In this study, we explored the atypical patterns of social synchronization during response to joint attention in children with ASD through utilizing the analytical method of cross-recurrence quantification analysis (CRQA). Consistent with our hypotheses, children with ASD was identified to possess the ability, to some extent, to synchronize their gaze shifts with the female model in the video during the RJA tasks. Compared to the TD children however, they displayed lower levels of synchronization and longer latency in this synchronized behavior.
First, we discovered that when compared with the baselines, both groups were looking at the same AOI with the female model at above-chance levels, indicating that they were not looking randomly during the RJA tasks. This further suggests that children with ASD can, to some extent, respond to social cues such as gaze shift and head turn. This finding is consistent with previous studies which demonstrate that children with ASD had the ability to follow gaze during RJA tasks [35, 36]. Upon comparing the two groups however, we found that the level of gaze-shift synchronization in children with ASD was still lower than that in the TD children. This suggests that the ability to synchronize the gaze shifts with social partners in children with ASD, although above the chance level, is still poorer than TD children. The level of synchronization here was determined by the length of time children spent on the same AOIs where the female model also looked. Here, we assume that that the level of synchronization could be influenced by two factors: gaze following accuracy (the children follow the female model’s gaze to the attended object), and the duration of fixations at the female model's face or the attended objects. Vivanti et al.  has found that children with ASD demonstrated decrease attention to the model’s face when the model looked straight during RJA tasks. Bedford et al.  and Falck-Ytter et al.  have also noted that the time they spent on the attended objects was shorter than that in the TD group despite the intact gaze following accuracy in the ASD group. These factors could all contribute to the low level of synchronization in the ASD group during the RJA tasks. The reason why children with ASD allocated less attention to the attended object could be attributed to their difficulty in understanding communicative meaning of the social referential cues such as gaze shift . Based on the previous study, the eye-looking time positively correlated with the subsequent attention on the object in TD children, but this is not the case in children with ASD. It reflected that these children were impaired in understanding the social meaning of gaze and could not actively utilize gaze cues to determine their attention like the TD children . Ultimately, our results suggest deficient synchronization performance during RJA in children with ASD, which may result in an insufficient process of social information during social interaction such as joint attention.
Second, a delayed social synchronization was identified during joint attention in ASD. Unlike TD children who promptly demonstrated gaze-shift synchronization in responding to the female model’s gaze shift, children with ASD displayed a longer latency to respond to it. In addition, the lag time at which the ASD group had the best synchronization performance was longer than that in TD group. Such a longer latency in gaze-shift behavior of children with ASD was also identified in some previous studies concerning RJA .These results together suggest that children with ASD had an impairment of synchronization in the dimension of time, and they were not as sensitive to social cues such as gaze shifts as compared with TD children. As responding to the social cues in a timely manner can help people effectively capture some useful but ephemeral information in social interactions, it is vital to explore the latency in social synchronization as well as the motivating reasons. Here, it was speculated that this latency could be accounted for by several factors. First, children with ASD may have difficulties in interpreting social cues [37, 38], so they need more time to process these cues before responding to them. Second, previous studies have found that children with ASD are resistant to distraction and may need more time to disengage from the model’s face or the attended object [54, 55]. Third, some research has suggested that children with ASD also suffer from gaze dyspraxia, suggesting that they have eye-motor difficulties in looking where another person is pointing or if they are asked to look . As a result, it may be the incapability of gaze movement rather than the deficit in intentionality to respond to the gaze shift that underlies the delayed latency of gaze-shift synchronization. The combination of these three factors could contribute to the delay in gaze-shift synchronization of children with ASD, and more future evidence is required to support this conclusion.
Third, we further reviewed whether group membership could be predicted from the percentage recurrence through using a machine learning method as a supplementary analysis to verify the traditional analytical methods applied in this study. Generally, it was found that the overall classification performance resulted in 60% accuracy which differed from chance. This implies that the performance of gaze-shift synchronization could be a potential indicator to differentiate children with ASD from TD children. Meanwhile however, considering the low sensitivity and specificity of the single indicator, it is necessary to combine other characteristic indicators of ASD, such as the geometric preference , the abnormality in face scanning [57, 58], and facial affect recognition . This is especially crucial in building a more effective and reliable model to effectively screen children with ASD.
In this study, it was found that children with ASD exhibited impairment of social synchronization during joint attention. However, certain questions still remain concerning the relationship between the impairments of social synchronization and joint attention in ASD. Here, it has been proposed that there are three possible relationships between them. First, the impairment of social synchronization could be an underlying mechanism of impairments of joint attention in children with ASD. Notably, some researchers suggest that the impairment of social synchronization ability during the neonatal period may disrupt the development of joint attention between the 6 to 9 month period. It should be noted that this could lead to difficulties in building social and emotional bonds with caregivers, as well as in understanding and responding to social cues . Second, there may be some common underlying mechanisms of social synchronization and joint attention. For instance, both require the perception and information processing from others’ actions as well as the social motivation to attend to and respond to the social interactions of others. Third, joint attention could play an imperative role in social synchronization. With the ability to synchronize with others’ movements, people need to attend to the rhythms of others while also understanding or even inferring their intentions through the ability of joint attention. Generally speaking, it is difficult to reach a unanimous conclusion about their relationship in the current study, and more future evidence is needed to reach a conclusion.
In our study, we explored the impairment of joint attention from a new perspective —social synchronization: through a classical RJA paradigm and attained direct evidence concerning the impairment of synchronization in joint attention. In previous studies, the gaze following of children with ASD was demonstrated in discrete experimental trials without considering social interaction to be a continuous adaptational and coordinated behavior. Furthermore, these studies also ignored the redirection of the attention to the partner’s face to form the mutual gaze after gaze following, which is suggested to be a critical component of joint attention. These limitations suggest a serious gap in obtaining a comprehensive understanding of how children with ASD coordinate and synchronize visual attention. This is especially with others in the complex social contexts of everyday life with moving heads and eye gaze in a continuous manner. Our study bridged this gap through using the CRQA method to investigate the continuous adaptation of the attention of autistic children to the visual attention of others. Through this analytic method, a more comprehensive understanding of social synchronization can be obtained compared with previous studies about social motor synchronizations [3, 6, 16]. In the dimension of time, we could explore the latency of gaze-shift synchronization during the RJA tasks. In the dimension of synchronization level, we will not only simultaneously understand the synchronization performance but also obtain crucial information of this at different lag times. Moreover, this method can be further applied in other social interactions such as social communicative exchanges, cooperation and imitation, and provide a new pathway to understanding the social defects in children with ASD.