PROJECT SUMMARY Attention is fundamental to parsing the dynamic, multisensory, and semantically rich environments we encounter in day-to-day life. The human sensory systems take in more information each moment than can be processed at once, so a subset must be prioritized for further processing through attentional mechanisms. Information about the same object or event can be initially processed by multiple sensory organs, so signals from different sensory systems must additionally be matched and integrated to create the perception of a coherent multisensory world. Disruptions to these attention and sensory integration mechanisms are thought to underlie the sensory processing issues commonly observed in neurodevelopment disorders (e.g., autism, ADHD). Patients either under-respond or over-respond to common sensory stimuli, causing distress and difficulty completing the tasks of daily life. The mechanisms underlying these sensory processing symptoms are ill defined because of critical gaps in our understanding of how attention operates in multisensory environments. Attention has largely been studied within each sensory system separately, thus many factors remain poorly understood in the audiovisual contexts that more closely resemble the environments we encounter in daily life. In the proposed research, I will investigate one of these factors, semantics, which is a critical guide of attention in vision but has only been studied narrowly in audiovisual contexts with stimuli that shared a source (e.g., a dog and its' bark). This narrow focus means the mechanism of audiovisual attention benefit remains unknown and could include any semantic relationship (a semantic-general mechanism) or be specific to the relationship of sharing a source (a source- specific mechanism). In Aim 1, I will characterize the degree to which semantic relatedness influences attentional prioritization, which will be measured both by visual search efficiency in study 1 and prioritization in early visual cortex at object locations in study 2. Attentional prioritization that scales with semantic relatedness would suggest a semantic-general mechanism. In Aim 2, I will identify the neural mechanism of the shared-source benefit in audiovisual search, specifically examining the time course to understand the relative contributions of attentional and semantic processing. Investigating these mechanisms will provide a more robust understanding of the role of semantics in guiding attention in real world environments, which are frequently both multisensory and semantically rich. Ultimately, understanding basic attentional principles will support future research into the sensory processing issues so common in neurodevelopment disease, including potential treatments that allow patients to better manage sensory under- and over-responsiveness.