Clinical Neurophysiology 126 (2015) 2045–2046
Contents lists available at ScienceDirect
Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph
Editorial
Are current blinding methods for transcranial direct current stimulation (tDCS) effective in healthy populations? See Article, pages 2181–2188
In their paper What Do You Feel if I Apply Transcranial Electrical Stimulation?, Fertonanai et al. (2015) present the largest quantitative analysis to date of the sensorial sensations and side-effects generated by varied forms of transcranial electrical stimulation (tES) within healthy populations. This group concludes that, by and large, transcranial direct current, alternating current, and random noise stimulation are safe and painless techniques. Interestingly, this group reports that anodal transcranial direct current stimulation (tDCS) generates an average discomfort rating 25% higher than sham stimulation. Though this difference was only nearly significant (p = 0.056), it leads to interesting questions regarding the effectiveness of current blinding techniques during tDCS experimentation within healthy individuals: an issue that has been raised several times previously in the literature. 1. Studies exploring tDCS blinding The first study to directly compare discomfort ratings of real vs. sham tDCS in healthy populations was published by Gandiga et al. (2006: Table 1). After comparing a total of 85 stimulation sessions, these authors reported no significant difference between sensations during anodal, cathodal, and sham stimulation. Despite concluding that ‘‘. . .[tDCS] can be successfully used in the setting of double-blind trials. . .’’ (p. 849), these authors did not explore the ability of tDCS administrators to differentiate between active and sham conditions; accordingly, this data only supports the notion of single-blinding efficacy (not double-blinding). Six years later, Kessler et al. (2012) compared sensations generated during 177 stimulation sessions. These authors reported significantly stronger tingling, itching, burning, and pain sensations during real tDCS than during sham (all p < 0.001). These authors conclude by suggesting that, ‘‘[Differences in sensory effects] may differentially affect task performance, raising the possibility that the current method of sham stimulation is not an adequate control condition’’ (p. 162). Again, though this study speaks to participant blinding, it did not explore tDCS administrator blinding. Next, O’Connell et al. (2012) compared 195 stimulation sessions to determine whether or not healthy, naïve participants could differentiate between real vs. sham stimulation (Table 1). These authors reported participants correctly judged the stimulation condition significantly more often than would be expected by chance (p < 0.01)
indicating inadequate participant blinding. In addition, the tDCS practitioners in this study noted redness at the electrode site significantly more often following real stimulation than following sham (p < 0.001) indicating inadequate assessor blinding as well. These authors conclude by stating ‘‘. . .our data show that both participant and assessor blinding is compromised at 2 mA intensity. . .’’ (p. 5). Finally, Russo et al. (2013) compared sensorial effects from 195 stimulation sessions in healthy participants (Table 1). These authors report that active stimulation was significantly less comfortable than sham stimulation (p < 0.05). However, these authors also report that participants were largely unable to discriminate between active and sham conditions, despite differences in comfort levels. This group concludes by stating, ‘‘[It appears that tDCS] can be successfully administered using a double-blind procedure. . .’’ (p. 951). However, these researchers did not explore whether the tDCS administrator could differentiate between active and sham conditions; as such, this data only supports the notion of single-blinding efficacy (not double-blinding). 2. Is tDCS blinding effective? In aggregate, the data suggest that sensations generated in healthy participants during active tDCS are significantly greater than sensations generated during sham tDCS. However, what impact this difference has on the ability of participants to determine which form of stimulation they are receiving is less certain; namely, whereas O’Connell et al. (2012) reports participant blinding is compromised due to these enhanced sensations, Russo et al. (2013) reports participant blinding is unaffected. Additional research exploring the explicit link between sensory magnitudes and participant blinding efficacy is required to better understand this relationship. In addition, the only data supplied with regards to tDCS practitioner blinding suggests that assessors are able to guess the stimulation condition with better accuracy than would be expected by chance (due, primarily, to visible vasodilation under the electrode following active stimulation). This suggests that a true doubleblind experiment may be difficult to achieve whilst using a 0.06 mA/cm2 current density. If future data reveals that both single- and double-blinding under current methodologies are not effective, this will raise important questions concerning research undertaken in healthy
http://dx.doi.org/10.1016/j.clinph.2015.04.001 1388-2457/Ó 2015 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
2046
Editorial / Clinical Neurophysiology 126 (2015) 2045–2046
Table 1 Studies directly exploring the sensory effects and/or blinding efficacy in real vs. sham tDCS. Study
# of sessions
Target electrode
References electrode
Electrode Size
Current Density
Sham Protocol
Findings
Gandiga et al. (2006)
Contents lists available at ScienceDirect
Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph
Editorial
Are current blinding methods for transcranial direct current stimulation (tDCS) effective in healthy populations? See Article, pages 2181–2188
In their paper What Do You Feel if I Apply Transcranial Electrical Stimulation?, Fertonanai et al. (2015) present the largest quantitative analysis to date of the sensorial sensations and side-effects generated by varied forms of transcranial electrical stimulation (tES) within healthy populations. This group concludes that, by and large, transcranial direct current, alternating current, and random noise stimulation are safe and painless techniques. Interestingly, this group reports that anodal transcranial direct current stimulation (tDCS) generates an average discomfort rating 25% higher than sham stimulation. Though this difference was only nearly significant (p = 0.056), it leads to interesting questions regarding the effectiveness of current blinding techniques during tDCS experimentation within healthy individuals: an issue that has been raised several times previously in the literature. 1. Studies exploring tDCS blinding The first study to directly compare discomfort ratings of real vs. sham tDCS in healthy populations was published by Gandiga et al. (2006: Table 1). After comparing a total of 85 stimulation sessions, these authors reported no significant difference between sensations during anodal, cathodal, and sham stimulation. Despite concluding that ‘‘. . .[tDCS] can be successfully used in the setting of double-blind trials. . .’’ (p. 849), these authors did not explore the ability of tDCS administrators to differentiate between active and sham conditions; accordingly, this data only supports the notion of single-blinding efficacy (not double-blinding). Six years later, Kessler et al. (2012) compared sensations generated during 177 stimulation sessions. These authors reported significantly stronger tingling, itching, burning, and pain sensations during real tDCS than during sham (all p < 0.001). These authors conclude by suggesting that, ‘‘[Differences in sensory effects] may differentially affect task performance, raising the possibility that the current method of sham stimulation is not an adequate control condition’’ (p. 162). Again, though this study speaks to participant blinding, it did not explore tDCS administrator blinding. Next, O’Connell et al. (2012) compared 195 stimulation sessions to determine whether or not healthy, naïve participants could differentiate between real vs. sham stimulation (Table 1). These authors reported participants correctly judged the stimulation condition significantly more often than would be expected by chance (p < 0.01)
indicating inadequate participant blinding. In addition, the tDCS practitioners in this study noted redness at the electrode site significantly more often following real stimulation than following sham (p < 0.001) indicating inadequate assessor blinding as well. These authors conclude by stating ‘‘. . .our data show that both participant and assessor blinding is compromised at 2 mA intensity. . .’’ (p. 5). Finally, Russo et al. (2013) compared sensorial effects from 195 stimulation sessions in healthy participants (Table 1). These authors report that active stimulation was significantly less comfortable than sham stimulation (p < 0.05). However, these authors also report that participants were largely unable to discriminate between active and sham conditions, despite differences in comfort levels. This group concludes by stating, ‘‘[It appears that tDCS] can be successfully administered using a double-blind procedure. . .’’ (p. 951). However, these researchers did not explore whether the tDCS administrator could differentiate between active and sham conditions; as such, this data only supports the notion of single-blinding efficacy (not double-blinding). 2. Is tDCS blinding effective? In aggregate, the data suggest that sensations generated in healthy participants during active tDCS are significantly greater than sensations generated during sham tDCS. However, what impact this difference has on the ability of participants to determine which form of stimulation they are receiving is less certain; namely, whereas O’Connell et al. (2012) reports participant blinding is compromised due to these enhanced sensations, Russo et al. (2013) reports participant blinding is unaffected. Additional research exploring the explicit link between sensory magnitudes and participant blinding efficacy is required to better understand this relationship. In addition, the only data supplied with regards to tDCS practitioner blinding suggests that assessors are able to guess the stimulation condition with better accuracy than would be expected by chance (due, primarily, to visible vasodilation under the electrode following active stimulation). This suggests that a true doubleblind experiment may be difficult to achieve whilst using a 0.06 mA/cm2 current density. If future data reveals that both single- and double-blinding under current methodologies are not effective, this will raise important questions concerning research undertaken in healthy
http://dx.doi.org/10.1016/j.clinph.2015.04.001 1388-2457/Ó 2015 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
2046
Editorial / Clinical Neurophysiology 126 (2015) 2045–2046
Table 1 Studies directly exploring the sensory effects and/or blinding efficacy in real vs. sham tDCS. Study
# of sessions
Target electrode
References electrode
Electrode Size
Current Density
Sham Protocol
Findings
Gandiga et al. (2006)
