Med Biol Soc, 2010, 2073-2076. Schmidt, S.L., Iyengar, A.K., Foulser, A.A., Boyle, M.R., & Frohlich, F. (2014). Endogenous cortical oscillations constrain neuromodulation by weak electric fields. Brain Stimulation). Siegel, M., Donner, T.H., & Engel, A.K. (2012). Spectral fingerprints of large-scale neuronal interactions. Nature Reviews Neuroscience, 13(2), 121-134. Sjostrom, P.J., Turrigiano, G.G., & Nelson, S.B. (2001). Rate, timing, and cooperativity jointly determine cortical synaptic plasticity. Neuron, 32(6), 1149-1164.
Tecchio, F., Cancelli, A., Cottone, C., Zito, G., Pasqualetti, P., Ghazaryan, A., et al. (2014). Multiple sclerosis fatigue relief by bilateral somatosensory cortex neuromodulation. Journal of Neurology, 261(8), 1552-1558.
687
Tecchio, F., Zappasodi, F., Porcaro, C., Barbati, G., Assenza, G., Salustri, C., et al. (2008). High-gamma band activity of primary hand cortical areas: A sensorimotor feedback efficiency index. NeuroImage, 40(1), 256-264.
691
Wach, C., Krause, V., Moliadze, V., Paulus, W., Schnitzler, A., & Pollok, B. (2013). Effects of 10 Hz and 20 Hz transcranial alternating current stimulation (tACS) on motor functions and motor cortical excitability. Behavioural Brain Research, 241, 1-6.
695
Zaghi, S., Acar, M., Hultgren, B., Boggio, P.S., & Fregni, F. (2010). Noninvasive brain stimulation with low-intensity electrical currents: Putative mechanisms of action for direct and alternating current stimulation. The Neuroscientist: A Review Journal Bringing Neurobiology, Neurology and Psychiatry, 16(3), 285-307.
699
Zaghi, S., de Freitas Rezende, L., de Oliveira, L.M., El-Nazer, R., Menning, S., Tadini, L. et al. (2010). Inhibition of motor cortex excitability with 15Hz transcranial alternating current stimulation (tACS). Neuroscience Letters, 479(3), 211-214.
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roo f
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Tecchio, F., Cancelli, A., Cottone, C., Tomasevic, L., Devigus, B., Zito, G., et al. (2013). Regional personalized electrodes to select transcranial current stimulation target. Frontiers in Human Neuroscience, 7, 131.
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Struber, D., Rach, S., Trautmann-Lengsfeld, S.A., Engel, A.K., & Herrmann, C.S. (2014). Antiphasic 40 Hz oscillatory current stimulation affects bistable motion perception. Brain Topography, 27(1), 158-171.
cte
659
Radman, T., Ramos, R.L., Brumberg, J.C., & Bikson, M. (2009). Role of cortical cell type and morphology in subthreshold and suprathreshold uniform electric field stimulation in vitro. Brain Stimulation, 2(4), 215-228, 28, e1-e3.
rre
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organization of the human brain with transcranial direct current stimulation. Human Brain Mapping, 32(8), 1236-1249.
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Cortical inhibition and excitation by bilateral transcranial alternating current stimulation
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Restorative Neurology and Neuroscience xx (20xx) x–xx DOI 10.3233/RNN-140411 IOS Press
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Andrea Cancellia,b , Carlo Cottonea,c , Giancarlo Zitoa,d , Marina Di Giorgiod , Patrizio Pasqualettie,f and Franca Tecchioa,f,∗
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Abstract. Purpose: Transcranial electric stimulations (tES) with amplitude-modulated currents are promising tools to enhance neuromodulation effects. It is essential to select the correct cortical targets and inhibitory/excitatory protocols to reverse changes in specific networks. We aimed at assessing the dependence of cortical excitability changes on the current amplitude of 20 Hz transcranial alternating current stimulation (tACS) over the bilateral primary motor cortex. Methods: We chose two amplitude ranges of the stimulations, around 25 A/cm2 and 63 A/cm2 from peak to peak, with three values (at steps of about 2.5%) around each, to generate, respectively, inhibitory and excitatory effects of the primary motor cortex. We checked such changes online through transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs). Results: Cortical excitability changes depended upon current density (p = 0.001). Low current densities decreased MEP amplitudes (inhibition) while high current densities increased them (excitation). Conclusions: tACS targeting bilateral homologous cortical areas can induce online inhibition or excitation as a function of the current density.
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of Electrophysiology for Translational neuroScience (LET’S) – ISTC – CNR, at Department of Neuroscience, Fatebenefratelli Hospital, Rome, Italy b Institute of Neurology, Department of Geriatrics, Neurosciences & Orthopaedics, Catholic University of Sacred Heart, Rome, Italy c Department of Neuroscience and Imaging, G. d’Annunzio University of Chieti – Pescara, Italy d Department of Clinical Neuroscience, Fatebenefratelli Hospital, Rome, Italy e Medical Statistics and Information Technology, Fatebenefratelli Foundation for Health Research and Education, AFaR Division, Rome, Italy f Unit of Neuroimaging, IRCCS San Raffaele Pisana, Rome, Italy
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Keywords: Neuromodulation, transcranial alternating current stimulation (tACS), neuronavigation, motor cortex (M1), superficial current density, personalized electrode
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1. Introduction
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Among neuromodulation techniques, transcranial alternating current stimulation (tACS) induces cor∗ Corresponding
author: Dr. Franca Tecchio, LET’S Laboratory of Electrophysiology for Translational neuroScience, ISTCCNR, Dipartimento di Neuroscienze, Osp. Fatebenefratelli, Isola Tiberina, 00186 Roma, Italy. Tel./Fax: +39 06 6837382; E-mail: [email protected].
tical excitability changes, depending on the current frequency and the target location (Guleyupoglu et al., 2013; Reato et al., 2013; Zaghi et al., 2010). tACS interacts with ongoing brain oscillations, causing facilitator responses of cortical neurons, provided that the frequency is properly delivered with dependence on the actual experimental circumstances, e.g. in idle state (Feurra et al., 2011, 20 Hz) or while executing motor tasks when stimulating M1 (Feurra et al., 2013, 5Hz)
0922-6028/15/$27.50 © 2015 – IOS Press and the authors. All rights reserved
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induced by tACS depending on the current density. In a realistic conductive model, Salvador and colleagues observed an inhomogeneity of the induced currents, in such a way that the magnitude of the current density measured in the depth of the cortical sulci beneath the central region was about twice that measured at the borders of the electrode. In our aforementioned study (Tecchio et al., 2013), to test both S1 and M1 personalized electrodes with the online protocol, we had positioned them slightly apart from the central sulcus to separate the otherwise contiguous electrodes. Thus, although other factors, such as the bilateral nature of the stimulation, cannot be excluded from the explanations for the discrepant results, in our opinion a smaller current reached the M1’s pyramidal neurons in our experiment than in Feurra’s. In the present work, we pursued the double aim of inducing an increased M1 excitability via our bilateral electrode and testing whether or not a bilateral tACS is also able to achieve both inhibition and excitation by applying appropriate current intensities.
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or in lit or darkened environments while stimulating visual areas (Kanai et al., 2008). Several studies confirm that the modulating effects are also topographically dependent because they are observed selectively when the oscillating currents are applied on the scalp overlying the cortical area involved in the task. The personalization of therapeutic interventions is considered worldwide the most promising frontier in overcoming the physiological variability of responses typically displayed by several people subjected to the same treatment. In transcranial electric stimulation (tES) procedures, the electrode focusing on a predetermined target is a relevant parameter susceptible to personalization. In particular, the personalization of specific cortical targeting by an ad hoc MRI-guided neuronavigation procedure has shown promising results (Tecchio et al., 2013). The aim of the present study was to modulate neuronal excitability patterns by targeting a bilateral cortical patch of homologous areas. This procedure consists of shaping and positioning the stimulating electrode according to the three-dimensional reconstruction of each subject’s structural MRI acquired at high spatial resolution. We engineered the stimulating electrodes to modulate the excitability of bilateral primary somatosensory cortices (S1) covering the whole body representation area ((Tecchio et al., 2014), Fig. 1). When searching for an enhancing effect on cortical excitability, an inhibitory (instead of excitatory) response was produced, in contradiction with our expectations (Tecchio et al., 2013). In particular, we had applied the same stimulation parameters (current intensity, frequency and surface area of the electrodes) as in the protocol introduced by Feurra and colleagues (Feurra et al., 2011); and as these authors did, we also measured the online effects induced on the amplitude of motor potentials evoked (MEPs) by the transcranial magnetic stimulation (TMS) through coil positioned over the tACS electrode positioned to cover the bilateral primary motor areas (M1). While Feurra and colleagues found an augmented excitability induced by transcranial alternating current stimulation (tACS) at 20 Hz, we observed a reduction (Tecchio et al., 2013). Among the possible explanations for such discordance, we hypothesize that a reduction in excitability might be due to a much lower current volume reaching the targeted area, thus integrating the results of two previous works (Moliadze et al., 2012, Salvador et al., 2010). Moliadze and colleagues documented that inhibitory vs. excitatory effects can be
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2. Material and methods
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We employed the inhibitory and excitatory current density values of Moliadze and colleagues (Moliadze et al., 2012) to test, by the protocol of (Feurra et al., 2011), whether or not analogous inhibition and excitation effects can be obtained via our bilateral electrode (with tACS at 20 Hz instead of 140 Hz to apply to the online protocol). From (Moliadze et al., 2012), we considered the tACS intensities able to inhibit (0.4 mA) or to facilitate (1.0 mA) MEPs. More precisely, we derived from Fig. 3B that the MEP amplitude ratio (vs. baseline) was 0.84 mA with SD = 0.13 after 0.4 mA stimulation and 1.37 mA with SD = 0.13 after 1.0 mA. Assuming these effect sizes as true, a sample size of eight subjects allows reaching powers of 0.85 mA and 0.99 mA (respectively) to be recognized as significant with a two-sided alpha level set at 0.05. Furthermore, we also performed a power analysis for the changes with respect to sham stimulation. Using once again the protocol of Moliadze et al., we computed that the 0.4 mA intensity induced a relative decrease vs. sham of 0.19 mA with SD = 0.19 and that 1.0 mA intensity induced a relative increase of 0.34 mA with SD = 0.15. Power analysis indicated that a sample of eight subjects
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Eight healthy, right-handed volunteers (five females, age range 25–47 years) with normal neurological examinations and medical history were included in the study, after signing the informed consent approved by the Ethics Committee of ‘San Giovanni Calibita’ Fatebenefratelli Hospital. None of the volunteers had been taking psychoactive drugs for the six months prior to the study.
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Best, Netherlands; MPRAGE contiguous sagittal slices with full brain coverage). The MRI data were elaborated by the SofTaxic Neuronavigation System ver. 2.0 (www.softaxic.com, E.M.S., Bologna, Italy) in order to guide the stereotaxic procedure for the electrodes’ personalization. We projected the central sulcus of each subject on the scalp surface (Tecchio et al., 2013). Parallelograms of 2 cm widths were then constructed on the obtained trace, starting from Cz, and maintained at the same length in the left and right hemispheres to set the electrode area to 35 cm2 . Finally, an individual electrode was obtained by cutting two strips from a sponge and sewing them together into a tube into which a copper wire was inserted for the current transmission. Electrode positioning was obtained by SofTaxic navigation in line with the central sulcus, 1.5 cm anteriorly and 0.5 cm posteriorly. The closing loop (‘reference’) electrode was a rectangle (7 × 10 cm2 ) positioned above Oz (Bikson et al., 2010).
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has a power of 0.68 to detect the inhibition and of 0.95 to detect the excitation. To further test the consistency of excitatory and inhibitory effects, we collected data in small ranges of current (about 2% steps) around the two levels (17.7 uA/cm2 inhibiting and 44.5 uA/cm2 enhancing). In this way, the consistency of the effects around inhibiting and enhancing values will increase the power reducing the standard error.
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Fig. 1. Experimental setup. A) For the 8 subjects, the tACS regional personalized electrodes shaped to target the bilateral individual M1 (see methods). B) The optical neuronavigation system (left) used to position the personalized electrode (right, reference landmarks are visible frontally). C) Experimental setup for the TMS session probing the online effects of tACS. The TMS focal coil overlies the tACS M1 personalized electrode, with its position monitored throughout the session once the OP hot-spot is identified. D) Representation of the tACS/TMS session: 20 Hz tACS stimulation (tACS on) lasted 1.5 min and was interspersed by 3–4 min (tACS off). For each tACS, the TMS was carried out during the 1.5 min of tACS on, and in the previous 1.5 min of tACS off providing the baseline MEPs’ values for the following stimulation. E) The seven tACS conditions, i.e. three levels of low/high current density: Low CD1, CD2, CD3 and High CD1, CD2, CD3 (corresponding to 850, 875, 900, 2175, 2200, 2225 uA respectively) and the sham, were delivered in random order across subjects.
A few days before the experimental session, each subject underwent a structural brain MRI exam with a 1.5 T scanner (Achieva, Philips Medical Systems,
2.4. transcranial Alternating Current Stimulation (tACS) The ‘active’ and ‘reference’ sponge electrodes were previously soaked in a saline solution and the contact
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areas in this frequency band (Engel et al., 2013; Polania et al., 2011, 2012). To probe differential tACS effects depending on current intensity, a single-pulse TMS was performed through a standard focal coil (diameter of each wing = 70 mm) connected to a SuperRapid module (The Magstim Company Ltd, Whitland, UK). Subjects were asked to stay relaxed while sitting comfortably in a reclining chair and looking at a fixed point in front of them with their arms fully relaxed in a natural position and their hands pronated. TMS-MEPs were recorded on the opponens pollicis (OP) of the right hand by surface electrodes in a belly-tendon montage (2.5 cm apart). The hot spot of the right OP muscle was identified while the TMS coil was positioned above the tACS electrode. Thereafter, the coil position was digitized and monitored throughout the session with the SofTaxic neuronavigator (Fig. 1B). For each subject, the TMS was applied at an intensity adjusted to produce OP MEP amplitudes of about 500 V in basal conditions (i.e., TMS applied through the tACS electrode, with tACS off). TMS stimuli were elicited with an inter-stimulus interval randomly changing between 5 s and 7 s (about 15 repetitions for each session), both while each tACS (real or sham) was active and during the last 1.5 min of tACS off, to constitute the baseline of the subsequent stimulation (Fig. 1C).
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surfaces were covered by a conductive gel (Aquasonic 100, Parker, Fairfield, NJ 07004 USA) and affixed to the subject’s head with an elastic net (Fig. 1). Impedances were below 10 k throughout the stimulations. We selected the stimulating current densities according to the results of Moliadze and colleagues (Moliadze et al., 2012). They observed the inhibitory effect for 0.4 mA and the excitatory from 1 mA across an electrode sized 4 × 4 cm2 (with a reference electrode of 6 × 14 cm2 ). This corresponded to a peak current superficial density of 25 A/cm2 and 63 A/cm2 for inhibitory and excitatory effects, respectively. Since our electrode area was 35 cm2 , we selected current intensities around 875 A and about 2200 A respectively. To be more confident about the stability of the effects, we delivered currents at three intensities around each of these two values: 850, 875, 900 A (called Low intensity — CD1, CD2, CD3) and 2175, 2200, 2225 A (High intensity — CD1, CD2, CD3). Stimulations with a sinusoidal modulated current at 20 Hz at the six levels of current amplitudes or a sham were delivered through a current stimulator charged with a battery (Eldith Stimulator by NeuroConn, Ilmenau, Germany). The effective currents (RMS = peak √ amplitude/ 2) were thus 603, 621, 638, 1543, 1560, 1578 A, corresponding to effective superficial current densities around 17.7 and 44.5 A/cm2 . The selected frequency of the alternating current, 20 Hz, was chosen to induce online effects (Feurra et al., 2011; Tecchio et al., 2013). The tACS stimulations lasted 1.5 min and were intermingled by about 3–4 min of tACS off (Fig. 1C). A sham stimulation was also provided (4 s of active stimulation at the beginning and end of each 1.5 min block). To circumvent the intra-session time variable, the 7 tACS blocks (the sham and six current intensities) were randomly ordered across subjects. At debriefing, no subject reported feeling any difference or discomfort across stimulations.
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2.5. Transcranial Magnetic Stimulation (TMS) probing the tACS current intensity-dependent effects We exploited an online protocol (Feurra et al., 2011, Tecchio et al., 2013) using the 20 Hz tACS, since it is efficacious in modulating M1 excitability; it is especially suitable for a bilateral M1 alternating current stimulation, as it takes advantage of the naturally phase-coupled activity of left and right sensorimotor
2.6. Statistical analysis After a logarithmic transformation and performing the Shapiro-Wilk test to check the fit of the Gaussian distribution, the mean transformed MEP amplitudes averaged across all repetitions for each tACS off and on were computed (Tecchio et al., 2008). The ratios of on and off mean values were calculated and submitted to a repeated measure Analysis of Variance (ANOVA) with Stimulation Intensity (Low, High) and Level of Stimulation Intensity (CD1, CD2, CD3) as within-subjects factors. In this design we assessed the direction of the induced changes on the individual small changes around the current densities expected to inhibit and enhance M1 excitability. For this reason we did not include sham at this analysis stage. In this case the comparison among each current intensity and sham was evaluated by a paired two-tailed t-test. To further test the changes induced by the stimulation, a single sample t-test against value 1 was calculated for the 7 tACS conditions.
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The mean TMS intensity was 68 ± 6% of the maximal stimulator output. Shapiro-Wilk tests indicated that no transformed MEP amplitude distributions differed from a Gaussian distribution (p > 0.200 consistently for the 7 stimulation values). Mean MEP amplitudes for all tACS intensities and sham are presented in Table 1, distinguishing the baseline (tACS off) and neuromodulated (tACS on) values for each. The ANOVA showed a clear Stimulation Intensity effect [F(1, 5) = 60.331, p = 0.001]. Post-hoc comparisons indicated that MEP amplitude was reduced with respect to the sham for all the lowest current intensities (Table 2, Fig. 2) while augmented for all the high current intensities (Table 2, Fig. 2). The lack of an interaction effect indicated that the inhibition and excitation were comparable for the three current densities in the low and high level ranges. A single sample t-test comparing the ratios during stimulation with respect to no-effect showed a clear difference with respect to value 1 for all current intensities, except for the sham (Table 2). ANOVA design applied to MEPs at baseline did not show any effect (Stimulation Intensity p > 0.200), denoting the absence of after-effects at any stimulation.
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4.1. Mechanisms enabling tACS-induced cortical inhibition and excitation Evidence shows that sinusoidal tACS modulates the neuronal membrane potential and can entrain neuronal oscillations, for which network interactions seem more crucial than single cell effects. In fact, AC fields can modulate rate and timing of spiking neurons and thus modulate recurrent interaction between neurons (Frohlich and McCormick, 2010). The effects we observed are probably mainly generated by pyramidal cells. A recent theoretical model (Ali et al., 2013) showed that the human brain includes synaptic inhibition randomly connected with pyramidal neurons, inhibitory interneurons and feedback inhibition circuits, and that the transcranial current is mainly injected into the pyramidal neurons, since these cells have elongated somato-dendritic axes that make them more susceptible to applied electric fields than the inhibitory interneurons (Lopez et al., 1991; Radman et al., 2009). Nevertheless, different pyramidal cell subpopulations, such as regular spiking and intrinsic bursting pyramidal cells, can show orthogonal forms of response to modulator events with facilitation or inhibition induced by the same intervention (Jacob et al., 2012). If we consider that these neuronal pools
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The main result of our study is the possibility to induce a reduction or an increase in cortical excitability by a bilateral tES, depending on the stimulating current density. Specifically, we documented the ability to inhibit or enhance M1 excitability during a bilateral tACS at 20Hz.
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Table 1 MEP amplitudes in baseline and during tACS. Mean and standard deviation of the MEP amplitude (V, transformed by natural logarithm) for each delivered current intensity and sham (tACS on) and the corresponding baseline condition (tACS off).
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4. Discussion
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The same ANOVA design was performed on baseline values to investigate possible current intensity after effects. tACS off MEP amplitudes were logarithmically transformed and submitted to ANOVA instead of the TACS on vs. tACS off ratios, i.e. with the Stimulation Intensity and Level of Stimulation Intensity within-subjects factors.
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tACS peak current intensity (A) 2200
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Efficacious current superficial density
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678.6 6.52 0.79
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820.6 6.71 0.82
652 6.48 0.88
454.9 6.12 0.95
478.2 6.17 0.99
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413.6 6.02 0.88
The means are also inversely retransformed (Exp(mean)) for comparisons with typical MEP amplitude values.
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Table 2 MEP amplitude stimulation vs. baseline ratios. Ratios of the means of logarithm transformed MEP amplitudes during stimulation (Stim) and in baseline (Bl) with standard deviation in brackets, for the 7 tACS current intensities tACS intensity (V) 850 875 900 2175 2200 2225 Sham
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ps t-test p (vs. sham)
0.61−0.13 0.75−0.21 0.66−0.1 1.96−0.72 1.81−0.86 1.57−0.47 0.98−0.19
0.000 0.012 0.000 0.007 0.033 0.011 0.826
0.007 0.065 0.019 0.006 0.018 0.034 –
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The third column shows the significance p in the single sample twotailed t-test (ss t-test) with respect to value 1. In the fourth column the significance p in the paired sample two-tailed t-test (ps t-test) with respect to sham are reported.
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tioning reduced M1 intracortical facilitation and did not change intracortical inhibition as assessed by a paired-pulse TMS protocol. This aftereffect seems to support changes to the global cortical network via modulation of local circuit neurons. With smaller current densities (peak current density range [1, 6–2,5] A/cm2 ), no changes were induced by tACS at frequencies similar to those of our study (1, 10, 15, 30, 45 Hz, (Antal et al., 2008)). Nevertheless, in that study, a 20 Hz frequency was not applied so we cannot exclude the possibility that inhibitory effects could also be observed by their current densities. Further efforts should be spent integrating non-invasive experimental data in humans with simulations, as well as in-vitro and ex-vivo findings (Schmidt et al., 2014) to better understand physiological mechanisms beneath tACS-induced neuronal excitability inhibition and enhancement. 4.2. Bilateral stimulation We observed that the bilateral M1 stimulation did induce effects, both inhibitory and excitatory, similar to those of monolateral M1 stimulation for similar current densities over the tested areas (present findings compared with (Moliadze et al., 2012)). Although it would be very interesting to test bilateral cortical
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have diverse activation thresholds, we can hypothesize that the inhibitory vs. excitatory effects are accounted for by different pyramidal cells’ sub-populations. To the best of our knowledge, while interventions typically induced cortical excitability enhancement (Chaieb et al., 2011, Feurra et al., 2011, 2013, Kanai et al., 2008), only (Zaghi et al., 2010) in addition to (Moliadze et al., 2012) observed inhibitory tACS effects. Authors (Zaghi et al., 2010) found that 15 Hz tACS (peak current density of 8 A/cm2 ) precondi-
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Fig. 2. Cortical excitability modulation induced by tACS dependent upon current intensity. For each peak current intensity, the single subject values (Left) and mean and the standard deviation (Right) of the two-tailed non-paired t values calculated on the amplitudes (natural log transformed) of the MEP repetitions with the tACS stimulation on and off.
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As opposed to the protocol used by Moliadze and colleagues, which investigated neuromodulation offline, we tested the tACS effects on the ongoing brain activity. In their study the stimulation at each intensity was delivered for 10 minutes and the neuromodulation effects tested immediately following stimulation, and at approximately 5, 10, 20, 30, 40, 50, 60 and 90 min post stimulation. The entire protocol was repeated for each tACS and tRNS intensities (Moliadze et al., 2012). We collected data during the 1.5 min of (20 Hz) tACS stimulation. The consistency of the results of these two studies implies three key concepts. 1. The bilateral transcranial current stimulation induces comparable inhibitory/excitatory effects to the monolateral stimulation in the same left M1 area, as long as the same superficial current density is delivered (we adjusted the current intensity in proportion to our 35 cm2 vs. their 16 cm2 electrode). 2. Different electrode configurations and spectral properties of the modulating current (tACS at 20 or 140Hz or tRNS) induce similar increases/decreases in the excitability of cortical pyramidal neurons for similar current superficial densities over the tested M1 area. In particular, (Moliadze et al., 2012) employed a 4 × 4 cm stimulation electrode over the left M1 and the reference electrode was 6 × 14 cm placed over the contralateral orbit. As these authors did, we also opted for a cephalic reference to ensure efficacy independent of the inter-electrode distance (Moliadze et al., 2010) and to be uniform with the conditions of the online protocol (Feurra et al., 2011, 2013; Tecchio et al., 2013). For this same reason, we preferred an occipital positioning instead of the frontal reference of (Moliadze et al., 2012). 3. The protocol introduced by Feurra et al. for assessing online effects is also suitable for testing the stimulation parameters for studies aimed at offline effects, as required in almost all clinical applications in order to obtain long-lasting effects. Notably, although similar effects in terms of inhibition and excitation were found, further attention to the timing of offline effects is required for offline protocols. For example, while amplitude enhancement induced by both tACS and tRNS was observed immediately after the stimulation, the low-density inhibition appeared 20 minutes after
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producing, respectively, amelioration and deterioration of behavioral execution quality in healthy people (Polania et al., 2011, 2012).
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excitability, as well as to further investigate local and inter-hemispheric inhibitory effects, our result discerns and rejects the hypothesis that a bilateral stimulation of a homologous area cancels out any net effect. This hypothesis of no-effect in bilateral M1 stimulation derives from the well-known motor system organization, which also relies on transcallosal inhibition of homologous M1 areas. Thus, one can conceive that trying to simultaneously enhance the excitability of both left and right M1 areas, the excitability increase of left (right) M1 is made negligible by the enhancement of inhibition acted by the right (left) M1, whose excitability is augmented as well. Overall, no such effects were observed. The opposite observation, of left M1 excitability modulated in similar ways by bilateral and monolateral stimulations, further strengthens the notion that the effects are mainly due to direct influences on pyramidal neurons (Artola et al., 1990; Bar-Ilan et al., 2012; Le Roux et al., 2006). In fact, the right (or left) M1 inhibits the left (or right) M1 through projections on local interneurons, so that if we had changed the excitability of these local networks the net effect measured by pyramidal neurons excitability would differ between bilateral and monolateral interventions. In favor of tACS effects induced directly on pyramidal neurons, we can also interpret the consistency of the directions of the neuromodulator effects induced by spectrally diverse interventions (tACS at 20 Hz, tACS 140 Hz or transcranial random noise stimulation — tRNS). This suggests that these interventions mainly act on the same neurons, since interneurons in the cerebral cortex are heterogeneous in biophysical properties (Gupta et al., 2000; Markram et al., 2004), so that they are modulated by different frequencies of stimulation as observed both in animal models (Lu et al., 2007; Sjostrom et al., 2001) and in humans (Benali et al., 2011). A mechanism that can promote the bilateral excitability enhancement that we found is an induced beta band in-phase stimulation of bilateral homologous motor areas, which are naturally coupled in this frequency range (Engel et al., 2001, 2013, Siegel et al., 2012). On the opposite side, the anti-phase intervention able to desynchronize the inter-hemispheric oscillations impaired behavioral performance, conceivably associated with a reduced excitability in homologous areas (Struber et al., 2014). Accordingly, tACS can be successfully used to artificially induce inter-areas coupling and decoupling of brain rhythms, concurrently
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The tDCS study performed by Nitsche and Paulus (Nitsche and Paulus, 2000) documented less efficacious excitability enhancements than with the 20 Hz tACS for similar efficacious current superficial densities (see Fig. 3). Furthermore, 20 Hz tACS induced a greater enhancement than 140 Hz tACS, which was stronger than that induced by tRNS (Moliadze et al.,
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In the direct current stimulations (tDCS), it was observed that the neuromodulation effects decrease with the current densities, so a reference much wider than the stimulation electrode is suggested to reduce the induced effects under the return electrode. In the tACS interventions, the size of the reference electrode should be chosen so that the current density is in the no-effect range, i.e. should not be too large since this will produce inhibitory effects.
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2012). In fact, while the inhibition was quite similar, the enhancement we observed for the higher current was stronger than that found by Moliadze (Fig. 3). Since the efficacious current superficial density was the same, we can conceive that the greater enhancement was due to the better entrainment ability of the 20 Hz than the 140 Hz tACS for M1 neurons. In light of the results found by Feurra and colleagues (Feurra et al., 2011), where the regional excitability modulation displayed a definite dependence on the frequency content (Gal and Marom, 2013), we argue that the neuromodulation intervention can be built to properly entrain the neuronal pool activity of the target area. Once the neuronal ensemble of interest is entrained, the modulation effects can be inverted depending on the current intensity. While networking properties revealed functional connectivity in a state of rest expressed by the correlation of power (Intrinsic Coupling Modes, ICM, (Engel et al., 2013), which was strongest in the gamma frequency range (8–32 Hz) for the sensorimotor structures (Hipp et al., 2012; Tecchio et al., 2008), beta band (in agreement with our results) appeared typical of local activity in motor areas (Pogosyan et al., 2009; Wach et al., 2013) and inter-hemispheric connectivity between homologous areas (Engel et al., 2013). Concurrently, the consistency of the direction of the effects induced by tDCS, tRNS, tACS at 140 Hz, tACS at 20 Hz and monolateral and bilateral electrode configurations seems to suggest that the effects are induced mainly on pyramidal neurons whose excitability is tested through MEPs evoked by supra-motor-threshold TMS protocols.
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stimulation ended in both tACS and tRNS (Moliadze et al., 2012).
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Fig. 3. Literature-based cortical excitability modulation dependent upon current density. A summary of the three works that studied the relation between the current intensity and the cortical excitability was tested by TMS-induced MEP amplitudes. Our data (diamonds) are reported alongside Nitsche & Paulus (2000, triangles) and Moliadze et al. (2012, squares), after the currents were re-calculated in terms of efficacious current superficial densities. The ratios of mean MEPs after/during stimulation and in baseline are plotted. Asterisks indicate significant changes.
4.6. Conclusion We documented that by properly defining the current density it is possible to induce inhibitory or excitatory effects through transcranial alternate current stimulation targeting bilateral M1 areas. Our results indicate that the bilateral stimulation of homologous areas induces common effects with monolateral interventions. The present finding enhances the usefulness of tACS interventions. The ability to invert the neuromodulation effects depending on current properties appears to be especially useful for all tACS and tRNS stimulations, where electrode polarity does not exist since the two electrodes deliver the same stimulation cancelling the distinction between anode and cathode.
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605
Jacob, V., Petreanu, L., Wright, N., Svoboda, K., & Fox, K. (2012). Regular spiking and intrinsic bursting pyramidal cells show orthogonal forms of experience-dependent plasticity in layer V of barrel cortex. Neuron, 73(2), 391-404.
609
Kanai, R., Chaieb, L., Antal, A., Walsh, V., & Paulus, W. (2008). Frequency-dependent electrical stimulation of the visual cortex. Current Biology : CB, 18(23), 1839-1843.
613
Le Roux, N., Amar, M., Baux, G., & Fossier, P. (2006). Homeostatic control of the excitation-inhibition balance in cortical layer 5 pyramidal neurons. Eur J Neurosci, 24(12), 3507-3518.
616
Lopez, L., Chan, C.Y., Okada, Y.C., & Nicholson, C. (1991). Multimodal characterization of population responses evoked by applied electric field in vitro: Extracellular potential, magnetic evoked field, transmembrane potential, and current-source density analysis. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 11(7), 1998-2010.
619
Lu, J.T., Li, C.Y., Zhao, J.P., Poo, M.M., & Zhang, X.H. (2007). Spike-timing-dependent plasticity of neocortical excitatory synapses on inhibitory interneurons depends on target cell type. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 27(36), 9711-9720.
625
Markram, H., Toledo-Rodriguez, M., Wang, Y., Gupta, A., Silberberg, G., & Wu, C. (2004). Interneurons of the neocortical inhibitory system. Nature Reviews Neuroscience, 5(10), 793-807.
630
Moliadze, V., Antal, A., & Paulus, W. (2010). Electrode-distance dependent after-effects of transcranial direct and random noise stimulation with extracephalic reference electrodes. Clinical Neurophysiology: Official Journal of the International Federation of Clinical Neurophysiology, 121(12), 2165-2171.
634
Moliadze, V., Atalay, D., Antal, A., & Paulus, W. (2012). Close to threshold transcranial electrical stimulation preferentially activates inhibitory networks before switching to excitation with higher intensities. Brain Stimulation, 5(4), 505-511.
640
Nitsche, M.A., & Paulus, W. (2000). Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation. J Physiol, 527(Pt 3), 633-639.
644
Frohlich, F., & McCormick, D.A. (2010). Endogenous electric fields may guide neocortical network activity. Neuron, 67(1), 129-143.
Pogosyan, A., Gaynor, L.D., Eusebio, A., & Brown, P. (2009). Boosting cortical activity at Beta-band frequencies slows movement in humans. Current Biology : CB, 19(19), 1637-1641.
647
Gal, A., & Marom, S. (2013). Entrainment of the intrinsic dynamics of single isolated neurons by natural-like input. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 33(18), 7912-7918.
Polania, R., Nitsche, M.A., Korman, C., Batsikadze, G., & Paulus, W. (2012). The importance of timing in segregated theta phasecoupling for cognitive performance. Current Biology: CB, 22(14), 1314-1318.
650
Guleyupoglu, B., Schestatsky, P., Edwards, D., Fregni, F., & Bikson, M. (2013). Classification of methods in transcranial electri-
Polania, R., Nitsche, M.A., & Paulus, W. (2011). Modulating functional connectivity patterns and topological functional
654
Artola, A., Brocher, S., & Singer, W. (1990). Different voltagedependent thresholds for inducing long-term depression and long-term potentiation in slices of rat visual cortex. Nature, 347(6288), 69-72. Bar-Ilan, L., Gidon, A., & Segev, I. (2012). The role of dendritic inhibition in shaping the plasticity of excitatory synapses. Front Neural Circuits, 6, 118. Benali, A., Trippe, J., Weiler, E., Mix, A., Petrasch-Parwez, E., Girzalsky, W., et al. (2011). Theta-burst transcranial magnetic stimulation alters cortical inhibition. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 31(4), 1193-1203. Bikson, M., Datta, A., Rahman, A., & Scaturro, J. (2010). Electrode montages for tDCS and weak transcranial electrical stimulation: Role of “return” electrode’s position and size. Clinical Neurophysiology: Official Journal of the International Federation of Clinical Neurophysiology, 121(12), 1976-1978.
roo f
550
Hipp, J.F., Hawellek, D.J., Corbetta, M., Siegel, M., & Engel, A.K. (2012). Large-scale cortical correlation structure of spontaneous oscillatory activity. Nature Neuroscience, 15(6), 884-890.
Antal, A., Boros, K., Poreisz, C., Chaieb, L., Terney, D., & Paulus, W. (2008). Comparatively weak after-effects of transcranial alternating current stimulation (tACS) on cortical excitability in humans. Brain Stimulation, 1(2), 97-105.
or P
549
uth
548
602
dA
547
Gupta, A., Wang, Y., & Markram, H. (2000). Organizing principles for a diversity of GABAergic interneurons and synapses in the neocortex. Science, 287(5451), 273-278.
Ali, M.M., Sellers, K.K., & Frohlich, F. (2013). Transcranial alternating current stimulation modulates large-scale cortical network activity by network resonance. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 33(27), 11262-11275.
cte
546
599
Chaieb, L., Antal, A., & Paulus, W. (2011). Transcranial alternating current stimulation in the low kHz range increases motor cortex excitability. Restorative Neurology and Neuroscience, 29(3), 167-175. Engel, A.K., Fries, P., & Singer, W. (2001). Dynamic predictions: Oscillations and synchrony in top-down processing. Nature Reviews Neuroscience, 2(10), 704-716.
rre
545
cal stimulation (tES) and evolving strategy from historical approaches to contemporary innovations. Journal of Neuroscience Methods, 219(2), 297-311.
Engel, A.K., Gerloff, C., Hilgetag, C.C., & Nolte, G. (2013). Intrinsic coupling modes: Multiscale interactions in ongoing brain activity. Neuron, 80(4), 867-886.
co
544
References
Feurra, M., Bianco, G., Santarnecchi, E., Del Testa, M., Rossi, A., & Rossi, S. (2011). Frequency-dependent tuning of the human motor system induced by transcranial oscillatory potentials. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 31(34), 12165-12170.
Un
543
9
Feurra, M., Pasqualetti, P., Bianco, G., Santarnecchi, E., Rossi, A., & Rossi, S. (2013). State-dependent effects of transcranial oscillatory currents on the motor system: What you think matters. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 33(44), 17483-17489.
600 601
603 604
606 607 608
610 611 612
614 615
617 618
620 621 622 623 624
626 627 628 629
631 632 633
635 636 637 638 639
641 642 643
645 646
648 649
651 652 653
655
663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682
Reato, D., Rahman, A., Bikson, M., & Parra, L.C. (2013). Effects of weak transcranial alternating current stimulation on brain activity-a review of known mechanisms from animal studies. Frontiers in Human Neuroscience, 7, 687. Salvador, R., Mekonnen, A., Ruffini, G., & Miranda, P.C. (2010). Modeling the electric field induced in a high resolution realistic head model during transcranial current stimulation. Conf Proc IEEE Eng Med Biol Soc, 2010, 2073-2076. Schmidt, S.L., Iyengar, A.K., Foulser, A.A., Boyle, M.R., & Frohlich, F. (2014). Endogenous cortical oscillations constrain neuromodulation by weak electric fields. Brain Stimulation). Siegel, M., Donner, T.H., & Engel, A.K. (2012). Spectral fingerprints of large-scale neuronal interactions. Nature Reviews Neuroscience, 13(2), 121-134. Sjostrom, P.J., Turrigiano, G.G., & Nelson, S.B. (2001). Rate, timing, and cooperativity jointly determine cortical synaptic plasticity. Neuron, 32(6), 1149-1164.
Tecchio, F., Cancelli, A., Cottone, C., Zito, G., Pasqualetti, P., Ghazaryan, A., et al. (2014). Multiple sclerosis fatigue relief by bilateral somatosensory cortex neuromodulation. Journal of Neurology, 261(8), 1552-1558.
687
Tecchio, F., Zappasodi, F., Porcaro, C., Barbati, G., Assenza, G., Salustri, C., et al. (2008). High-gamma band activity of primary hand cortical areas: A sensorimotor feedback efficiency index. NeuroImage, 40(1), 256-264.
691
Wach, C., Krause, V., Moliadze, V., Paulus, W., Schnitzler, A., & Pollok, B. (2013). Effects of 10 Hz and 20 Hz transcranial alternating current stimulation (tACS) on motor functions and motor cortical excitability. Behavioural Brain Research, 241, 1-6.
695
Zaghi, S., Acar, M., Hultgren, B., Boggio, P.S., & Fregni, F. (2010). Noninvasive brain stimulation with low-intensity electrical currents: Putative mechanisms of action for direct and alternating current stimulation. The Neuroscientist: A Review Journal Bringing Neurobiology, Neurology and Psychiatry, 16(3), 285-307.
699
Zaghi, S., de Freitas Rezende, L., de Oliveira, L.M., El-Nazer, R., Menning, S., Tadini, L. et al. (2010). Inhibition of motor cortex excitability with 15Hz transcranial alternating current stimulation (tACS). Neuroscience Letters, 479(3), 211-214.
705
roo f
662
683
or P
661
Tecchio, F., Cancelli, A., Cottone, C., Tomasevic, L., Devigus, B., Zito, G., et al. (2013). Regional personalized electrodes to select transcranial current stimulation target. Frontiers in Human Neuroscience, 7, 131.
dA
660
Struber, D., Rach, S., Trautmann-Lengsfeld, S.A., Engel, A.K., & Herrmann, C.S. (2014). Antiphasic 40 Hz oscillatory current stimulation affects bistable motion perception. Brain Topography, 27(1), 158-171.
cte
659
Radman, T., Ramos, R.L., Brumberg, J.C., & Bikson, M. (2009). Role of cortical cell type and morphology in subthreshold and suprathreshold uniform electric field stimulation in vitro. Brain Stimulation, 2(4), 215-228, 28, e1-e3.
rre
658
organization of the human brain with transcranial direct current stimulation. Human Brain Mapping, 32(8), 1236-1249.
co
657
Un
656
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