# Abstract

 

## Background Working memory (WM) often is impaired in autism spectrum disorder (ASD). Such impairment may underlie core deficits in cognition and social functioning. Transcranial direct current stimulation (tDCS) has been shown to enhance WM in both healthy adults and clinical populations, but its efficacy in ASD is unknown. We predicted that bifrontal tDCS would improve WM performances of adults with high-functioning autism during active stimulation compared to sham stimulation and that such enhancement would generalize to an untrained task. 

## Methods Twelve adults with high-functioning ASD engaged in a battery of WM tasks that included backward spatial span, backward digit span, spatial *n*-back and letter *n*-back. While engaged, 40 min of 1.5 mA bifrontal stimulation was applied over the left and the right dorsolateral prefrontal cortices (DLPFC). Using a single-blind crossover design, each participant received left anodal/right cathodal stimulation, right anodal/left cathodal stimulation, or sham stimulation, in randomized counterbalanced order on three separate days. Following tDCS, participants again engaged in letter and spatial *n*-back tasks before taking the Brief Test of Attention (BTA). We used repeated-measures ANOVA to compare overall performance on the WM battery as measured by a composite of *z*-scores for all five measures. Post hoc ANOVAs, *t* tests, Friedman's tests, and Wilcoxon signed-rank tests were used to measure the online and offline effects of tDCS and to assess performances on individual measures. 

## Results Compared to sham stimulation, both left DLPFC anodal stimulation (*t*~11~ = 5.4, *p* = 0.0002) and right DLPFC anodal stimulation (*t*~11~ = 3.57, *p* = 0.004) improved overall WM performance. Left anodal stimulation (*t*~11~ = 3.9, *p* = 0.003) and right anodal stimulation (*t*~11~ = 2.7, *p* = 0.019) enhanced performances during stimulation. Enhancement transferred to an untrained task 50 min after right anodal stimulation *(z*~11~ = 2.263, *p* = 0.024). The tasks that showed the largest effects of active stimulation were spatial span backward (*z*~11~ = 2.39, *p* = 0.017) and BTA (*z*~11~ = 2.263, *p* = 0.024). 

## Conclusions In adults with high-functioning ASD, active bifrontal tDCS given during WM tasks appears to improve performance. TDCS benefits also transferred to an untrained task completed shortly after stimulation. These results suggest that tDCS can improve WM task performance and could reduce some core deficits of autism. 

## Trial registration [NCT01602263](#) # Results 

## Primary outcome: overall WM performance As shown in the final row of Table [1](#), the composite *z*-scores (full battery) were −0.25 (SD 0.71) for sham stimulation, 0.13 (SD 0.82) for left anodal stimulation, and 0.11 (SD 0.81) for right anodal stimulation, which repeated measures ANOVA showed to be significantly different (*F*~2,22~ = 12.85, *p* = 0.0002). Post hoc, paired-sample *t* tests revealed that performances associated with both left anodal active stimulation (*t*~11~ = 5.4, *p* = 0.0002) and right anodal active stimulation (*t*~11~ = 3.57, *p* = 0.004) were better than performances associated with sham tDCS. For composite *z*-scores (full battery), Cohen's *d*~*s*~ effect sizes were 0.50 for left anodal stimulation and 0.47 for right anodal stimulation. There was no difference in overall WM performance between the two active stimulation conditions (*t*~11~ = 0.26, *p* = 0.796). To assess the role of session order on full-battery composite *z*-scores, we compared them across sessions. We found composite *z*-scores (full battery) of −0.03 (SD 0.81) for session 1, −0.06 (SD 0.75) for session 2, and 0.09 (SD 0.83) for session 3. Repeated measures ANOVA showed no significant practice effects across sessions (F~2,22~ = 0.88, *p* = 0.428). ::::table-wrap

::: caption
Behavioral data and statistics: Composite *z*-score (online) is the mean
of four *z*-scores: spatial span backward maximum length, digit span
backward maximum length, online letter *n*-back accuracy, and online
spatial *n*-back accuracy. Composite *z*-score (offline) is the mean of
three *z*-scores: offline letter *n*-back accuracy, offline spatial
*n*-back accuracy, and BTA raw score. Composite *z*-score (full battery)
is the mean of five *z*-scores: spatial span backward maximum length,
digit span backward maximum length, letter *n*-back accuracy (mean of
online and offline percentages), spatial *n*-back accuracy (mean of
online and offline percentages), and BTA raw score. Also included are
means and standard deviations for 1-back, 2-back, and 3-back for letter
and spatial *n*-back, both online and offline
:::

```
                                                                            Sham            Left anodal     Right anodal    Repeated measures ANOVA F(2,22)   Friedman's Test χ2   *p* value
```

______________________________________________________________________

WMS-III, spatial span backward, longest span (blocks) 4.8 (0.9) 6.2 (1.8) 6.0 (0.9) 9.24 0.01
WMS-III, digit span backward, longest span (digits) 5.3 (1.5) 5.5 (1.6) 5.4 (2.0) 0.12 0.891
Spatial *n*-back accuracy (online) 62.4% (25.0%) 68.6% (22.1%) 66.7% (23.2%) 3.17 0.205
1-back 74.7% (23.1%) 80.0% (19.9%) 79.6% (21.4%)\
2-back 61.4% (27.9%) 68.5% (26.4%) 66.2% (25.5%)\
3-back 49.7% (25.8%) 55.6% (22.9%) 52.7% (25.0%)\
Letter *n*-back accuracy (online) 69.6% (30.2%) 76.2% (22.5%) 76.3% (26.8%) 0.5 0.779
1-back 77.3% (26.6%) 82.9% (19.9%) 84.8% (21.7%)\
2-back 70.4% (32.4%) 76.0% (26.0%) 76.2% (29.3%)\
3-back 61.1% (33.1%) 69.7% (24.2%) 68.1% (29.7%)\
Composite *z*-score (online) −0.25 (0.74) 0.15 (0.87) 0.09 (0.81) 7.68 0.003
Spatial *n*-back accuracy (offline) 63.9% (24.9%) 70.6% (20.9%) 67.7% (24.2) 0.55 0.758
1-back 76.2% (21.9%) 85.1% (20.2%) 78.4% (21.7%)\
2-back 63.2% (29.0%) 70.5% (24.3%) 69.8% (26.8%)\
3-back 52.5% (26.5%) 56.3% (21.8%) 54.6% (26.0)\
Letter *n*-back accuracy (offline) 72.3% (29.8%) 78.5% (22.5%) 76.5% (25.9%) 0.17 0.92
1-back 80.7% (23.2%) 86.1% (20.7%) 84.3% (21.6%)\
2-back 71.1% (33.2%) 77.8% (22.3%) 77.8% (27.2%)\
3-back 65.1% (33.6%) 71.6% (26.2%) 67.4% (30.0)\
Brief Test of Attention (offline) 6.8 (2.5) 7.4 (2.2) 7.9 (2.5) 7.09 0.029
Composite *z*-score (offline) −0.18 (0.92) 0.09 (0.77) 0.09 (0.93) 7.17 0.028
Spatial *n*-back accuracy combined (mean of online and offline percentages) 63.2% (24.7%) 69.6% (20.7%) 67.3% (23.6%) 0.67 0.717
Letter *n*-back accuracy combined (mean of online and offline percentages) 71.0% (29.8%) 77.4% (22.1%) 76.4% (26.3%) 0.17 0.92
Composite *z*-score (full battery) −0.25 (0.71) 0.13 (0.82) 0.11 (0.81) 12.85 0.0002:::: The composite *z*-scores (online) were −0.25 (SD 0.74) for sham stimulation, 0.15 (SD 0.87) for left anodal stimulation, and 0.09 (SD 0.81) for right anodal stimulation, which repeated measures ANOVA showed to be significantly different (*F*~2,22~ = 7.68, *p* = 0.003). Post hoc, paired-sample *t* tests revealed that performances as measured by composite *z*-scores of the four working memory tests given during both left anodal active stimulation (*t*~11~ = 3.9, *p* = 0.003, Cohen's *d*~*s*~ = 0.52) and right anodal active stimulation (*t*~11~ = 2.7, *p* = 0.019, Cohen's *d*~*s*~ = 0.46) were better than performances during sham stimulation. Performances during the two active stimulation conditions (*t*~11~ = 0.60, *p* = 0.563) were not significantly different. The composite *z*-scores (offline) were −0.18 (SD 0.92) for sham stimulation, 0.09 (SD 0.77) for left anodal stimulation, and 0.09 (SD 0.93) for right anodal stimulation, which Friedman's test showed to be significantly different (χ^2^ = 7.17, *p* = 0.028). Follow-up Wilcoxon signed-rank tests showed that after left anodal stimulation performances were not significantly better compared to after sham stimulation (*z* = 1.88, *p* = 0.06, *r* = 0.27, Cohen's *d*~*s*~ = 0.33), but performances after right anodal stimulation were significantly better than after sham stimulation (*z* = 2.35, *p* = 0.019, *r* = 0.34, Cohen's *d*~*s*~ = 0.30). Although performances after right anodal stimulation were marginally better than after left anodal stimulation, the difference was not significant (*z* = 1.49, *p* = 0.136). 

## Secondary outcomes: individual WM measures Mean (SD) scores for each of the five measures that made up our overall WM composite are presented in Table [1](#). As shown in the first row, the participants' longest backward spatial span was 4.8 (SD 0.9) blocks during sham stimulation compared to 6.2 (SD 1.8) blocks during left anodal stimulation and 6.0 (SD 0.9) blocks during right anodal stimulation. Friedman's test showed that these differed significantly (χ^2^ = 9.24, *p* = 0.010). Post hoc Wilcoxon signed-rank tests indicate that left anodal stimulation (*z*~11~ = 2.21, *p* = 0.027, *r* = 0.32) and right anodal stimulation (*z*~11~ = 2.39, *p* = 0.017, *r* = 0.34) were both associated with longer maximum spans than sham stimulation. Cohen's *d*~*s*~ effect sizes were 1.03 for left anodal stimulation and 1.39 for right anodal stimulation. Performances during the two active stimulation conditions were not different (*z*~11~ = 0.30, *p* = 0.762). We examined the role of practice effects on all tests for which significant differences were found between stimulation conditions. Participants' longest spatial span was 5.6 blocks (SD 1.2) during session 1, 5.5 blocks (SD 1.1) during session 2, and 5.9 blocks (SD 1.8) during session 3. Friedman tests showed no significant practice effects across sessions (χ^2^ = 0.05, *p* = 0.973). Also as shown in Table [1](#), participants' longest backward digit span averaged 5.3 (SD 1.5) digits during sham compared to 5.5 (SD 1.6) digits during left anodal and 5.4 (SD 2.0) during right anodal stimulation. Friedman's test showed no significant differences in longest digit span between conditions (χ^2^ = 0.41, *p* = 0.814). Spatial *n*-back accuracy was calculated as (hits + correct negatives)/(total items). Online *n*-back accuracy was 62.4% (SD 25.0%) during sham stimulation, 68.6% (SD 22.1%) during left anodal stimulation, and 66.7% (SD 23.2%) during right anodal stimulation. Friedman's test showed no significant differences between conditions (χ^2^ = 3.17, *p* = 0.205). Offline spatial *n*-back accuracy was 63.9% (SD 24.9%) after sham stimulation, 70.6% (SD 20.9%) after left anodal stimulation, and 67.7% (SD 24.2%) after right anodal stimulation, which were not significantly different (χ^2^ = 0.55, *p* = 0.758). Letter *n*-back accuracy was calculated as (hits + correct negatives)/(total items). Online *n*-back accuracy was 69.6% (SD 30.2%) during sham stimulation, 76.2% (SD 22.5%) during left anodal stimulation, and 76.3% (SD 26.8%) during right anodal stimulation. These were not significantly different (χ^2^ = 0.5, *p* = 0.779). Offline letter *n*-back accuracy was 72.3% (SD 29.8%) after sham stimulation, 78.5% (SD 22.5%) after left anodal stimulation, and 76.5% (SD 25.9%) after right anodal stimulation. These also were not significantly different (χ^2^ = 0.17, *p* = 0.920). Brief Test of Attention (BTA) raw scores averaged 6.8 (SD 2.5) after sham stimulation compared to 7.4 (SD 2.2) after left anodal and 7.9 (SD 2.5) after right anodal stimulation. We found the predicted main effect of condition on BTA performance with Freidman's test (χ^2^ = 7.09, *p* = 0.029). Follow-up Wilcoxon signed-rank tests revealed that BTA performance after left anodal stimulation was not significantly different from BTA performance after sham stimulation (*z*~11~ = 1.63, *p* = 0.102, *r* = 0.24, Cohen's *d*~*s*~ = 0.27), but BTA performance after right anodal stimulation was better than after sham (*z*~11~ = 2.26, *p* = 0.024, *r* = 0.33, Cohen's *d*~*s*~ = 0.46). Additionally, BTA performance was better after right than left anodal stimulation (*z*~11~ = 2.45, *p* = 0.014, *r* = 0.35, Cohen's *d*~*s*~ = 0.22). There was no effect of practice on BTA performance from session 1 to session 3 (*z*~11~ = −1.12, *p* = 0.265). The results of our secondary analyses are depicted in Fig. [3a--c](#). 

## Blinding We hypothesized that participants would tolerate tDCS and remain blind to condition. All participants completed the three sessions with no adverse events. After each session, all 12 participants were asked to indicate (yes/no) whether they thought they had received real stimulation. Eleven participants endorsed left anodal stimulation as real, nine endorsed right anodal stimulation as real, and 10 endorsed sham as real. McNemar's test of related samples showed no differences in endorsement rates between sham and both left (*p* > 0.99) and right (*p* > 0.99) active anodal stimulation.


Extracted trial result:
Outcome: overall working memory (WM) performance in autism spectrum disorder (ASD) (continuous)
Intervention: left dorsolateral prefrontal cortex (DLPFC) anodal transcranial direct current stimulation (tDCS)
Comparator: sham stimulation
Intervention group size: 12
Intervention mean: 6.2
Intervention standard deviation: 1.8
Comparator group size: 12
Comparator mean: 4.8
Comparator standard deviation: 0.9