Horvath and colleagues published recently two large meta-analysis of effects of tDCS on cognitive measures and neurophysiological measures [1,2]. Essentially, they claim that tDCS has no reliable effects besides MEP modulation. And the effect size of this modulation appears to be decreasing since the first studies on tDCS fifteen years ago. This sounds like very bad news for tDCS and it took not long for other scientists comments [3,4].On the other side, we have the current hype on DIY-tDCS and consumer markets. From i scientific perspective, i welcomed Horvaths endevaour. Yet, i am currently supervising a student for some related study on tdCS and in that role, I was looking into some specific neurophysiological effects, i.e. EEG power changes in the alpha-range following M1 tDCS. During this literature research, i came to different conclusions compared to Horvath and colleagues.
Horvath et al. report that three papers study effects of anodal M1 stimulation on alpha power [5,6,7]. First, they report that Matsumoto (2010) showed no effects on alpha power. But this paper did not look into this aspect at all and simply did not test this hypothesis . Horvath et al. report that Pellicciari (2013) found a significant increase in alpha power after anodal stimulation. Please note that Pelliciari compared anodal and cathodal M1 stimulation, and showed the effects for post versus pre stimulation . It appears that this finding was included in the review. Horvath et al. now report that Notturno (2013) found no significant increase in alpha power after anodal stimulation. Given, Notturno did not find differences between anodal and sham, but between pre and post anodal stimulation , similar to Pelliciari. In that regard, Horvath et al. used two different measures (pre versus post, anodal versus sham). If Horvath and colleagues would have used the same contrast, it would have resulted in two studies showing increased alpha power after anodal stimulation over M1.
Furthermore, for some reasons (maybe because it was MEG, or published too late to be included), Horvath et al. apparently chose not to include two papers [8,9]. Venkatakrishnan et al (2011) researched tDCS effects on MEG ICA and found that independent from polarity, alpha power in a centro-parietal component decreased from the timepoint immediately after stimulation to 20 minutes post stimulation . Roy et al (2014) used a 4x1 HD setup, and found that relative to sham stimulation, alpha power was increased during anodal stimulation, and decreased following stimulation . Furthermore, since the review, at least four paperson tDCS and M/EEG have been published. Regarding anodal stimulation over M1, one is a methods paper , the other gives some evidence for increased alpha power during the rest period of a motor imagery task . Another paper finds evidence for increased increased power in the delta-theta-alpha range following frontal stimulation . Another paper that finds increased alpha power during and after anodal stimulation over posterio parietal cortex , and is in that regard replicating and expanding on similar findings of an earlier study on PPC .
Taken together, this gives me the impression that there is sufficient evidence to expect that anodal M1 stimulation might increase alpha-power. Additionally, radial versus tangential components as well as cathodal versus anodal stimulation needs more research. Some authors appear to have claimed that Horvath and colleagues are guilty of mischaracterization and inappriate data selection . I would be hesitant to make such a claim. There are many studies on tDCS with too many different protocols and additionally, the field is developing quickly. In agreement with probably many other researchers, i believe that just as the current hype about neuroenhancement is problematic, it is currently also too early for any hype about a zero effect claim.
Put shortly: Taken together, it currently looks as if there is evidence that anodal stimulation over M1 might increase alpha power, but I strongly suggest you to do your own literature research, and come to your own conclusions. The field is developing too quickly to trust a review that is only a year old.
1. Horvath, Jared Cooney, Jason D. Forte, and Olivia Carter. “Quantitative Review Finds No Evidence of Cognitive Effects in Healthy Populations from Single-Session Transcranial Direct Current Stimulation (tDCS).” Brain Stimulation, January 2015. doi:10.1016/j.brs.2015.01.400.
2. Horvath, Jared Cooney, Jason D. Forte, and Olivia Carter. “Evidence That Transcranial Direct Current Stimulation (tDCS) Generates Little-to-No Reliable Neurophysiologic Effect beyond MEP Amplitude Modulation in Healthy Human Subjects: A Systematic Review.” Neuropsychologia 66C (January 2015): 213–36. doi:10.1016/j.neuropsychologia.2014.11.021.
3. Price, Amy R., and Roy H. Hamilton. “A Re-Evaluation of the Cognitive Effects From Single-Session Transcranial Direct Current Stimulation.” Brain Stimulation, March 2015. doi:10.1016/j.brs.2015.03.007.
4. Nitsche, Michael A., Marom Bikson, and Sven Bestmann. “On the Use of Meta-Analysis in Neuromodulatory Non-Invasive Brain Stimulation.” Brain Stimulation, April 2015. doi:10.1016/j.brs.2015.03.008.
5. Matsumoto, Jun, Toshiyuki Fujiwara, Osamu Takahashi, Meigen Liu, Akio Kimura, and Junichi Ushiba. “Modulation of Mu Rhythm Desynchronization during Motor Imagery by Transcranial Direct Current Stimulation.” Journal of NeuroEngineering and Rehabilitation 7, no. 1 (2010): 27. doi:10.1186/1743-0003-7-27.
6. Pellicciari, Maria Concetta, Debora Brignani, and Carlo Miniussi. “Excitability Modulation of the Motor System Induced by Transcranial Direct Current Stimulation: A Multimodal Approach.” NeuroImage 83 (December 2013): 569–80. doi:10.1016/j.neuroimage.2013.06.076.
7. Notturno, Francesca, Laura Marzetti, Vittorio Pizzella, Antonino Uncini, and Filippo Zappasodi. “Local and Remote Effects of Transcranial Direct Current Stimulation on the Electrical Activity of the Motor Cortical Network.” Human Brain Mapping 35, no. 5 (May 2014): 2220–32. doi:10.1002/hbm.22322
8. Venkatakrishnan, Anusha, José L Contreras-Vidal, Marco Sandrini, and Leonardo G Cohen. “Independent Component Analysis of Resting Brain Activity Reveals Transient Modulation of Local Cortical Processing by Transcranial Direct Current Stimulation.” Conference Proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 2011 (2011): 8102–5. doi:10.1109/IEMBS.2011.6091998.
9. Roy, Abhrajeet, Bryan Baxter, and Bin He. “High-Definition Transcranial Direct Current Stimulation Induces Both Acute and Persistent Changes in Broadband Cortical Synchronization: A Simultaneous tDCS-EEG Study.” IEEE Transactions on Bio-Medical Engineering 61, no. 7 (2014): 1967–78. doi:10.1109/TBME.2014.2311071
10. Soekadar, Surjo R., Matthias Witkowski, Eliana G. Cossio, Niels Birbaumer, Stephen E. Robinson, and Leonardo G. Cohen. “In Vivo Assessment of Human Brain Oscillations during Application of Transcranial Electric Currents.” Nature Communications 4 (June 21, 2013). doi:10.1038/ncomms3032.
11. Kasuga, Shoko, Yayoi Matsushika, Yuko Kasashima-Shindo, Daiki Kamatani, Toshiyuki Fujiwara, Meigen Liu, and Junichi Ushiba. “Transcranial Direct Current Stimulation Enhances Mu Rhythm Desynchronization during Motor Imagery That Depends on Handedness.” Laterality, January 19, 2015, 1–16. doi:10.1080/1357650X.2014.998679.
12. Miller, Joe, Barbara Berger, and Paul Sauseng. “Anodal Transcranial Direct Current Stimulation (tDCS) Increases Frontal-Midline Theta Activity in the Human EEG: A Preliminary Investigation of Non-Invasive Stimulation.” Neuroscience Letters 588 (February 19, 2015): 114–19. doi:10.1016/j.neulet.2015.01.014.
13. Mangia, Anna L., Marco Pirini, and Angelo Cappello. “Transcranial Direct Current Stimulation and Power Spectral Parameters: A tDCS/EEG Co-Registration Study.” Frontiers in Human Neuroscience 8 (August 7, 2014). doi:10.3389/fnhum.2014.00601.
14. Spitoni, Grazia F., Rocco L. Cimmino, Chiara Bozzacchi, Luigi Pizzamiglio, and Francesco Di Russo. “Modulation of Spontaneous Alpha Brain Rhythms Using Low-Intensity Transcranial Direct-Current Stimulation.” Frontiers in Human Neuroscience 7 (2013): 529. doi:10.3389/fnhum.2013.00529.