Yuna Kwak¹ (), Lillian Behm¹, Juliana E. Trach¹, David J. Lewkowicz¹, Nicholas B. Turk-Browne¹; ¹Yale University

Adults show clear behavioral facilitation for multisensory stimuli (e.g., faster and more accurate detection) relative to unisensory stimuli, supported by brain regions such as the superior temporal sulcus (STS). Although similar behavior emerges in infancy, how multisensory integration is supported across the infant brain is unknown. In an ongoing study, we are using fMRI to characterize audiovisual binding in awake infants (3-24 months) during a naturalistic movie paradigm and to compare their neural responses to those of adults. We played three versions of the same movie clip to each participant in both age groups: audiovisual, video-only, and audio-only. Our main analysis relies on intersubject correlation (ISC), which identifies brain regions with reliable neural responses to the stimulus in each condition by quantifying the similarity of fMRI time-series across individuals. In adults, ISC was higher (1) for video-only and audio-only conditions in early visual and auditory cortex, respectively, relative to other brain regions and (2) in audiovisual versus unisensory conditions — evidence of multisensory facilitation — in known multisensory areas such as STS and the temporo-parieto-occipital junction, as well as in the medial temporal lobe. Decoding analyses support these findings and further suggest that the observed facilitation effect reflects the fusion of unisensory inputs rather than a summation of independent signals. Preliminary results from infants show initial evidence of (1) unisensory coding in early visual and auditory cortex and (2) audiovisual facilitation across the neural hierarchy, as in adults. The full sample will be needed to identify the specific regions involved and to understand how closely infant processing aligns with adults. Overall, these findings indicate that a wide network of regions supports multisensory integration in adults and suggest that this network begins emerging early in development.