Autism spectrum conditions (ASC) encompass a range of sensory, motor, and social-communicative differences, reflecting the considerable heterogeneity within the autism spectrum. This diversity underscores the limitations of categorical diagnostic approaches, which often fail to capture the individualized manifestations of autism. Advances in genetics and neuroscience have driven a shift towards dimensional frameworks that emphasize the spectrum nature of autism and the broad autism phenotype (BAP). BAP encapsulates subclinical traits that mirror those of autism in the general population, challenging the conventional boundaries between clinical and non-clinical populations. Furthermore, sensorimotor differences, which are particularly prevalent in individuals with ASC, follow a spectrum-like pattern similar to the BAP and are predictive of developmental outcomes related to social participation, communication, and overall quality of life in people with and without ASC. However, specific descriptions of these relationships are lacking.

This dissertation investigated the complex relationships between sensorimotor differences and autistic traits (ATs). Through a series of five interconnected studies, we examined broad sensory processing patterns and specific sensory modalities, namely auditory processing and motor/proprioception, to explore their roles in autistic phenotypes.

Study 1 of the dissertation validated a Swedish translation of the Broad Autism Phenotype Questionnaire (BAPQ). By examining parents' BAP traits, the study highlighted significant associations between higher ATs and having a child with ASC. Furthermore, it confirmed the presence of all three AT domains—social interaction (AT_SOC), communication (AT_COM), and cognitive rigidity (AT_RIG), reinforcing the genetic and phenotypic continuity between clinical and subclinical ATs.

Study 2 and Study 3 served as broader investigations into all seven sensory modalities and their associations with ATs. Study 2 explored these modality-specific associations, using Bayesian stochastic search variable selection (SSVS) and dominance analysis. This study highlighted auditory processing difficulties as the most consistent predictor of all three AT domains. Additionally, proprioceptive and tactile processing difficulties were specifically associated with AT_COM and AT_SOC, respectively.

Study 3 extended this analysis to a developing population, focusing on the relationship between sensorimotor processing, ATs, and anxiety in children aged 6-11 years. Identical to Study 2, we found tactile symptoms as a predictor of AT_SOC, proprioceptive symptoms for AT_COM, and auditory symptoms for AT_RIG. In addition, olfactory symptoms were selected as a predictor of AT_COM, and motor coordination was a consistent predictor of all AT domains. Using SSVS, this study also identified that auditory and olfactory processing difficulties were strong predictors of anxiety symptoms.

Building on the previous studies, Study 4 narrowed the focus to auditory processing differences, investigating specific auditory problems and their associations with the AT domains. All AT domains significantly predicted affective reactions to sounds, while difficulties with speech perception, spatial perception, and auditory stream segregation were most strongly predicted by AT_COM.

Study 5 focused on the previously found links between motor coordination and proprioceptive processing and AT_COM. Using causal mediation analysis within a counterfactual framework, this study found that cerebellar error correction deficits, measured through a finger tapping task, significantly impacted AT_COM through motor skills in childhood.

Together, this dissertation provides a comprehensive overview of the sensory processing dimensions related to the core AT domains. Specifically, the studies underscored the clinical significance of monitoring auditory and olfactory complaints in children, as these were predictive of anxiety, and emphasized that early motor deficits impact social communication development. The findings advocate for the inclusion of detailed sensory and motor assessments in neurodevelopmental evaluations to identify children at risk for poor mental health outcomes. Future research should continue to explore the mechanisms underlying sensory processing differences. Particular focus should be placed on auditory and motor/proprioceptive functions and their contributions to ATs and clinical outcomes, such as anxiety. Emphasis should also be given to longitudinal studies that track these relationships over time.