PROJECT SUMMARY Our world is highly multisensory, and we acquire information about it via a number of distinct sensory systems. Typically, objects or events are specified by more than a single sense, and the integration of this multisensory information confers powerful and adaptive perceptual and behavioral advantages such as faster and more accurate responses. These advantages become pivotal when navigating complex environments where motion is ubiquitous, as is the case in the real world. However, multisensory processing also presents a computational challenge for the brain: to carry it out efficiently, the brain needs to not only decide which pieces of sensory information belong to the same event (and thus should be integrated or bound), but also which information needs to be segregated. Although great strides have been made in recent years to further our understanding of multisensory perception and its neural correlates, there are still significant gaps in our knowledge with regards to processing more ecologically-valid stimuli, such as those containing motion. One of these gaps revolves around how motion information is transformed in the presence of modulatory, cross-modal input as it makes its way through successive stages of the cortical processing hierarchy, and how these transformations map on to behavior/perception. The experiments outlined in the current proposal begin to address this issue using behavioral paradigms that we have developed in which macaques signal the direction of an auditory, visual, or audiovisual motion stimulus. During performance of the task, we will record neural activity in two cortical domains reflecting successive levels in the processing hierarchy: the medial temporal (MT) and medial superior temporal (MST) areas. The first aim will examine how modality and motion strength within audiovisual stimuli impact discrimination behavior and contribute towards causal inference. The second aim seeks to characterize responses to auditory, visual, and audiovisual motion information in these areas with the overarching hypothesis that as motion information ascends from MT to MST, there will be an increase in the role of modulatory auditory input, reflective of a gradual shift from encoding low-level stimulus features such as signal strength toward the encoding of features relevant to goal-oriented behavior such as stimulus direction and task demands. Collectively, the work will shed great light on the mechanistic underpinnings of multisensory perception in nodes critical to motion processing. Additionally, success in these experiments would challenge how we think about the modularity of the sensory cortical processing hierarchy. Such knowledge is of increasing importance given the growing recognition of altered multisensory function in those with neurodevelopmental conditions and/or sensory function loss, as well as the value of brain-informed algorithms for naturalistic virtual and augmented reality technology.