Participants

The initial sample included 40 boys with ASD and 38 typically developing (TD) boys recruited at rehabilitation centers affiliated with the Moscow University of Psychology and Education and from the local community, respectively. The exclusion criteria were a known chromosomal syndrome (e.g., Down Syndrome, Fragile X syndrome), or a diagnosed neuropsychiatric disorder other than ASD (e.g., epilepsy). The diagnosis of ASD was confirmed by an experienced psychiatrist and was based on the Diagnostic and Statistical Manual of Mental Disorder-5 criteria as well as an interview with the parents/caregivers. Additionally, parents of all children were asked to fill in the Russian translation of the Autism spectrum Quotient (AQ) for children (Auyeung et al., 2008). The TD children who had scores above the cut-off of 76 (n = 2) and children with ASD who scored below this cut-off (n = 2) were excluded from the analysis. All participants with ASD were verbal. Subject's IQ was assessed using the Kaufman Assessment Battery for Children KABC II (Kaufman and Kaufman, 2004). All children had normal or corrected to normal vision according to the available medical records. The analysis was repeated for a subsample of children carefully matched on IQ. Approximately half of our ASD subjects had IQ scores below 94, the lowest value in the TD sample. Therefore, as a first step of IQ-matching ASD subjects with IQ scores above 90 were selected. As the next step, each subject of this subsample (n = 12) was individually matched with a TD participant in a way that a pair's Kaufman composite scores could differ by no more than 5 points. If several TD subjects could be matched to a particular ASD participant, then the TD subject with the closest IQ and age values was taken.

The previous studies suggest significant sex differences in etiological factors and the behavioral manifestation of ASD (Lai et al., 2015). Since the gender ratio in ASD is highly biased toward boys (Baio, 2012), a recruiting of the sufficient number of ASD girls for testing of the effect of gender might be problematic. Therefore, in our study we focused only on male participants, leaving the question about generalizability of our results across gender for the future studies.

The investigation was approved by the local ethics committee of the Moscow University of Psychology and Education and was conducted in accordance with the ethical principles of the Declaration of Helsinki regarding human experimentation. All children provided their verbal consent to participate in the study and were informed about their right to withdraw from the study at any time during the testing. Written informed consent was also obtained from a parent/guardian of each child.