1. N. Bridges1, A. McKinley2, C. Walters2, M. Weisend3 ,C. Goodyear1
Session : #205
Sham Stimulation Effectiveness and Its Impact on Transcranial Direct Current Stimulation (TDCS) Learning-Enhancement Effects
N. Bridges1, A. McKinley2, C. Walters2, M. Weisend3 ,C. Goodyear1
1 Infoscitex, Dayton, OH, 2Air Force Research Laboratory, Dayton OH, 3Mind Research Network, Albuquerque, NM
Introduction
Brain stimulation studies often use a sham stimulation condition that mimics the physical sensation experienced during stimulation (e.g. itching). This pseudo-stimulation/placebo condition serves to contrast the stimulation condition being studied and differentiate physiologic, behavioral and cognitive tDCS-effects from psychological effects. Typically, sham stimulation consists of a significantly lower amperage which lasts either the amount of time it takes actual stimulation to dissipate or throughout the duration of the study. AFRL researchers used the former approach in a study investigating whether tDCS could enhance learning in a threat identification synthetic aperture radar training task. This analysis served to assess the effectiveness of the sham condition by examining reported sensation ratings and each subject’s ability to distinguish between the two stimulation conditions. Such findings elucidate the potential psychological factors tDCS might play in the performance results found in the study.
Results
Discussion
Figure 2:
Differences between active and sham conditions were greater in session 2 compared to session 1
This suggests that previous stimulation exposure has an effect on objective sensation ratings
Figure 3
Itchiness was the predominant sensation experienced by subjects during active stimulation
Overall, discomfort ratings were low which suggests itching was present but not predominant
Table 1
61% of subjects could tell the difference between active and sham stimulation conditions (similar findings exist2)
This is likely due in large part to differences in itchiness between the two stimulation conditions
Figure 4
Active stimulation improved performance on the first test (where subjects where naïve to stimulation)
This suggests that stimulation exerted physiologic effects outweighed any possible psychological effects due to sensation (similar findings exist1,2)
Differences between active and sham stimulation ratings were greater in session 2
Ratings are higher in the active compared to sham condition overall * * * * * * * * * * * *
Figure 3. Overall sensation ratings.
Green and gold lines represent the average sensation ratings for active and sham conditions respectively. P-values are from an F-test for the condition x block interaction resulting from a three factor mixed design ANOVA (condition, block, stimulation group) performed for each rating type separately and for the average across rating type. * * * *
Figure 2. Sensation ratings.
The values above represent the mean ratings (SQ1-SQ4) from the four training modules within session 1 and session 2. Values in red represent the AS group (n=8) while values in blue represent the SA group (n=8). Values with an identically colored asterisk (found above the data point) are significantly different from each other (two-sample t-test, p<0.05).
Test 1 performance is greater in the active versus sham condition
Methods mA EQ
Training Block + Test
Session 1
Session 2 t
SQ1
SQ2
SQ3
SQ4
Break
TM1
TM2
TM3
TM4
Group 2
SA mA
0.1
2.0
t=30 seconds
t=30 minutes
Real
Active
Group 1
AS Re
Sham
Subject’s ability to detect stimulation condition is greater than by chance *
Correct
Incorrect
No Difference
Total
Number 11 1 6 18
Percent
61% 6%
33%
39% *
Table 1. Stimulation condition detection percentages.
Subjects were provided a questionnaire (EQ) at the end of the study asking them which session they thought they received active and sham stimulation. The above table displays the percentage values of their responses.
Figure 4. Task Performance.
Values in red represent the AS group (n=9) while values in blue represent the SA group (n=10). In test 1, performance in the active condition is significantly greater than task performance in the sham condition (p=0.0272). SA group performance increased significantly from test 1 to test 2 (p=0.0116).
Conclusion
61% of subjects could distinguish between active and sham stimulation conditions. Physiologic effects, however, outweighed any potential sensation/psychological effects that may exist. Such findings suggest that brain stimulation can effectively be used to enhance learning but that potential psychological/sensation factors should be taken into account when interpreting sham stimulation conditions. Future sham improvements should take approaches that reduce itchiness into consideration.
Subjects Experience More Sensation in the Active Compared to Sham Condition
Sham
Active F p
Mean
0.49
1.08
7.36
0.0161
Collaboration
Figure 1: Experimental design
Half of the subjects received active (A) and sham (S) stimulation during the first and second blocks (AS) respectively while the other half received vice versa (SA).
A sensation questionnaire that asked subjects to rate sensations of itchiness, pain, heat and discomfort on an 11-point Likert scale (SQ1-SQ4) preceded each of the four training modules (TM1-TM4).
At the end of the entire learning task, we asked subjects to identify which of the two blocks they thought they received real stimulation using an exit questionnaire (EQ).
Subjects Experience Varying Degrees of Sensation Depending on Sensation Type
Itching
Pain
Heat
Discomfort F p
Sham
0.74
0.31
0.4
0.5
6.12
0.0013
Active
1.59
0.75
0.79
1.2
7.57
0.0003
References:
Bullard, L. M., Browning, E. S., Clark, V. P., Coffman, B. A., Garcia, C. M., Jung, R. E., et al. (2011). Transcranial direct current stimulation's effect on novice versus experienced learning. Exp Brain Res, 213(1), 9-14.
Clark, V. P., Coffman, B. A., Mayer, A. R., Weisend, M. P., Lane, T. D., Calhoun, V. D., et al. (2010). TDCS guided using fMRI significantly accelerates learning to identify concealed objects. Neuroimage, 59(1),
Subjects Experience Less Sensation Over Time
SQ1
SQ2
SQ3
SQ4 F p
Sham
1.46
0.19
0.1
19.08
0.0001
Active
1.53
1.09
0.88
0.83
9.32