Participants were categorised in one of the four experimental groups (typically developing controls; ASD-only; ADHD-only; comorbid ADHD+ASD), by analysing information collected from parents and teachers, and the direct assessment of clinical symptoms of ASD. A diagnosis of ASD and/or ADHD was confirmed using combined information from the clinical measures presented in this paragraph, including:
• Conners’ Rating Scales (CRS-3)
• Social Communication Questionnaire (SCQ)
• Autism Diagnostic Observation Schedule (ADOS-2) • Development and Well-Being Assessment (DAWBA)
2.2.1.1. ADHD symptoms: Conners' Rating Scales, Third Edition (CRS-3)
The evaluation of symptoms of ADHD was primarily derived from the CRS-3 (Conners, 2008), which were completed by children’s parents and teachers, and gave, for each participant, a profile of different behavioural symptoms associated with ADHD symptomatology. Besides giving information about problems associated to inattentiveness and hyperactivity/impulsivity, the CRS provide a set of output measures about executive functioning, learning problems, aggression and relations with peers or family members. The manual of the CRS-3 suggests that a cut-off T-score of 65 on these scales is likely to differentiate individuals with behavioural problems associated with clinically significant ADHD symptomatology from those who show non-clinical levels of ADHD-like behaviours (Conners, 2008).
2.2.1.2. ASD symptoms: Social Communication Questionnaire (SCQ)
The SCQ (Berument et al., 1999; Rutter et al., 2003) is a commonly used screening measure of ASD symptomatology, which showed high sensitivity (96%) and specificity (80%) in discriminating between patients showing symptoms of ASD from individuals showing no signs of this condition (Chesnut et al., 2017). Specifically, a total score of 15 on SCQ has been suggested as the threshold to differentiate between people at-risk vs people not-at-risk of ASD (Rutter et al., 2003). Participating children’s parent and teacher completed the SCQ: while parents completed the SCQ-Lifetime version, which identifies behavioural signs of ASD during early infancy and childhood, the SCQ-Current version was completed by teachers, who were asked to answer referring to the child’s behaviour in the past 3 months.
2.2.1.3. Autism Diagnostic Observation Schedule, Second Edition (ADOS-2)
The ADOS-2 (Lord et al., 2012) is a semi-structured, standardised tool used to indicate the presence of clinical symptoms of ASD in children and adolescents. This measure is widely used in academic and clinical practice, since it has been recognised a gold standard for the diagnostic evaluation of ASD, especially among children and adolescents (Kamp-Becker et al., 2018). It is comprised of different activities, involving play and verbal questioning, but also stimulating social interaction with the examiner. It provides an objective measure of social, communicative, play and stereotyped behaviours which are part of the ASD phenotype. Specifically, the coding of the entire assessment by trained researchers, provides a diagnostic label of ASD, ASD spectrum or ‘no autism’, according to different cut-offs which are dependent on the ADOS- module and age of participants, but also dimensional scales of different aspects of ASD symptomatology.
2.2.1.4. Development and Well-Being Assessment (DAWBA) and Strengths and Difficulties Questionnaire (SDQ)
The DAWBA (Goodman, Ford, Richards, Gatward, & Meltzer, 2000) is a battery of questionnaires and interviews which was completed by the children’s parents and gave a computer-generated summary of prediction for different psychiatric conditions. Within the DAWBA assessment, parents filled the Strengths and Difficulties Questionnaire (Goodman, 2001), which gives a measure of children and adolescents’ prosocial behaviours and psychopathology. Computer generated DAWBA diagnostic predictions and SDQ scores, were evaluated and confirmed or overturned by experienced clinical practitioners (CH and PK). The DAWBA has been shown to be effective in discriminating patients showing psychiatric or psychological symptoms from people who did not show any sign of these conditions, with high specificity (89%) and sensitivity (92%) in recognising the presence of clinical signs of psychopathology in children and adolescents (Goodman et al., 2000).
2.2.1.5. Wechsler Abbreviated Scale of Intelligence, Second Edition, (WASI- II)
The WASI-II (Wechsler, 2011) was used to obtain a complete and reliable measure of cognitive functioning across the sample of participating children. The WASI-II is a revision of the WASI, which has been reported to show high validity and reliability (McCrimmon et al., 2012). It includes 4 subtests, assessing verbal (Vocabulary and Similarities sub-tests) and perceptual reasoning (Block Design and Matrix Reasoning sub-test) abilities. Three output measures can be obtained, namely full-scale IQ (FSIQ), verbal comprehension index (VIQ) and perceptual reasoning index (PIQ).
2.2.1.6. Child Sensory Profile 2
The Child Sensory profile, Second Edition (Dunn, 2014) is a standardized evaluation of sensory processing behaviours in childhood, and it was used to obtain a parent-based measure of children’s sensory issues and atypicalities, which could not be noticeable in the experimental setting. Little et al. (2017) used this tool to assess sensory processing mechanisms in children with ASD and ADHD, showing that the Sensory Profile is a reliable and valid measure to compare different sensory processing behaviours of individuals with these conditions and typically developing controls. Information about four characteristics of information processing, associated with sensitivity to sensory stimulation and self-regulation strategies (Dunn, 1997), are the main dimensional outcomes derivable from this tool, as following: seeking, avoiding, sensitivity and registration. Furthermore, information about sensory processing mechanisms is collected from parents regarding child’s auditory, visual, touch, and oral sensory modalities, besides patterns of movement and body positioning in the space.
2.2.1.7. Socio-economic status
A short semi-structured interview, indicated as the first choice by the UK government for both official statistics and academic research, i.e., the National Statistics Socio-economic Classification, NS-SEC (Rose et al., 2005), was carried out during the collection of general information about the children from their parent, with the aim of evaluating the children’s family socio-economic status.