Whole-brain multisensory neural dynamics in the fly are bounded by anatomical connectivity

Many natural behaviors are driven by multisensory inputs that engage multiple pathways and brain regions simultaneously, but how the brain's neural network combines information across sensory modalities remains poorly understood. Here, we examine brain-wide audio-visual processing in the context of fruit fly courtship, a social behavior in which the female fly combines auditory and visual cues from a male suitor to inform her own behavior. Employing 2-photon functional imaging in head-fixed, behaving female flies presented with fictive auditory and visual cues, we find widespread activity correlated with both auditory, visual, and multisensory stimulus presentations which cluster into spatially segregated subpopulations with distinct temporal dynamics, consistent with a distributed, population-level encoding of multisensory stimuli. Notably, the multisensory response map includes brain regions not activated by unisensory stimuli, with widespread with widespread nonlinear (super- and sublinear) multisensory responses, pointing to neuron populations which combine unisensory signals. Aligning these whole-brain functional data to the FlyWire whole-brain connectome, we directly compare function and structure at brain scale, and find that anatomical connectivity places an upper bound on functional correlation between regions, indicating that that wiring constrains but does not fully determine function. By directly linking whole-brain neural dynamics to the anatomical connectome, this work provides an empirical basis for connectome-constrained models of brain-scale sensory computation.