Abstract
BackgroundWorking 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.MethodsTwelve 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.ResultsCompared to sham stimulation, both left DLPFC anodal stimulation (t11 = 5.4, p = 0.0002) and right DLPFC anodal stimulation (t11 = 3.57, p = 0.004) improved overall WM performance. Left anodal stimulation (t11 = 3.9, p = 0.003) and right anodal stimulation (t11 = 2.7, p = 0.019) enhanced performances during stimulation. Enhancement transferred to an untrained task 50 min after right anodal stimulation (z11 = 2.263, p = 0.024). The tasks that showed the largest effects of active stimulation were spatial span backward (z11 = 2.39, p = 0.017) and BTA (z11 = 2.263, p = 0.024).ConclusionsIn 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 registrationNCT01602263
Highlights
Working memory (WM) often is impaired in autism spectrum disorder (ASD)
Pairedsample t tests revealed that performances associated with both left anodal active stimulation (t11 = 5.4, p = 0.0002) and right anodal active stimulation (t11 = 3.57, p = 0.004) were better than performances associated with sham Transcranial direct current stimulation (tDCS)
Recent reviews suggest that spatial WM is more impaired than verbal WM in individuals with ASD [2], which may explain why we found larger effects of prefrontal tDCS on spatial WM performance than have previously been reported in healthy adults [100, 114]
Summary
Working memory (WM) often is impaired in autism spectrum disorder (ASD) Such impairment may underlie core deficits in cognition and social functioning. 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. Working memory is critical for many complex cognitive functions including language [15, 16], general intelligence, and reasoning [17]; WM deficits likely produce profound effects in individuals with autism. Poor WM likely contributes to social problems in people with ASD [18] because maintaining continually changing social information in temporary storage (WM) is necessary for social flexibility [19].
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