Project Summary Individuals with autism spectrum disorder (ASD) exhibit multiple differences in sensory perception, which have now been recognized as a core feature of the condition. Among these sensory differences, decreased sound tolerance (DST; i.e., an inability to cope with everyday sounds) is particularly salient, with a lifetime prevalence of 50–70% in the ASD population. Despite both first-person accounts and empirical studies indicating that DST is a major source of distress and functional impairment in ASD, little is known about the phenomenology or physiologic underpinnings of this symptom cluster, and no evidence-based treatments for DST in ASD are currently available. Some researchers have suggested that the adverse reactions seen in ASD are manifestations of hyperacusis, reflecting disordered loudness perception. However, others contend that exaggerated emotional responses to specific acoustic stimuli underlie these behaviors, indicating that DST in ASD could be a form of misophonia, a psychiatric disorder characterized by excessive emotional reactions to specific “trigger” sounds. To date, little empirical work has tested these hypotheses against one another, and it remains an open question whether DST in ASD reflects hyperacusis, misophonia, or a combination of the two. Furthermore, it remains unknown whether these symptoms are associated with alterations in the peripheral or central auditory system. The proposed study aims to answer these questions using a two-stage approach. In stage 1, we will construct a novel self-report questionnaire that assesses a wide range of DST symptoms spanning the four theoretical domains of loudness, pain, annoyance (i.e., misophonia), and fear. Available measures of DST typically assess only the loudness (hyperacusis) or annoyance (misophonia) dimensions, failing to address a number of other clinically significant symptoms. Utilizing large online samples of adults with and without ASD, we will refine and psychometrically validate our questionnaire for use as a quantitative measure of transdiagnostic DST symptoms. In stage 2, we will recruit adults with ASD (both with and without DST) and healthy controls from the community, characterizing their auditory function using a battery of psychoacoustic and physiological tests. Auditory perception will be assessed using pure tone audiometry, loudness discomfort level testing, and categorical loudness scaling. Underlying auditory physiology will be assessed from the middle ear to auditory cortex using a combination of tympanometry, acoustic reflex testing, otoacoustic emission suppression, brainstem/cortical auditory event-related potentials, and auditory steady-state responses. Objective auditory measures will be compared between diagnostic groups, and relationships between these measures and subjective DST symptomatology (based on established surveys and the novel self-report developed in stage 1) will be explored across the full stage 2 sample. This pr...