Participants
Thirty high-functioning ASD and 49 TD Japanese subjects were tested. Participants from two discontinuous age groups were included – preschool children ages 3 to 6 years who never had any previous classroom experience and adolescents aged 11 to 15 years who had attended elementary school or junior high school. Thus, we compared the eye gaze behavior of four groups: preschool children with TD (n = 25; 7 males), preschool children with ASD (n = 12; 9 males), adolescents with TD (n = 24; 11 males), and adolescents with ASD (n = 18; 11 males). Participants with any past or present psychiatric illness and problems in eyeball movement or visual function and those who could not accomplish a ten-minute experiment were excluded. Written informed consents of all participants were obtained from their parents.
High-functioning ASD was diagnosed by specialists in the field of pediatric neurology and/or developmental pediatrics according to the following criteria: (i) autistic disorder or pervasive developmental disorder based on the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision [4]; (ii) a full scale intelligence quotient (FSIQ) score of ≥70 in the Wechsler Intelligence Scale for Children, Fourth Edition [15], for children >5 years old and a developmental quotient (DQ) of ≥70 in the Kyoto Scale of Psychological Development [16] for children less than 5 years old; and (iii) a score of ≥25.5 in the Childhood Autism Rating Scale (CARS) [17] or a score above the cutoff value for the relevant age group in the Parent-Interview ASD Rating Scale, Text Revision (PARS-TR) [18].
The CARS score indicates the severity of autism. Originally, scores < 30 indicated no autism and scores > 30 indicated mild-to-moderate or severe autism [17]. However, Tachimori et al. [19] recently reported that children diagnosed with Asperger’s syndrome could be distinguished from those without ASD using cutoff values of 25.5/26.0. We also used this criterion in differentiating children with ASD from children with TD. The Japanese version of the PARS-TR [18] was administered in a semi-structured interview together with a parent or family member of the child. It evaluates both the current symptoms and the most pronounced symptoms during infancy (the peak symptoms scale). There was a significant correlation between PARS-TR scores and Autism Diagnostic Interview-Revised (ADI-R) scores, particularly between qualitative abnormalities in reciprocal social interaction in the ADI-R score and social communication in the PARS-TR score [20].
Ethical approval
The research was approved by the Ethics Committee of Kansai Medical University (No. 1100).
Experimental procedure
Experiments were conducted in a quiet, well-lit room at Kansai Medical University Medical Center. Participants were seated in front of a 48 × 30 cm monitor for the presentation of visual stimuli, and their chins were placed on a chin rest to minimize head movement. The distance between the monitor and the chin rest was 60 cm. Partitions were placed to ensure that only the monitor was within the participant’s field of vision. On the monitor, two social images—a smiling human face and a classroom scene in a high school setting (Figure 1)—were presented sequentially, once for each, with no sound. Each stimulus was shown for nine seconds followed by an intertrial interval of one second. The duration of the whole experiment was approximately ten minutes (Figure 1, row 1). The participants were instructed to freely watch the static visual images on the monitor. The eye gaze position was measured at 250 Hz using an infrared camera attached to the bottom of the monitor (iView X RED, SensoMotoric Instruments, Teltow, Germany). Eye tracking data were analyzed using a custom software written in MATLAB (Mathworks, Natick, MA, USA).
Four of 79 participants for a human face and one participant for a classroom scene could not accomplish a ten-minute experiment, hence were excluded from the following analyses.
Statistical analysis
To compare the eye gaze behavior between children with TD and children with ASD and between preschool children and adolescents, we first identified the visual areas that were regions of interest (ROIs) (Figure 1, second row). The ROIs were set as eyes and mouths for the human face and the face, finger, pointed-at object, and wall for the high school classroom. We compared the eye gaze time for each ROI using a two-way analysis of variance (ANOVA) with two factors: age (preschool children vs. adolescents) and development (ASD vs. TD). Within the nine seconds of each stimulus presentation, we measured the duration of the gaze in each ROI. Statistical analyses were performed using SPSS version 22.0 (IBM Corp., Armonk, NY, USA).