A Single Blind, Randomized Controlled Trial of Anodal Transcranial Direct-Current Stimulation Against Cathodal and Sham Stimulation in Adults with High-Functioning Autism

Background: Evidence suggests that Transcranial Direct Current Stimulation (tDCS) on the left dorsolateral prefrontal cortex (DLPFC) leads to an increase in working memory (WM) performance in typically developed individuals (TD) (Fregni et al., 2005; Jo et al., 2009; Andrews et al., 2011) and individuals with psychiatric disorders, such as depression (Oliveira et al., 2013), schizophrenia (Papazova et al., 2018) and ADHD (Nejati et al., 2017). However, it has not been properly looked at when it comes to autism spectrum disorder (ASD), a population which experience WM impairments.

Objectives: Does anodal tDCS lead to an improvement in WM accuracy scores when administered over the left DLPFC and compared to sham in adults with high functioning Autism? Secondly, are the observed effect of tDCS over the left DLPFC and WM scores dependent on polarity?

Methods: 23 individuals with ASD between the ages of 18–35 (M=26; SD=5.20) were recruited for this study. The participants had IQ scores that ranged from 74 to 138 (M = 109.48; SD= 15.41) on the Wechsler Abbreviated Scale of Intelligence and thus were classified as high functioning. Participants underwent a three-back working memory task from which their accuracy, reaction time and errors were tracked before, during and after receiving 1.5 mA for 15 minutes over the DLPFC. Additionally, participants completed a self-administered WM questionnaire (WMQ; Vallat-Azouvi, Pradat-Diehl and Azouvi, 2012) which asks about everyday WM difficulties. The ASD group scores were compared to TD individuals, with no significant difference between IQ and age.

Results: WM performance increased in individuals with ASD following anodal stimulation compared to baseline (t=-4.66, df=22, P<0.001), sham (t=2.82, df=22, P=0.010), cathodal (t=3.62, df=22, P=0.002), and, during anodal stimulation (t=4.13, df=22, P<0.001). There was a significantly less errors made between post anodal stimulation and baseline (t=4.87, df=22, P<0.001), anodal stimulation and sham (t=2.97, df=22, P=0.007), anodal stimulation and cathodal stimulation (t=2.59, df=22, P=0.017), however, there was no significant difference between during and post anodal stimulation (t=-2.05, df=22, P=0.053). Furthermore, the analysis showed that there was no significant difference in reaction time during any of the conditions (P>0.05).

Moreover, the WMQ analysis demonstrated that individuals with ASD (M=51.78; SD=20.64) had worse scores on the WMQ than the TD group (M=16.14; SD= 8.36) showing that individuals with ASD report greater difficulties in everyday WM (t=7.53, df=43, P<0.001).

Conclusions: Our results indicate that only anodal stimulation of the left prefrontal cortex, but not cathodal stimulation of left DLPFC increases the accuracy of the task performance when compared to sham stimulation of the same area. This accuracy enhancement during active stimulation cannot be accounted for by slowed responses, as response times were not changed by stimulation. Furthermore, this effect depends on the stimulation polarity. These findings may be helpful to develop future interventions. The proposed intervention can feasibly be applied in a clinical setting, and we thus expect the experiment to lead ultimately to an optimised treatment of WM impairments in individuals with ASD to enhance WM impairments and the quality of life for individuals with ASD.