Intrainsular Connectivity and Somatosensory Responsiveness in Young Children with ASD

Background:  The human somatosensory system comprises dissociable paths for discriminative and affective touch, reflected in separate peripheral afferent populations and distinct cortical targets. Developmental differences in behavioral and neural responses to affective touch may play an important role in early social experiences, which may be relevant for autism spectrum disorder (ASD). Integrity of two sensory-related white matter pathways may provide insight into developmental differences in affective touch. The thalamocortical tract (sensory thalamus to primary somatosensory cortex) carries discriminative tactile information, while the intrainsular tract (posterior to anterior insula) represents the cortical projection target of unmyelinated tactile afferents mediating affective touch (posterior insula) and integration of sensory and visceral inputs to interpret emotional salience of sensory stimuli (anterior insula). We hypothesized structural integrity in these pathways might mediate variability in behavioral responses to affective touch in young children with ASD.

Objectives:  To determine structural correlates of behavioral responses to affective sensory stimulation in children with ASD.

Methods:  Using diffusion tensor imaging and probabilistic tractography, we investigated the structural integrity of the thalamocortical and intrainsular tract by comparing fractional anisotropy, mean diffusivity, and tract volume in a group of young children with ASD (n=29, ages 5-8) and a group of typically developing (TD, n=26) peers. We assessed tactile discrimination and affective response to social touch using the Tactile Defensiveness and Discrimination Test-Revised (TDDT-R). We examined group differences in tract integrity and behavioral assessments (Student’s t or Mann-Whitney), as well as relationships between white matter integrity and behavioral responses (Spearman’s and linear regression).

Results:  There were significant group differences in white matter integrity in both tracts investigated, such that individuals with ASD had higher mean diffusivity (MD, lower tract integrity) than individuals in the TD group in both tracts (intrainsular, p=.039; thalamocortical, p=.026). Consistent with previous findings, the ASD group exhibited impairment in tactile discriminative ability and aberrant affective responses (both positive and negative in valence) to touch. Associations between tactile seeking behavior, characterized by positive affective behavioral response and unusually intense interest in tactile stimuli, and intrainsular integrity significantly differed by group (significant group by intrainulsar MD interaction, p=.0032). In the ASD group, increased intrainsular tract integrity was associated with more seeking behaviors while the opposite was true in the TD group. Reduced integrity in thalamocortical tracts was associated with increased tactile defensiveness across both groups.

Conclusions:  Individuals with ASD had reduced integrity in both sensory-related tracts, correlating with responses to affective touch, suggesting altered sensory responses may propagate across both discriminative and affective touch pathways in ASD. These results are in line with previous findings that positive affective response to touch is mediated by somatosensory input to the posterior insular cortex (Olausson 2002). Relationships between intrainsular tract integrity and sensory seeking also differed by group, suggesting unique mechanisms may contribute to altered sensory seeking behaviors in ASD.