We investigated the feasibility of using Gazefinder on infants from 4 to 11 months of age. The purpose of this study was to confirm the reciprocity with previously reported gaze patterns of infants under 12 months in measurements with this device. Despite the many insights into ASD development stemming from eye tracking research, difficulty when comparing results from different eye tracking studies of children has been noted [25]. One systematic review [26], investigated stimuli dimensions and experimental paradigms in eye-tracking research for young children at risk for ASD, highlights variability in eye-tracking protocols and heterogeneity of stimuli as factors that weaken the value of eye-tracking as an objective, reliable screening tool. Although the gaze patterns data itself in our study may not be new to previous studies, it is necessary to confirm that the data obtained by this device and protocol, were consistent with previous reports at this developmental stage, before conducting future large-scale studies. There were few reports that examine the validity of measurement protocols themselves, and this point was one of a novelty of our study.
The mean fixation time percentage of all sequences was 81.0% and no significant difference among the four age groups. In previous research on children between the ages of 1 year and 6 months to 2 years and 2 months [14], the Modified Checklist of Autism for Toddlers (M-CHAT) and Gazefinder gaze fixation data showed a significant correlation with the Algorithm Diagnosis. In this algorithm, the Youden Index (sensitivity + specificity − 1) was highest when using gaze data that was ≥ 50% of the fixation time percentage of all sequences. The fixation time of all sequences in our study may be enough to evaluate the gaze patterns data༎In the following, we discuss the gaze patterns in each movie.
The movies I-V were human face stimuli. From birth, infants show interest in human faces [26]. Previous studies have suggested that children with ASD have less attention than children with TD to the human eyes [3, 7, 9, 13, 21]. However, a study that tracked ASD infants from 2 to 24 months, suggests that at 6 months fixations towards the eyes region are just beginning to decline, with maximal reductions in eye region fixation not occurring until 2 years [7]. Another study reported infants with ASD consistent gaze to the eyes region at 6 months, and greater amounts of fixation to the mouth during live interaction, predicted higher levels of expressive language at outcome and greater rates of growth [12]. The study using Gazefinder, also showed the moving mouth attracted the attention of young children with or without ASD [15]. Thus, infants in this age group, with or without ASD, possibly show more preference to human eyes than mouth, and are easy to pay attention by the moving mouth. Our results were consistent with previous reports and the participants identified human eyes and mouths on the monitor as targets of gaze starting at four months of age. The gaze behavior to the human eyes and mouth may not provide early biomarkers for autism around 1 year of age but can be used in predicting language acquisition.
The movie VI presented the biological motion. TD infants preferentially attend to biological motion within the first days of life, and preferential attention to biological motion is thought as a precursor to the capacity for attributing intentions to others [10, 16, 17, 21]. A recent study that tested gaze patterns in repeatedly low ASD risk infants from 2 until 24 months of age, reported preference for upright biological motion that emerges by 3 months and continues to increase until 24 months [21]. In our study, infants from 4 to 11 months of age showed consistent preference to upright biological motion, but the aging effect, greater preference to biological motion by age, was not shown. This may depend on the quality of the stimulus movies and fewer sample sizes to detect the preference. Moreover, the previous study [21] provided longer trial time (mean duration = 61s), with analyses focused more on the second half of the trials, than our study (11s), and it may not be sufficient time for interstimulus shifting and visual exploration for the infants. Previous studies reported, by two-three years old, children with ASD gaze at upright biological motion for shorter periods than did the TD children [10, 16, 17], while other studie reported preschool children with ASD gazed at upright biological motion approximately the same as TD children [13]. The preference to biological motion under passive view conditions for children seems to be susceptible to the quality of stimuli and their age. Moreover, there have been few studies that reported changes in the gaze patterns of infants under 1year old with ASD over time.
The movies VII and VIII simultaneously presented human and geometry. ASD children prefer to gaze at highly repetitive images (geometric patterns) rather than social images (human) [8, 20], and it was associated with increased symptom severity of ASD [8]. In previous research using Gazefinder to children between the ages of 1 year and 6 months to 2 years and 2 months, the M-CHAT and the fixation data of human and geometry (Movie VII and VIII), showed a significant correlation [14]. However, it has been known that TD individuals visually fixated on geometric images were enhanced with increasing age [8] and the studies using Gazefinder reported the same trend [13, 15, 19]. One report showed the fixation to the human gradually decreased in the TD individuals and stabilized after around 5 years of age [15]. This trend is considered that during very early development TD infants and toddlers are strongly drawn to the human face [27], while TD individuals may become able to alter their attention to non-social stimuli with increasing age [15]. Thus, enhanced gaze preference for geometric pattern is indicated as a developmental biomarker of ASD only in early childhood. In our study, in movie VII (same size), there was a positive association of age with gaze at human and a negative association with geometric patterns gaze, while it was not seen in movie VIII [small window] and participants showed consistent preference to gaze at human. Regarding movie VII, in 4–5 months group, the fixation percentage of geometric patterns were higher than human. This difference may be because the geometric patterns in Movie VIII occupies a relatively small area than Movie VII and then it was too small to identify the geometric patterns for early infants due to their limited visual acuity. Although, infants may show interest in geometric patterns at age 4–5 months, there were few studies that reported the preference of human and geometry. It was also needed to consider that the participants in our study did not accurately represent infants with TD.
Movie IX present joint attention that is a shared attention state between two individuals focused on an object or event of interest and is critically associated with language acquisition in TD children [22] and with social deficits in ASD [11]. In our study, Group B (pointed object) indicated the emergence of joint attention, and the fixation percentage of pointed object increased with age. Infants develop joint attention around 10 months of age [11, 12], and we may measure the enhancement of joint attention skills with age. Joint attention has also been considered an important factor in learning and acquiring vocabulary in infants [22], the stimuli may also be used as a quantitative predictor of vocabulary acquisition in infants.
Limitations
This study was conducted in a small region, therefore, the generalizability of these results may be limited due to the potential impact of welfare and living conditions in this particular region. The number of subjects was small. We did not have prior assessments of high-risk infants or cohorts. In terms of epidemiology [2], approximately 1% of participants were likely to receive a future diagnosis of ASD. Therefore, the data in this study did not accurately represent infants with TD.