Project Summary/Abstract Autism Spectrum Disorder (ASD) is a critical public health concern, given its negative quality of life impacts, increasing prevalence rates, and high health care utilization costs. Two-thirds of the ASD population have co-occurring psychiatric disorders. These individuals require intensive interventions to address psychiatrically related aberrant behaviors (e.g., irritability and hyperactivity), which are the source of much of the disability and cost for this population. The goal of this project is to assess physiological mechanisms underlying Therapeutic Horseback Riding’s (THR) previously observed significant positive effects on ASD youth, particularly those with co-occurring psychiatric disorders, and to further evaluate durability, dose, and sub-population effects of the intervention. Aim 1: Test the hypothesis that physiological response patterns of salivary cortisol, cardiovascular, and electrodermal activity account for our previously observed significant outcomes (i.e., reduced irritability and hyperactivity, and improved social and communication), and additional outcomes (emotion regulation and caregiver quality of life), in youth ages 6-16 yrs. with ASD and co-occurring psychiatric diagnoses randomized to a 10-week manualized THR intervention compared to a no-horse Barn Activity (BA) control. Our preliminary findings suggest the psychiatrically challenged ASD majority may be more likely to benefit from THR, particularly if outcomes are mediated by physiological arousal. Aim 2: Evaluate the durability of Aim 1 outcomes in the THR group compared to the BA control group six-months after the intervention period, given our preliminary evidence for maintenance of initial outcome gains in a six-month follow-up. Aim 3: Explore dose and sub-population effects of THR and BA interventions by comparing effect size differences in THR and BA groups to a: (1) 10-week wait-list control group; (2) Hybrid intervention group (five weeks BA followed by five weeks THR); and (3) subsample of the THR study population randomized following psychiatric hospitalization. Our preliminary results, obtained at both PATH international premiere accredited riding centers proposed in the current application demonstrate that peripheral physiological arousal data collection is feasible with ASD youth while engaged in THR. We have also partially replicated our previous results and demonstrated that cortisol is a viable target mediator of THR effects on aberrant behaviors in ASD youth that merits further investigation. Our proposal has the potential to advance the field of human-animal interaction (HAI), specifically THR, for individuals with ASD and co-occurring psychiatric disorders. It also stands to guide future researchers interested in better understanding the physiological mechanisms associated with HAI. Our long-range goal is to empirically establish THR as an ecologically valid, transdiagnostic intervention that targets underlying psychoph...