5.1 Abnomal sensory processing of ASD
As we all know, sensory system contains many dimensions such as touch, oflactory, taste, auditory and vision, and even sensory abnormality has many manifestations, which makes it difficult to distinguish and evaluate the abnormal sensory behavior of ASD individuals. The parental assessment questionnaire used in this study accurately grasped the sensory characteristics of ASD and TD children in different classification. In the seven sensory domains and multisensory integration included in SSP questionnaire, the degree and proportion of sensory abnormality in ASD group were more significant compared with TD group. Our results were consistent with Rogers who have found that 95% of the sample of children with ASD demonstrated some degree of sensory processing dysfunction on the SSP total score[24]. ASD individuals' sensory problem has many implications in behavior. For example, they may hate physical contact, fear light, and extremely sensitive to tiny sounds, etc. These sensory challenges, which exist in ASD childhood, may cause difficulties for children to develop normal communication and social ability.
The high incidence of sensory abnormality has been generally agreed by researchers. However, there are still more features behind this appearance, which are worthy of further consideration. The current results show that although ASD children have amazing damage in many senses, the abnormal proportion of each sense is different, the high abnormal rate of auditory filtering has attracted our attention. In the process of communication, the exposed environment often faces the simultaneous input of various sounds, which requires the ability to extract and receive useful information from complex background sounds, which is also an important manifestation of auditory function[25]. In this study, most parents indicated that ASD children could not concentrate and do things normally in noisy environment. This shows that it is very difficult for these individuals to filter out useless noises. In the past literature, some researchers pointed out that hearing filtration disorder is very high in ASD patients[26]. Groen has investigated the level of disyllabic words extracted from various auditory backgrounds by individuals with autism, and found that these people need a higher signal-to-noise ratio than the control group in order to perceive the sounds in pink noise[27]. Sensory functions and symptoms are mutually mapped. The general anomaly of ASD in a certain dimension just reveals the reasons of their puzzling behaviors on weekdays. Therefore, the key to improve the poor performance is to start with those patients who have sensory injuries, which is likely to save more prognosis.
The environment in which human beings live is complex and changeable, the source of sensory stimulation is never single, and the sensory system is also very complex. In our investigation, the vast majority of ASD individuals not only have one sensory abnormality, but also are not limited to sensitivity or dullness, but a variety of them coexist. All kinds of sensory abnormalities that co-exist among ASD individuals may play their own "roles" and produce various behavioral effects. For example, children with both tactile and auditory filtering difficulties are likely to resist the approach of others and often fail to respond to their calls. This is likely to cause great obstacles to the smooth communication, which is likely to aggravate the symptoms. In addition to the single influence, when external environmental stimuli are applied to human body, they are often mixed with a variety of sensory information, which requires individuals to have the ability to process these inputs at the same time. Just as daily conversation involves the rapid integration of auditory and visual information to create a whole and dynamic perception. Failure to integrate these sensory cues in time will lead to defects in speech perception[28–29]. In the questionnaire evaluation, we found that up to 30% of ASD children have visual/auditory and auditory filtering abnormalities. A previous study by Noel found that ASD children's audio-visual time combination window is four times larger than that of typical developing children, which indicates that ASD children are completely deficient in audio-visual acuity[30]. The sensory disturbance of ASD in more than one dimension has made them difficult to cope with, however, the change of reaction mode is an important reason for their abnormal behavior. Sensory hypersensitivity and unresponsiveness are the criteria of ASD diagnosis. Besides these two models, we also evaluated over-seeking. Among the people with only one abnormal reaction (42%), hypersensitivity still accounted for the majority (37%), the proportion of people with multiple reactions also can not be ignored (57%). The results of high sensitivity are consistent with Taylor's research[31]. These different reaction modes bring ASD's sensory processing to two extremes, extreme excitement or negativity. The lack of proper feelings may also make them infatuated with the pleasure brought by excessively seeking certain feelings, and more likely lead to rigid behavior. A great deal of research has been done on the causes of different patterns, which may be affected by cortical excitability, abnormal receptor function at molecular level and genes[32–34]. Cross-disciplinary anomalies and complicated reaction patterns make it difficult to distinguish ASD's sensory behavior and seriously affect their other skills.
5.2 ASD sensory abnormality predicts social disorder
Information integration across multiple sensory organs is the foundation of cognition. As social animals, interaction with each other is the most essential feature of human beings, which requires them to give full play to the function of sensory system in order to obtain more valuable information and carry out social activities when facing the world composed of chaotic information[35]. However, aberrant responses of ASD individuals to sensory stimuli may contribute to aggravate social disorders of autism. From the statistical point of view, the current study analyzes the relationship between the scores of sensory questionnaire and the scores of clinical scale reflecting autism behavior. The results show that there is a negative correlation between the sensory score in SSP and SRS score, as well as the scores of subscales for evaluating social ability in ADI-R and ADOS, which further proves that with the aggravation of the abnormal degree of perception, the social defects of autism are more obvious. And the state of each sensory field has different degrees of correlation with social interaction.
As one of the important ways to explore with the external environment as early as infancy, tactile is very important to the development of individuals. Tactile processing dysfunction is not only related to the emotion of ASD individuals, but also may cause social disorders. Touch, especially social contact expressing friendly nature, such as hug, is very necessary in the process of communication. Moreover, gentle emotional touch plays an important role in relieving individual anxiety and improving social level[36]. However, autistic people often show excessive defense to mild touch stimulation, but low response ability to painful stimulation[37–38], which contributes to the failure of detecting social clues. The current study found that SSP tactile score is negatively correlated with ADI-R non-verbal communication score, which is similar to Foss-Feig's[39] research conclusion. It can be seen that the abnormality of tactile sensation can affect the nonverbal communication ability of ASD, which is the basis of social communication[40]. The relationship between tactile and ASD social symptoms is also reflected in the fact that tactile information is crucial in distinguishing the relationship between self and others, and the distinction of self -others is the basis of social cognition[41]. For example, as described in SRS questionnaire, autistic children may not be able to understand the interrelation of things (such as causes and causes) like their peers, which is the prompt of social cognitive defects and ultimately affects the overall social ability of autism.
Besides touch, the influence of vision and auditory on social behavior can not be ignored. "Listening" and "observing" are two indispensable "ties" to communicate with others. When these feelings appear obstacles, it is likely to affect norm al social interaction. Our results suggest that the abnormality of audio-visual perception is inversely related to social function from three perspectives: social perception, social communication and social motivation. Emotion recognition is an important part of social cognition, including understanding facial expressions and emotion rhythm[42–43], which just depend on the intact visual-auditory ability. Up to 80% of environmental information is transmitted to the brain through vision[44], and visual processing is related to social behaviors such as joint attention imitating others[45–46]. However, the current study found that these abilities of ASD group were weak by SRS scale and correlated with abnormal visual performance. Most ASD individuals have difficulty in looking at their faces, and they often pay more attention to the local and grasp the whole, which makes them tired of dealing with complex and subtle social stimuli. For example, ASD children usually pay too much attention to other people's lip movements in the process of dialogue with people, instead of understanding social cues such as emotions through comprehensive observation of facial expressions[47]. Moreover, conversational communication must be the most important way in the social process. Besides facial expressions, language elements play an important role in the transmission of emotions and attitudes. But due to the auditory abnormality, ASD children's excessive obsession with certain language features (such as voice or intonation) during communication makes it difficult for them to filter out socially irrelevant information from other people's words, and then integrating social clues-such as language information expressing emotions. And excessive hearing sensitivity may cause children's excessive fear of a certain voice, further limiting their social progress. Our SSP assessment found that ASD children are often difficult to concentrate in noisy environments, emphasizing the obstacles in their auditory filtering and auditory attention. And this weakness is more obvious in the recognition of human voice[48], which may be an important explanation for autistic individuals' lack of attention preference for social stimulation[49].
Anorexia, picky eaters and difficulty in eating are common in autistic children, which implies abnormal oflaction and taste. However, another important finding of this study is that this abnormality not only affects ASD's eating behavior, but also contributes to social difficulties. In the past, it was thought that terrestrial mammals usually use their sense of smell to identify their peers' emotions and engage in social activities. Recently, more evidence shows that human beings have similar phenomena, and sociochemical signals can transmit emotions such as aggression, happiness and fear[50]. Abnormal oflaction may may affect the correct detection of these signals. In the correlation analysis, we found that smell/taste expression is not only related to communication ability, but also leads to the occurrence of autism behavior. The definition of autism behavior in SRS scale includes unacceptable changes in daily habits, unusual perceptual interests, and being considered weird or unusual by other children. For ASD individuals, these performances not only interfere with their emotions, but also seem to be incompatible with others and become an important obstacle to integrating into social environment. As Lahera's research found, smell perception has a great relationship with emotional behavior and cognitive ability[51].
The total score of SSP questionnaire is the sum of scores obtained by subjects in all sensory dimensions, which is a comprehensive evaluation of their sensory performance and reflects the overall perceptual function. The obvious correlation between the total score and SRS scale gives us an important hint: besides analyzing individual feelings, the influence of multi-sensory integration function on social level is also very significant. In the face of complex social environment, individuals often need to receive multiple sensory stimuli at the same time, which requires multiple senses to be mobilized at the same time to cope with a large amount of rushing information. Audio-visual integration needs a lot of employment in the process of communication, in order to fully analyze the information expressed by each other's facial movements, gestures and voice output. In this study, we found that some ASD children have visual and auditory difficulties at the same time, which may make it difficult for them to relate other people's language and facial expressions, which also explains that many autistic children have obstacles in understanding lip language. And will affect their language development, thus developing into interpersonal frustration[52]. Even some children in the sample have tactile, gustatory/olfactory and audio-visual problems at the same time, all of these senses are helpful to identify emotions, and the sensory integration disorder of ASD children is undoubtedly a great destruction of their social skills.
5.3 Predictive effect of social function by model
The severity of social function is an important factor affecting the quality of life and prognosis of ASD group. The evaluation methods of ASD communication ability in clinic usually include questionnaire survey, psychological measurement and evaluation by appraisers during the game[53–55]. However, these methods are usually influenced by subjective factors more or less. In recent years, with the development of computer technology, big data analysis methods are widely used in scientific research, and machine learning is increasingly appearing in ASD field. At present, machine learning algorithms are involved in many researches, including finding effective biomarkers for diagnosis, classifying ASD by various phenotypes and predicting risk genes[56–58]. In this study, this method is innovatively used to judge how much sensory behavior can predict social function. Our results significantly prove that both random forest and SVM models, including 10 features and 7 features, can reliably and stably distinguish ASD social interaction (AUC>0.8). And the 10-feature model has higher sensitivity, while the 7-feature model shows higher specificity. This result provides a new perspective for future research. SRS scale, as an output factor in our model, is widely used in clinical practice, and it is one of the necessary tools to correctly grasp ASD diagnosis and children's social assessment. The characteristic factors used in the model construction all come from the scores of each sensory field in SSP questionnaire, as well as demographic characteristics such as gender and age. Our findings provide evidence for predicting the possibility of diagnosis and socialization through sensory expression.
We rank the contribution of characteristic factors to the model from low to high by the method of data dimension reduction, and find that the top three are low response/sensory seeking, total score of multi-feeling and age. It can be seen that these factors play a higher role in predicting social function. However, the importance of scores in each sensory sub-field ranks behind the total score. This may also suggest that the influence of feeling on social interaction may be the result of the synthesis of various senses, and the total effect is far greater than the single effect. The influence of cross-multisensory fusion on individual behavior has attracted wide attention as a separate field, especially for autistic people. In order to improve adaptive behavior, multi-sensory-based interventions and treatments have also been carried out in ASD[59]. Electrophysiological and neuroimaging studies have found that neural circuits and brain structure are the factors that affect multisensory integration, which provides a basis for biological research between sensation and social interaction[60]. The predictive effect of demographic characteristics shows that age is the robust factor of the model, while the importance of gender is the least among many characteristics. The predictive effect of demographic characteristics shows that age is the robust factor of the model, while the importance of gender is the least among many characteristics.
Limitations and Future Research
The limitation of this article lies in the fact that our method of evaluating children's sensory behavior only depends on the way parents report, so there is a lack of measurement of children's subjective feelings of stimulation. Future research should further use psychometrics and other more objective methods to evaluate the sensory performance of ASD children. In addition, although we have revealed the relationship between sensation and ASD symptoms, the biological reasons and causal relationship between them are still unclear. We need to devote ourselves to exploring the neurobiological mechanism in order to expect a deeper understanding of the mechanism. In the construction of the model, this research innovatively links feeling and social interaction, and further clarifies the existence of the opposite relationship between them. However, it is only limited to the predictive function, and the proof of causal relationship is still somewhat inadequate.