The effect of anodal transcranial direct current stimulation over the primary motor or somatosensory cortices on somatosensory evoked magnetic fields.

Clinical Neurophysiology xxx (2014) xxx–xxx

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The effect of anodal transcranial direct current stimulation over the primary motor or somatosensory cortices on somatosensory evoked magnetic fields Kazuhiro Sugawara a,⇑, Hideaki Onishi a, Koya Yamashiro a, Sho Kojima a, Shota Miyaguchi a, Hikari Kirimoto a, Atsuhiro Tsubaki a, Hiroyuki Tamaki a, Hiroshi Shirozu b, Shigeki Kameyama b a b

Institute for Human Movement & Medical Sciences, Niigata University of Health and Welfare, 1398 Shimami-cho, Kita-ku, Niigata 950-3198, Japan Department of Neurosurgery, Nishi-Niigata Chuo National Hospital, 14-1-1 Masago, Nishi-ku, Niigata 950-2085, Japan

a r t i c l e

i n f o

Article history: Accepted 24 April 2014 Available online xxxx Keywords: Transcranial direct current stimulation Somatosensory evoked magnetic fields Primary motor cortex Primary somatosensory cortex Magnetoencephalography Median nerve stimulation

h i g h l i g h t s We investigated the effect of anodal transcranial direct-current stimulation (tDCS) on somatosensory

evoked magnetic fields. The source strengths for the P35m and the P60m increased after tDCS was applied over the primary

motor cortex (M1) and increased for the P60m after tDCS was applied over the primary somatosensory cortex (S1). The mean equivalent current dipole (ECD) location for the P35m was between 10 and 20 min after anodal tDCS was applied over M1 and was located significantly anterior to that of the N20m.

a b s t r a c t Objectives: The purpose of this study was to investigate the effect of anodal transcranial direct-current stimulation (tDCS) applied over the primary motor (M1) or the primary somatosensory (S1) cortices on somatosensory evoked magnetic fields (SEFs) following median nerve stimulation. Methods: Anodal tDCS was applied for 15 min on the left motor or somatosensory cortices at 1 mA. SEFs were recorded following right median nerve stimulation using a magnetoencephalography (MEG) system before and after the application of tDCS. SEFs was measured and compared before and after tDCS was applied over M1 or S1. Results: The source strengths for the P35m and P60m increased after tDCS was applied over M1 and that for the P60m increased after tDCS was applied over S1. The mean equivalent current dipole (ECD) location for the P35m was located significantly anterior to that of the N20m, but only during post 1 (10–20 min after tDCS was applied over M1). Conclusion: Our results indicated that the anodal tDCS applied over M1 affected the P35m and P60m sources on SEF components, while that applied over S1 influenced the P60m source. Significance: We demonstrated anodal tDCS applied over M1 or S1 can modulate somatosensory processing and components of SEFs, confirming the hypothesis for locally distinct generators of the P35m and P60m sources. Ó 2014 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.

1. Introduction Transcranial direct-current stimulation (tDCS) modulates cortical excitability by applying weak electrical currents in the form of ⇑ Corresponding author. Tel.: +81 25 257 4723; fax: +81 25 257 4498. E-mail address: [email protected] (K. Sugawara).

direct current brain polarization. Neuronal firing rates increase or decrease depending on direct current (DC) polarity, presumably due to DC-induced changes in resting membrane potentials (Liebetanz et al., 2002; Nitsche et al., 2003b). For example, studies investigating the excitability of the primary motor cortex (M1) have shown that anodal tDCS led to an increase of excitability within the stimulated area, whereas cathodal tDCS decreased

http://dx.doi.org/10.1016/j.clinph.2014.04.014 1388-2457/Ó 2014 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.

Please cite this article in press as: Sugawara K et al. The effect of anodal transcranial direct current stimulation over the primary motor or somatosensory cortices on somatosensory evoked magnetic fields. Clin Neurophysiol (2014), http://dx.doi.org/10.1016/j.clinph.2014.04.014

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K. Sugawara et al. / Clinical Neurophysiology xxx (2014) xxx–xxx

cortical excitability (Nitsche and Paulus, 2000; Suzuki et al., 2012). Additionally, the effect of changed excitability was sustained for approximately 1 h following the application of the tDCS stimulation for more than 10 min, depending on stimulus duration (Matsunaga et al., 2004; Miyaguchi et al., 2013; Nitsche and Paulus, 2001). Studies have shown that the effects of tDCS on cortical excitability were modulated by gamma-amino-butyric acid (GABA) receptors and N-methyl-D-aspartate (NMDA) receptordependent processes (Nitsche et al., 2004, 2005). Furthermore, human studies suggested that tDCS influenced and induced longterm potentiation (LTP) and long-term depression (LTD)-like mechanisms (Liebetanz et al., 2002; Nitsche et al., 2003a). As the generator sources for the somatosensory evoked potentials (SEPs) and the somatosensory evoked magnetic fields (SEFs) in humans during median nerve stimulation, the primary sensorimotor cortex contralateral to the stimulation is the main area which generates both the cortical early (20–40 ms) and late (60–120 ms) response (Greenwood and Goff, 1987; Huttunen et al., 1992, 1987, 2006; Kawamura et al., 1996; Lin et al., 2005; Okajima et al., 1991; Theuvenet et al., 2005; Waberski et al., 1999). Matsunaga et al. (2004) reported that amplitudes for P25/ N33, N33/P40 (for parietal components) and P22/N30 (for frontal component) of SEPs following median nerve stimulation were increased after anodal tDCS was applied over the sensorimotor cortex, whereas P14/N20, N20/P25 (for parietal components) and N18/P22 (for frontal components) were unaffected. In their study, anodal tDCS affected SEP waveforms while cathodal tDCS did not change SEPs following median nerve stimulation (Matsunaga et al. (2004). However, Dieckhofer et al. (2006) reported that there was no effect on SEP following median nerve stimulation after anodal stimulation applied over the primary somatosensory cortex (S1). In Dieckhofer et al.’s study, cathodal tDCS affected SEP waveforms while anodal tDCS did not influence SEPs following median nerve stimulation. These studies were different in that the application of tDCS was over S1 in Dieckhofer’s study and over M1 in Matsunaga’s study. Scalp SEPs do not accurately reflect brain electrical activities because their electric fields are widely distributed over the scalp using volume conductors (Ishibashi et al., 2000; Kakigi et al., 2000). Therefore, magnetoencephalography (MEG) is a noninvasive technique that can detect bioelectrical functions of the brain and in theory has several advantages over the older electroencephalography (EEG) technique for localizing cortical sources of bioelectrical functions due to lower effects in volume currents on magnetic fields recorded on the scalp compared to electrical fields (Kakigi et al., 2000). Additionally, MEG offers good localization accuracy within a few millimeters, especially for superficial cortical sources such as those located in the primary motor and somatosensory cortex (Papadelis et al., 2011). Thus, it has been successfully used for the localization of neural sources activated during somatosensory responses after median nerve stimulation (Forss et al., 1994; Inui et al., 2004; Kakigi, 1994). SEF deflections following median nerve stimulation consist of three main components, the N20m, the P35m and the P60m. Previous studies have reported that N20m deflections reflect excitatory postsynaptic potentials (EPSPs) in the pyramidal neurons of Brodmann’s area 3b (Forss et al., 1994; Huttunen et al., 2008; Kakigi, 1994). However, P35m and P60m deflections have been found to be different from the neural population activated during N20m, as well as different from one another (Huang et al., 2000; Huttunen et al., 2006; Kawamura et al., 1996; Lauronen et al., 2002; Lin et al., 2005). Kawamura et al. (1996) reported that the P35m was a response of the motor cortex. Huttunen et al. (2006) suggested that P60m deflection reflected activation within areas 1 and 2. Therefore, we hypothesized that anodal tDCS applied over M1 would modulate P35m and anodal tDCS applied over S1 would

modulate P60m because the P35m and P60m components reflected differential cortical activation. Thus, the purpose of the present study was to clarify the effect of anodal tDCS applied over the M1 or S1 cortices on SEF components. 2. Methods 2.1. Participants Nineteen healthy participants (age range, 20–30 years; mean ± standard deviation, 23.6 ± 3.3 years; eighteen right-handed and one left-handed) participated in the present study. None of the participants engaged in recreational drug use, or used medication which affects their central nerve system. All participants gave their written informed consent. The study conformed to the Declaration of Helsinki and the Code of Ethics of the World Medical Association, and was approved by the ethics committee at the Niigata University of Health and Welfare. 2.2. Somatosensory evoked magnetic field (SEF) recordings SEFs were recorded following electrical stimulation of the right median nerve at the wrist at 0.5 Hz with a pulse width of 0.2 ms. The intensity of stimulation was fixed at 1.2 times that of the motor threshold and was monitored throughout the duration of the experiment. The mean intensity for the SEF was 6.2 mA (range, 4.0–9.6 mA). 2.3. Experiment 1 (effects of tDCS applied over M1 on SEFs) The effects of tDCS applied over M1 on peak latency, equivalent current dipole (ECD) location, and ECD strength of SEFs were investigated in thirteen participants (nine males, four females; age range, 21–30 years; mean ± standard deviation, 23.6 ± 3.3 years). tDCS was delivered using a direct current stimulator (Eldith, NeuroConn GmbH, Ilmenau, Germany) through a pair of saline-soaked surface sponge electrodes (5 7 cm, 35 cm2). The anodal electrode was placed on the left scalp over the area representing the right abductor pollicis brevis (APB) muscle, as identified by a single pulse transcranial magnetic stimulation (TMS); the cathodal electrode was placed above the contralateral orbit. The duration of the stimulation was 15 min at a current strength of 1 mA. The fade-in/fade-out time was 5 s. 2.4. Experiment 2 (effects of tDCS applied over S1 on SEFs) The effects of tDCS applied over S1 on peak latency, ECD location, and ECD strength of SEFs were investigated in eleven participants (eight males, three females; 20–27 years, 22.7 ± 2.4 years; including five subjects from the same subjects from Experiment 1). For participants who took part in Experiment 1, there was a break of at least 1 week between experiments. To investigate the effect of anodal tDCS applied over S1 on SEFs, the center of the tDCS electrode was placed 2 cm posterior to the C3 region (C3, based on the international 10-20 system) (Dieckhofer et al., 2006; Sehm et al., 2013). Additionally, we confirmed that MEP did not emerge over the area. For Experiment 2 conducted on S1, we followed the same exact protocol as in Experiment 1 over M1 regarding the size of the electrodes, duration time, and the current strength of anodal tDCS. 2.5. Experiment 3 (sham tDCS) The effects of a sham tDCS applied over M1 on peak latency, ECD location, and the ECD strength of SEFs were investigated in



seven participants (four males, three females; 20–27 years, 21.7 ± 1.7 years; including three subjects from the same pool of subjects as in Experiment 1 and six subjects from the same pool of subjects from Experiment 2). For participants who took part in Experiments 1 and 2, we allowed a break of at least 1 week between experiments. For Experiment 3, we followed the same exact protocol as in Experiment 1 regarding the size of the electrodes and the duration of time that tDCS was applied over M1. 2.6. Experimental procedure The peak latencies, ECD strengths, and ECD locations for the N20m, P35m, and P60m on SEFs were measured and compared before and after tDCS was applied over M1 or S1, as well as before and after sham tDCS was applied over M1. Each component of the SEFs before tDCS was used as a baseline measure (pre tDCS). For statistical purposes, measurements after tDCS were carried out in two separate time periods, 10–20 min after the end of tDCS (post 1, after tDCS), and subsequently 25–35 min after the end of tDCS (post 2, after tDCS). Participants’ heads were cleaned using alcohol-soaked cotton after tDCS, before they entered the MEG shield room again; therefore, we measured SEFs after confirming that the electrical stimulation strength was the same as that in pre tDCS, and post 1 after tDCS was set to 10 min. 2.7. Data acquisition Neuromagnetic signals were recorded using a 306-channel whole-head MEG system (Vectorview; Elekta, Helsinki, Finland). This 306-channel device contains 102 identical triple sensors, each housing two orthogonal planar gradiometers and one magnetometer. This configuration of gradiometers specifically detects the signal just above the source current. Continuous MEG signals were sampled at 1000 Hz using a band-pass filter ranging between 0.03 and 330 Hz. The participants were comfortably seated inside a magnetically shielded room (Tokin Ltd., Sendai, Japan). MEG recordings were acquired from 50 ms before to 300 ms after median nerve stimulation for analyzing SEFs. The average of more than 150 recordings (153.4 ± 3.2 recordings) was obtained during each session. Before MEG measurements, three anatomical fiducial points (nasion and bilateral preauricular points) and four indicator coil locations on the scalp were digitized using a three-dimensional digitizer (Polhemus, Colchester, VT, USA). The fiducial points provided the spatial information necessary for the integration of magnetic resonance (MR) images and MEG data, while the indicator coils determined the position of the participant’s head in relation to the helmet. T1-weighted MR images were obtained using a 1.5-T system (Signa HD; GE Healthcare, Milwaukee, WI, USA).

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sulcus, termed ‘‘area 3b’’ (Huttunen et al., 2006; Kawamura et al., 1996; Lin et al., 2005). For the analysis of the effect of tDCS applied over M1 or S1, sham tDCS was applied over M1 on somatosensory evoked responses, peak latencies, ECD strengths, and ECD locations for N20m, P35m, and P60m were compared using one-way repeated measures analysis of variance (ANOVA) using tDCS period (pre vs. post 1 vs. post 2) as a factor. Post hoc analyses with Turkey HSD were used for multiple comparisons. Also, the Friedman test with post hoc analysis by the Wilcoxon signed rank test was used to test for significant differences in the ECD x, y, and z coordinates among the N20m, the P35m, and the P60m. P < 0.05 was taken as the significance threshold. 3. Results The responses to stimulation of the right median nerve consisted of three main deflections at the left primary sensorimotor cortex, N20m, P35m, and P60m. Clear response deflections were found in all participants. Figs. 1 and 2 present SEF grand averaged waveforms at the pre, post 1, and post 2 conditions after tDCS was applied over M1 and S1. Fig. 3 presents these waveforms at pre, post 1, and post 2 after the sham tDCS was applied over M1. We used the largest amplitude on each waveform within the sensors of interest in the SEF over the sensorimotor area. For the present analyses, source modeling took into account the peak measurements on the waveforms. The latencies for all components before and after tDCS were not significantly different (see Tables 1–3). The mean ECD locations for P35m and P60m relative to those for N20m during pre, post 1, and post 2 after tDCS was applied over M1 are shown in Fig. 4(A–C). Although the mean ECD locations for P35m in all conditions were significantly medial (pre, 6.9 ± 1.3 mm, P = 0.003; post 1, 5.2 ± 0.9 mm, P = 0.025; post 2, 4.9 ± 1.3 mm, P = 0.020) (mean ± standard errors) to those for the N20m in the medial–lateral direction, the mean ECD location for P35m was located significantly anterior to that for the N20m in the anterior–posterior direction, but only during post 1 after tDCS (Fig. 4B) (0.9 ± 1.3 mm, P = 0.030). The mean ECD locations for P60m in all conditions were significantly posterior (pre, 2.2 ± 0.7 mm, P = 0.022; post 1, 2.6 ± 1.0 mm, P = 0.030; post 2, 2.3 ± 1.0 mm, P = 0.042) to those for P35m.

2.8. Data analysis For analysis of SEFs, the band-pass filter was set between 0.5 Hz and 100 Hz. The 20-ms period preceding stimulation was used as the baseline. The source components of interest for MEGs were estimated as ECDs using a least-squares search within a subset of 16–22 channels over the peak response area for the left hemisphere. We used source modeling software (Elekta) to estimate the sources. The ECD locations were calculated using the fiducial points (nasion and bilateral preauricular points). We accepted ECDs corresponding to peak amplitudes from sensor levels and a goodness-of-fit (g) of >90% for analyses. In addition, the ECD location for the first peak response that occurred approximately 20 ms after median nerve stimulation (N20m) was used as the cortical reference location (Onishi et al., 2011). Previous MEG studies demonstrated that N20m source of the SEF response to median nerve stimulation arises from activity in the posterior wall of the central

Fig. 1. The grand averaged waveforms from a channel showing maximum signals from the left sensorimotor area following right median nerve stimulation in tDCS applied over M1. The responses consisted of three deflections peaking at N20m, P35m and P60m, respectively, after stimulus onset (vertical line). Dashed line: pre tDCS condition. Solid line: post 1 after tDCS condition. Gray line: post 2 after tDCS condition.


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K. Sugawara et al. / Clinical Neurophysiology xxx (2014) xxx–xxx Table 3 The latencies at the peak amplitude for each component for sham tDCS (ms). tDCS

N20m

P35m

P60m

Pre Post 1 Post 2

21.3 (0.6) 21.6 (0.6) 21.9 (3.2)

33.1 (0.6) 32.9 (0.4) 32.0 (3.6)

62.9 (3.2) 65.5 (3.6) 66.1 (3.6)

Values are presented as mean (SE).

Fig. 2. The grand averaged waveforms from a channel showing maximum signals from the left sensorimotor area following the right median nerve stimulation in tDCS applied over S1. The responses consisted of three deflections peaking at N20m, P35m and P60m, respectively, after stimulus onset (vertical line). Dashed line: pre tDCS condition. Solid line: post 1 after tDCS condition. Gray line: post 2 after tDCS condition.

Fig. 3. The grand averaged waveforms from a channel showing maximum signals from the left sensorimotor area following the right median nerve stimulation during the sham tDCS experiment. The responses consisted of three deflections peaking at N20m, P35m and P60m, respectively, after stimulus onset (vertical line). Dashed line: the pre tDCS condition. Solid line: post 1 after the tDCS condition. Gray line: post 2 after the tDCS condition.

Table 1 The latencies at the peak amplitude for each component for tDCS applied over M1 (ms). tDCS

N20m

P35m

P60m

Pre Post 1 Post 2

21.3 (1.4) 21.6 (2.9) 21.6 (1.8)

33.1 (2.9) 33.0 (5.5) 32.8 (7.8)

67.6 (10.7) 65.4 (7.8) 69.7 (10.5)


Table 2 The latencies at the peak amplitude for each component for tDCS applied over S1 (ms). tDCS

N20m

P35m

P60m

Pre Post 1 Post 2

20.8 (1.7) 21.1 (2.4) 20.9 (1.4)

34.0 (1.8) 33.7 (4.7) 34.9 (5.9)

68.4 (9.4) 66.9 (8.4) 68.9 (9.7)


The mean ECD locations for P35m and P60m relative to those for the N20m during pre, post 1, and post 2 after tDCS was applied over S1 are shown in Fig. 5(A–C). Similar to when the tDCS was applied over M1, the mean ECD locations for the P35m during pre, post1, and post 2 after tDCS was applied over S1 were significantly medial (pre, 4.6 ± 1.1 mm, P = 0.002; post 1, 4.1 ± 1.3 mm, P = 0.042; post 2, 3.7 ± 1.1 mm, P = 0.042) to that for the N20m in the medial–lateral direction. The mean ECD locations for the P60m in all conditions were significantly posterior (pre, 2.9 ± 0.8 mm, P = 0.009; post 1, 2.0 ± 1.9 mm, P = 0.042; post 2, 2.0 ± 2.2 mm, P = 0.042) to those for the P35m in the anterior– posterior direction. The mean ECD locations for the P35m during pre, post 1, and post 2 after the sham tDCS was applied were significantly medial (pre, 4.7 ± 0.8 mm, P = 0.010; post 1, 4.8 ± 0.9 mm, P = 0.030; post 2, 5.2 ± 0.8 mm, P = 0.010) to that for the N20m in the medial– lateral direction. The mean ECD locations for the P60m in all conditions were significantly posterior (pre, 2.5 ± 2.6 mm, P = 0.023; post 1, 2.7 ± 1.9 mm, P = 0.023; post 2, 2.5 ± 2.3 mm, P = 0.030) to that for the P35m in the anterior–posterior direction. The ECD strengths in pre, post 1, and post 2 after tDCS was applied over M1 for each component in all conditions are presented in Table 4. A one-way repeated measures ANOVA showed a significant main effect on the ECD strengths for the P35m (F (2, 24) = 8.542, P = 0.002) and the P60m (F (2, 24) = 10.351, P = 0.001) during pre, post 1, and post 2 after tDCS was applied over M1, but no significant main effects were observed for the N20m (F (2, 24) = 0.695, P = 0.509). Post hoc analysis showed that the ECD strengths for the P35m (P = 0.001) and the P60m (P = 0.001) during post 1 after tDCS was applied significantly increased compared to that during the pre tDCS. The ECD strengths for the pre, post 1, and post 2 conditions after tDCS was applied over S1 for each component in all conditions are presented in Table 5. A one-way repeated measures ANOVA showed a significant main effect on the ECD strength for the P60m during pre, post 1, and post 2 after tDCS applied over S1(F (2, 20) = 4.354, P = 0.027), but no significant main effects were observed for N20m (F (2, 20) = 0.019, P = 0.981) and P35m (F (2, 20) = 0.425, P = 0.660). Post hoc analysis showed that the ECD strength for the P60m during post 1 after tDCS significantly increased compared to that during the pre tDCS (P = 0.037). The ECD strengths during pre, post 1, and post 2 after the sham tDCS was applied for each component in all conditions are presented in Table 6. A one-way repeated measures ANOVA showed no significant main effect on the ECD strength for any of the components during pre, post 1, and post 2 after the sham tDCS condition (N20m, F (2, 12) = 0.333, P = 0.723; P35m, F (2, 12) = 0.52, P = 0.949; P60m, F (2, 12) = 0.900, P = 0.915).

4. Discussion Our results showed that a stimulus strength of 1 mA anodal tDCS applied over the motor cortex, mainly over M1, resulted in increments of P35m and P60m source components of the SEFs after contralateral median nerve stimulation. Additionally, anodal tDCS applied over S1 resulted in increments of only P60m source



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Fig. 4. A shows mean ECD locations for all participants at P60m (N) and P35m (j) in pre tDCS applied over M1 relative to ECD for N20m (). B shows mean ECD locations for all participants at P60m (N) and P35m (j) in post 1 after tDCS applied over M1 relative to those for N20m (). C shows ECD locations for all participants at P60m (N) and P35m (j) in post 2 after tDCS was applied over M1 relative to those for N20m (). Additionally, ECD locations in the upper and lower panels are shown in both axial and coronal planes. Error bars indicate SE of the mean. The mean ECD locations for P35m in pre, post 1, and post 2 after tDCS over M1 were significantly medial to that for N20m (single asterisk, P < 0.05). The mean ECD location for P35m was significantly anterior to that for N20m, but only in post 1 after tDCS (triple asterisk, P < 0.05). The mean ECD locations for P60m in all conditions were significantly posterior to those for P35m (double asterisk, P < 0.05). (X = medial–lateral, Y = anterior–posterior, Z = superior–inferior).

component of the SEFs. No change was observed in the N20m source component when anodal tDCS was applied over M1 or S1. The effect of tDCS is influenced by the dimension and location of electrodes as well as the intensity, density, and duration of the stimulus (Jeffery et al., 2007; Nitsche and Paulus, 2000, 2001). The present study used a current of 1 mA and electrodes with a surface area of 35 cm2 for the application of tDCS over M1 or S1, with stimulation applied for 15 min. Prior work has reported that anodal tDCS applied for 15 min enhances corticomotor excitability, which persists for 90 min after tDCS (Nitsche and Paulus, 2001). Consistent with previous literature, the present findings also showed cortical excitability 10–20 min (post 1) after anodal tDCS. Many previous studies reported that the N20m component of SEFs following median nerve stimulation elicited a response in area 3b (Forss et al., 1994; Huttunen et al., 1992, 1987, 2006; Kakigi, 1994; Kakigi et al., 2000; Lin et al., 2005). Since for the present study anodal electrodes were applied over the representational field of the right APB muscle, as determined by a single-pulse TMS, it might be possible that the posterior margin of the electrode was placed near the central sulcus. Matsunaga et al. (2004) reported that early components that generated 3b following median nerve stimulation were unaffected by anodal tDCS with 1 mA, since at area 3b (Allison et al., 1989, 1991; Huttunen et al., 2006; Kawamura et al., 1996; Lin et al., 2005), which is within the depths of the central sulcus, cells experience a smaller voltage gradient from the applied tDCS. It has been assumed that anodal tDCS applied over M1 did not cover area 3b and tDCS applied over S1

did not affect area 3b in the depths of the central sulcus in anodal tDCS applied over S1 in this study. Furthermore, it was suggested that the N20m was unchanged by anodal tDCS applied over both M1 and S1. The present findings showed that the ECD strength for the P35m increased after anodal tDCS was applied over M1, but there was no effect on the P35m after anodal tDCS was applied over S1. The current generation source for the P35m is still debated, but previous studies have reported response activation in areas 3b or 4 (Huttunen et al., 2001; Kawamura et al., 1996; Wikstrom et al., 1996). Cooper et al. (1975) recorded event-related potentials from cortical electrodes in humans and reported that not only the postcentral area, but also the precentral area activated for median nerve stimulation at the contralateral wrist. An additional study, where SEPs and associated multiple unit activity (MUA) were recorded from a series of epidural and intracortical locations in a monkey, reported that the precentral area and postcentral area were activated at approximately the same latency following stimulation of the contralateral median nerve (Arezzo et al., 1981). Recently, Frot et al. (2013) conducted intracortical recordings of potentials following median nerve stimulation in humans. Their study clearly demonstrated that both responses for the precentral area and postcentral area occurred at the same latency of 22 ms. This indicated the presence of area 4 responses due to median nerve stimulation. In a previous study using MEG, the N20m and P35m may have reflected differential activation, since the ECD for the P35m was located medial to that of the N20m source


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Fig. 5. A shows mean ECD locations for all participants at P60m (N) and P35m (j) in pre tDCS applied over S1 relative to those for N20m (). In addition, B shows ECD locations for all participants at P60m (N) and P35m (j) in post 1 after tDCS was applied over S1 relative to those for N20m (). C shows locations for all participants at P60m (N) and P35m (j) in post 2 after tDCS was applied over S1 relative to that for N20m (). ECD locations in the upper and lower panels are shown in both axial and coronal planes. The mean ECD locations for P35m in pre, post 1 and post 2 after tDCS over S1 were significantly medial to that for N20m (single asterisk, P < 0.05). The mean ECD locations for P60m in all conditions were significantly posterior to that for P35m (double asterisk, P < 0.05).

Table 4 The ECD strengths for tDCS applied over M1 (nAm). tDCS

N20m

Pre Post 1 Post 2

18.7 (0.9) 18.3 (0.8) 19.0 (0.8)

P35m 21.3 (1.0) 27.0 (1.1)* 23.8 (1.3)

P60m 27.9 (1.6) 34.8 (1.4)* 28.0 (1.6)

Values are presented as mean (SE). * P < 0.05.

Table 5 The ECD strengths for tDCS applied over S1 (nAm). tDCS

N20m

Pre Post 1 Post 2

24.2 (2.7) 24.6 (2.7) 24.3 (2.5)

P35m 24.5 (1.3) 26.1 (1.3) 25.8 (2.0)

P60m 32.4 (2.0) 40.3 (2.0)* 33.8 (1.9)

Values are presented as mean (SE). * P < 0.05.

Table 6 The ECD strengths for sham tDCS (nAm). tDCS

N20m

Pre Post 1 Post 2

22.8 (4.6) 24.4 (5.4) 24.3 (4.5)


P35m 33.0 (8.2) 33.6 (7.7) 32.6 (6.0)

P60m 37.8 (7.5) 37.8 (5.7) 39.6 (6.0)

(Ishibashi et al., 2000; Lin et al., 2005); the P35m as reported by Kawamura et al. (1996) located 3.7 mm medial to the ECD location for the N20m was proposed as a response of the motor cortex. Using multiple source modeling of magnetic fields, Huang et al. (2000) reported a temporal overlap of 20 ms and 30 ms at the precentral and postcentral areas following median nerve stimulation. The ECD location for the P35m in the present study was significantly medial relative to that for the N20m, which fell in line with results from previous studies (Ishibashi et al., 2000; Kawamura et al., 1996; Lin et al., 2005), and the ECD strength was increased after tDCS was applied over M1. In addition, the ECD location for the P35m was significantly anterior relative to that for the N20m during post 1 after tDCS was applied over M1. These results suggested that the P35m deflection may reflect the activation of area 4 anterior to the central sulcus. In present study, the ECD for the P60m source was located posterior to that of the P35m, which was consistent with findings from Huttunen et al. (2006). Moreover, the ECD strengths for the P35m and P60m increased after tDCS was applied over M1, but only the P60m increased after tDCS was applied over S1. Thus, the P60m behaved differently from the P35m. For example, in the pediatric degenerative disease CLN5, known as the Finnish variant form of late infantile neuronal ceroid lipofuscinosis, the N20m and P35m displayed greatly enlarged waveforms compared to that of healthy participants, while the P60m could not be detected at all (Lauronen et al., 2002). These results suggested that the P35m and P60m reflected differential cortical activation. The results from the present study showed that the ECD strength for the P60m increased



after tDCS was applied over M1 or S1. Huttunen et al. (2006) suggested that P60m deflection reflected the activation of an extended source, possibly involving generators within areas 1 and 2 because the ECD location for the P60m was located significantly posterior to that for the P35m. Anodal tDCS in this study may cover M1 as well as the motor association area, the premotor cortex (PMC), because we stimulated the hotspot of APB with tDCS using a large electrode (35 cm2) (Kirimoto et al., 2011). M1 and PMC at the precentral area connected with areas 1 and 2, which occupy S1 at the crown of the postcentral gyrus (Jones et al., 1978). Consequently, we speculated that the excitatory improvement of the primary motor area and premotor area by anodal tDCS applied over M1 improved the activity of areas 1 and 2 by anodal tDCS, and presumably the ECD strength of the P60m was increased. Moreover, areas 1 or 2 located in the postcentral gyrus was directly affected by anodal tDCS applied over S1 as references from Dieckhofer et al. (2006) and Sehm et al. (2013) study. However, MEG cannot detect activities from radial dipoles such as those in the crown of gyri. Hence, the results from the present study cannot distinguish activation between areas 1 and 2 (Huttunen et al., 2006; Inui et al., 2004). The sustained effect of anodal tDCS in this study was observed only in post 1 after tDCS (between 10 and 20 min after tDCS). No meaningful changes were detected in SEF waveforms between pre tDCS and post 2 after tDCS (from 25 to 35 min after tDCS). Although in the present study it was difficult to assess why post 2 had no effect on SEF after tDCS was applied, one possibility may be that we carried out median nerve stimulation for more than 150 times during post 1 after tDCS. Thus, the cortical excitability effect of the anodal tDCS may have been attenuated. This finding falls in line with a previous report that demonstrated that the excitatory effect of M1 was not produced by performing peripheral electrical stimulation during the anodal tDCS (Schabrun et al., 2013). The influence of the cathodal electrode placed above the contralateral orbit on M1 and S1 is not negligible (Kincses et al., 2004; Nitsche et al., 2007), but it is thought to be minimal effects (Nitsche and Paulus, 2000). Therefore we plan to investigate the influence of the cathodal electrode position on M1 and S1 in future study. In summary, our results indicated that anodal tDCS applied over M1 affected the P35m and P60m source of the SEF components, and that applied over S1 influenced the P60m source. In future studies, we plan to investigate the change in SEF waveforms when cathodal tDCS is applied over M1 or S1 and when M1 or S1 is stimulated through anodal tDCS using smaller electrodes with the ability for more localized stimulation.

References Allison T, McCarthy G, Wood CC, Darcey TM, Spencer DD, Williamson PD. Human cortical potentials evoked by stimulation of the median nerve. I. Cytoarchitectonic areas generating short-latency activity. J Neurophysiol 1989;62:694–710. Allison T, Wood CC, McCarthy G, Spencer DD. Cortical somatosensory evoked potentials. II. Effects of excision of somatosensory or motor cortex in humans and monkeys. J Neurophysiol 1991;66:64–82. Arezzo JC, Vaughan Jr HG, Legatt AD. Topography and intracranial sources of somatosensory evoked potentials in the monkey. II. Cortical components. Electroencephalogr Clin Neurophysiol 1981;51:1–18. Cooper R, Crow HJ, Papakostopoulos D. Response of the motor cortex to sensory input in man. J Physiol 1975;245:70P–2P. Dieckhofer A, Waberski TD, Nitsche M, Paulus W, Buchner H, Gobbele R. Transcranial direct current stimulation applied over the somatosensory cortex – differential effect on low and high frequency SEPs. Clin Neurophysiol 2006;117:2221–7. Forss N, Hari R, Salmelin R, Ahonen A, Hamalainen M, Kajola M, et al. Activation of the human posterior parietal cortex by median nerve stimulation. Exp Brain Res 1994;99:309–15.

7

Frot M, Magnin M, Mauguiere F, Garcia-Larrea L. Cortical representation of pain in primary sensory-motor areas (S1/M1)—a study using intracortical recordings in humans. Hum Brain Mapp 2013;34:2655–68. Greenwood PM, Goff WR. Modification of median nerve somatic evoked potentials by prior median nerve, peroneal nerve, and auditory stimulation. Electroencephalogr Clin Neurophysiol 1987;68:295–302. Huang MX, Aine C, Davis L, Butman J, Christner R, Weisend M, et al. Sources on the anterior and posterior banks of the central sulcus identified from magnetic somatosensory evoked responses using multistart spatio-temporal localization. Hum Brain Mapp 2000;11:59–76. Huttunen J, Hari R, Leinonen L. Cerebral magnetic responses to stimulation of ulnar and median nerves. Electroencephalogr Clin Neurophysiol 1987;66:391–400. Huttunen J, Ahlfors S, Hari R. Interaction of afferent impulses in the human primary sensorimotor cortex. Electroencephalogr Clin Neurophysiol 1992;82:176–81. Huttunen J, Jaaskelainen IP, Hirvonen J, Kaakkola S, Ilmoniemi RJ, Pekkonen E. Scopolamine reduces the P35m and P60m deflections of the human somatosensory evoked magnetic fields. Neuroreport 2001;12:619–23. Huttunen J, Komssi S, Lauronen L. Spatial dynamics of population activities at S1 after median and ulnar nerve stimulation revisited: an MEG study. Neuroimage 2006;32:1024–31. Huttunen J, Pekkonen E, Kivisaari R, Autti T, Kahkonen S. Modulation of somatosensory evoked fields from SI and SII by acute GABA A-agonism and paired-pulse stimulation. Neuroimage 2008;40:427–34. Inui K, Wang X, Tamura Y, Kaneoke Y, Kakigi R. Serial processing in the human somatosensory system. Cereb Cortex 2004;14:851–7. Ishibashi H, Tobimatsu S, Shigeto H, Morioka T, Yamamoto T, Fukui M. Differential interaction of somatosensory inputs in the human primary sensory cortex: a magnetoencephalographic study. Clin Neurophysiol 2000;111:1095–102. Jeffery DT, Norton JA, Roy FD, Gorassini MA. Effects of transcranial direct current stimulation on the excitability of the leg motor cortex. Exp Brain Res 2007;182:281–7. Jones EG, Coulter JD, Hendry SH. Intracortical connectivity of architectonic fields in the somatic sensory, motor and parietal cortex of monkeys. J Comp Neurol 1978;181:291–347. Kakigi R. Somatosensory evoked magnetic fields following median nerve stimulation. Neurosci Res 1994;20:165–74. Kakigi R, Hoshiyama M, Shimojo M, Naka D, Yamasaki H, Watanabe S, et al. The somatosensory evoked magnetic fields. Prog Neurobiol 2000;61:495–523. Kawamura T, Nakasato N, Seki K, Kanno A, Fujita S, Fujiwara S, et al. Neuromagnetic evidence of pre- and post-central cortical sources of somatosensory evoked responses. Electroencephalogr Clin Neurophysiol 1996;100:44–50. Kincses TZ, Antal A, Nitsche MA, Bartfai O, Paulus W. Facilitation of probabilistic classification learning by transcranial direct current stimulation of the prefrontal cortex in the human. Neuropsychologia 2004;42:113–7. Kirimoto H, Ogata K, Onishi H, Oyama M, Goto Y, Tobimatsu S. Transcranial direct current stimulation over the motor association cortex induces plastic changes in ipsilateral primary motor and somatosensory cortices. Clin Neurophysiol 2011;122:777–83. Lauronen L, Huttunen J, Kirveskari E, Wikstrom H, Sainio K, Autti T, et al. Enlarged SI and SII somatosensory evoked responses in the CLN5 form of neuronal ceroid lipofuscinosis. Clin Neurophysiol 2002;113:1491–500. Liebetanz D, Nitsche MA, Tergau F, Paulus W. Pharmacological approach to the mechanisms of transcranial DC-stimulation-induced after-effects of human motor cortex excitability. Brain 2002;125:2238–47. Lin YY, Chen WT, Liao KK, Yeh TC, Wu ZA, Ho LT, et al. Differential generators for N20m and P35m responses to median nerve stimulation. Neuroimage 2005;25:1090–9. Matsunaga K, Nitsche MA, Tsuji S, Rothwell JC. Effect of transcranial DC sensorimotor cortex stimulation on somatosensory evoked potentials in humans. Clin Neurophysiol 2004;115:456–60. Miyaguchi S, Onishi H, Kojima S, Sugawara K, Tsubaki A, Kirimoto H, et al. Corticomotor excitability induced by anodal transcranial direct current stimulation with and without non-exhaustive movement. Brain Res 2013;1529:83–91. Nitsche MA, Paulus W. Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation. J Physiol 2000;527:633–9. Nitsche MA, Paulus W. Sustained excitability elevations induced by transcranial DC motor cortex stimulation in humans. Neurology 2001;57:1899–901. Nitsche MA, Fricke K, Henschke U, Schlitterlau A, Liebetanz D, Lang N, et al. Pharmacological modulation of cortical excitability shifts induced by transcranial direct current stimulation in humans. J Physiol 2003a;553:293–301. Nitsche MA, Liebetanz D, Antal A, Lang N, Tergau F, Paulus W. Modulation of cortical excitability by weak direct current stimulation—technical, safety and functional aspects. Suppl Clin Neurophysiol 2003b;56:255–76. Nitsche MA, Jaussi W, Liebetanz D, Lang N, Tergau F, Paulus W. Consolidation of human motor cortical neuroplasticity by D-cycloserine. Neuropsychopharmacology 2004;29:1573–8. Nitsche MA, Seeber A, Frommann K, Klein CC, Rochford C, Nitsche MS, et al. Modulating parameters of excitability during and after transcranial direct current stimulation of the human motor cortex. J Physiol 2005;568:291–303. Nitsche MA, Doemkes S, Karakose T, Antal A, Liebetanz D, Lang N, et al. Shaping the effects of transcranial direct current stimulation of the human motor cortex. J Neurophysiol 2007;97:3109–17.


8


Okajima Y, Chino N, Saitoh E, Kimura A. Interactions of somatosensory evoked potentials: simultaneous stimulation of two nerves. Electroencephalogr Clin Neurophysiol 1991;80:26–31. Onishi H, Oyama M, Soma T, Kirimoto H, Sugawara K, Murakami H, et al. Muscleafferent projection to the sensorimotor cortex after voluntary movement and motor-point stimulation: an MEG study. Clin Neurophysiol 2011;122:605–10. Papadelis C, Eickhoff SB, Zilles K, Ioannides AA. BA3b and BA1 activate in a serial fashion after median nerve stimulation: direct evidence from combining source analysis of evoked fields and cytoarchitectonic probabilistic maps. Neuroimage 2011;54:60–73. Schabrun SM, Chipchase LS, Zipf N, Thickbroom GW, Hodges PW. Interaction between simultaneously applied neuromodulatory interventions in humans. Brain Stimul 2013;6:624–30. Sehm B, Hoff M, Gundlach C, Taubert M, Conde V, Villringer A, et al. A novel ring electrode setup for the recording of somatosensory evoked potentials during transcranial direct current stimulation (tDCS). J Neurosci Methods 2013;212:234–6.

Suzuki K, Fujiwara T, Tanaka N, Tsuji T, Masakado Y, Hase K, et al. Comparison of the after-effects of transcranial direct current stimulation over the motor cortex in patients with stroke and healthy volunteers. Int J Neurosci 2012;122:675–81. Theuvenet PJ, van Dijk BW, Peters MJ, van Ree JM, Lopes da Silva FL, Chen AC. Whole-head MEG analysis of cortical spatial organization from unilateral stimulation of median nerve in both hands: no complete hemispheric homology. Neuroimage 2005;28:314–25. Waberski TD, Buchner H, Perkuhn M, Gobbele R, Wagner M, Kucker W, et al. N30 and the effect of explorative finger movements: a model of the contribution of the motor cortex to early somatosensory potentials. Clin Neurophysiol 1999;110:1589–600. Wikstrom H, Huttunen J, Korvenoja A, Virtanen J, Salonen O, Aronen H, et al. Effects of interstimulus interval on somatosensory evoked magnetic fields (SEFs): a hypothesis concerning SEF generation at the primary sensorimotor cortex. Electroencephalogr Clin Neurophysiol 1996;100:479–87.


Transcranial Direct Current Stimulation Over the Primary and Secondary Somatosensory Cortices Transiently Improves Tactile Spatial Discrimination in Stroke Patients.

Effects of High-Definition Anodal Transcranial Direct Current Stimulation Applied Simultaneously to Both Primary Motor Cortices on Bimanual Sensorimotor Performance.

Effect of transcranial static magnetic field stimulation over the sensorimotor cortex on somatosensory evoked potentials in humans.

Non-invasive modulation of somatosensory evoked potentials by the application of static magnetic fields over the primary and supplementary motor cortices.

No Effect of Anodal Transcranial Direct Current Stimulation Over the Motor Cortex on Response-Related ERPs during a Conflict Task.

Anodal transcranial direct current stimulation over premotor cortex facilitates observational learning of a motor sequence.

Polarity-Specific Cortical Effects of Transcranial Direct Current Stimulation in Primary Somatosensory Cortex of Healthy Humans.

Simultaneous acoustic stimulation of human primary and secondary somatosensory cortices using transcranial focused ultrasound.

Modulation of Cortical Inhibitory Circuits after Cathodal Transcranial Direct Current Stimulation over the Primary Motor Cortex.

Patient-conducted anodal transcranial direct current stimulation of the motor cortex alleviates pain in trigeminal neuralgia.

Polarity-independent effects of transcranial direct current stimulation over the bilateral opercular somatosensory region: a magnetoencephalography study.

The effect of anodal transcranial direct current stimulation on multi-limb coordination performance.

Anodal transcranial direct current stimulation over the dorsolateral prefrontal cortex improves anorexia nervosa: A pilot study.

Anodal transcranial direct current stimulation applied over the supplementary motor area delays spontaneous antiphase-to-in-phase transitions.

Commentary: Systematic assessment of duration and intensity of anodal transcranial direct current stimulation on primary motor cortex excitability.

Attenuation in detection of somatosensory stimuli by transcranial magnetic stimulation.

Parietal transcranial direct current stimulation modulates primary motor cortex excitability.

Rapid-rate paired associative stimulation over the primary somatosensory cortex.

The effect of etomidate on motor evoked potentials induced by transcranial magnetic stimulation in the monkey.

Effect of anodal transcranial direct current stimulation on autism: a randomized double-blind crossover trial.

Effects of repetitive transcranial magnetic stimulation on the somatosensory cortex during prism adaptation.

The effect of transcranial direct current stimulation of the motor cortex on exercise-induced pain.

Influence of anodal transcranial direct current stimulation (tDCS) over the right angular gyrus on brain activity during rest.

State-space models of impulse hemodynamic responses over motor, somatosensory, and visual cortices.