Response Time Intra-Subject Variability: Commonalities Between Children with Autism Spectrum Disorder and Children with Children with ADHD

Background: The high prevalence of ADHD symptoms in individuals with ASD has been widely reported in clinical and epidemiological samples. Yet, to date, the mechanisms underlying ADHD–like symptoms in ASD and their potential overlap with those present in children with typical ADHD are under-explored. A potential candidate for investigation is response time intra-subject variability (RT-ISV) given consistent findings of increased RT-ISV in typical individuals with ADHD. A few studies have directly compared RT-ISV in children with ASD and ADHD yielding contradictory results. 

Objectives: We aimed to examine whether RT-ISV distinguishes children with ASD from those with typical ADHD and whether it characterizes the subgroup of children with ASD and ADHD–like symptoms (ASD⁺).

Methods: A group of 133 boys aged between 7 and 14.9 years participated in this study. They included 55 with ASD, 47 with ADHD, and 31 typically developing (TD). RT data were collected during a fixed-sequence 5.5-min version of the Sustained Attention to Response Task (SART). We measured RT-ISV both as standard deviation of RT (SD-RT) and as amplitude of frequency fluctuations measured with Morlet wavelet transform. Specifically, based on theoretical models of neuronal oscillations, we selected a priori the following frequency bands: Slow-2 [0.2-0.34Hz], Slow-3 [0.073 – 0.2 Hz], Slow-4 [0.027-0.073 Hz], and Slow-5 [0.010-0.027 Hz]. We first compared the three groups (ASD, ADHD, TD) with one-way ANOVA. We then conducted post-hoc pair-wise group comparisons, Bonferroni corrected.Secondary group comparisons were also conducted after dividing the ASD group in 29 children with ASD⁺ and 26 ASD^- per DSM-IV total T-score ≤ 65 on the Conners Parent Rating Scales (CPRS). 

Results: The three groups differed in SD-RT with moderate effect size (167±52 vs. 178±54, and 151±45 ms, in ASD, ADHD, and TD children, respectively; Cohen’s d: 0.4). Additionally, both children with ASD and children with ADHD showed increased RT fluctuations at the fastest frequencies detected by the SART (Slow-2; 33207±19959 vs. 34501±21550, and 21647±10297 in the ASD, ADHD, and TD groups, respectively; p<0.01). Relative to TD children increased amplitudes of Slow-2 characterized both ADHD and ASD groups which, in turn, did not differ from each other. Secondary analyses showed that only the ASD⁺ and ADHD groups showed significantly increased amplitude of Slow-2 relative to TD. Children with ASD^- did not differ significantly from any of the other groups. Amplitude of frequencies slower than 0.2 Hz did not differ between groups.

Conclusions: Our results suggest that 1) increases of RT fluctuations > 0.2 Hz (i.e., cycles of ~5 sec) can serve as a potential marker of ADHD symptoms regardless of diagnostic categorical boundaries, and 2) children with ADHD and those with ASD⁺ may, at least in part share common physiopathological mechanisms. Fluctuations in RT may reflect intrinsic brain activity occurring in similar frequency ranges. Future work will need to directly examine the relationship between RT-ISV and fluctuations of brain intrinsic spontaneous activity.