## 1 Introduction
Nevertheless, studies on physiological responses to odour stimuli in ASD are still scarce and utterly different in terms of population and odour type, thus not allowing direct comparisons (e.g., [26; 27; 28]). For instance, [PERSON] et al. [27] found comparable facial muscular behaviour (measured with action units) and cardiac and electrodermal activities in children with high-functioning ASD and children with TD exposed to CO. However, while children with TD showed a congruency between their facial expression and hedonic judgment of the odours, this effect was less frequent in children with ASD. The authors suggested that children with ASD might have difficulties reporting their emotional response to olfactory stimuli, while their automatic responses and facial behaviour might be preserved [27]. Of foremost relevance to the present study, [PERSON] et al. [28] proposed that automatic responses to odour stimuli might be a potential biomarker of ASD. In this study, 18 children with ASD (mean age = 7.0) and 18 children with ASD (mean age = 6.7) were exposed to pleasant (rose or shampoo) and unpleasant (sour milk or rotten fish) CO while they were viewing a cartoon. Sniff responses were collected as a measure of automatic olfactory perception, using a pediatric olfactometer that allowed to simultaneously deliver the odours and measure nasal airflow. Firstly, while children with TD adjusted their sniff response based on odour valence, this effect was not observed in children with ASD. Secondly, abnormal sniff response was associated with the severity of social impairment in ASD, suggesting that altered sensory coordination in sniff response is related to abnormal olfactory perception, which in turn might contribute to social impairment in ASD. Lastly, using a leave-one-out analysis with data on sniff volume and duration, the authors were able to classify 17 of the 18 children with TD and 12 of the 18 children with ASD (i.e., 81% accuracy). Later, 20 adults with ASD and 20 adults with TD were exposed to fearful BO during two experimental tasks: the Faces task, in which participants rated the fearfulness of 27 faces, and the emotional Stroop task, in which participants indicated the colour of emotional words. Electrodermal activity (EDA), nasal airflow, and heart rate were collected [26]. While fearful BO led to a significant reduction in nasal inhalation in both groups, the authors observed a dissociation in EDA: EDA increased significantly in adults with TD but not in adults with ASD. Finally, using a discriminant-analysis classifier, 16 of the 20 adults with TD and 12 of the 20 adults with ASD were correctly classified (i.e., 70% of accuracy). These findings are particularly relevant because they provide a novel view on the use of olfaction as a potential diagnostic tool in ASD that does not require instructions, attention, and explicit perception [29]. Using functional magnetic resonance imaging, differences in olfaction between 18 adults with ASD and 18 TD controls were investigated [23]. After testing odour threshold and identification, individuals underwent a structural scan as well as functional scanning while perceiving two different odours. Multivariate analysis using the Support Vector Machine classification technique was performed to differentiate odour-evoked neural patterns in each region of interest between the two groups. Results showed that individuals with ASD had decreased function for odour thresholds and identification and decreased activation in the piriform cortex, suggesting that alterations in olfaction in ASD are already present in the primary olfactory cortex. In the present study, we aim to extend previous findings [26; 28] on odour-related biomarkers of ASD by providing preliminary evidence of the potential use of physiological responses to classify ASD in adults, using both social and non-social odours.