The larval Drosophila mushroom body balances lateralized sensing and interhemispheric integration.

Bilaterian animals must integrate sensory input from both sides of their bodies to make coherent perceptual decisions. In Drosophila larvae, olfactory receptor neurons target the ipsilateral brain hemisphere. Using calcium imaging, unilateral sensory perturbations, connectome analysis, and optogenetics, we identified the mushroom body (MB) as a key substrate for interhemispheric integration. Kenyon cell odor responses were almost entirely ipsilateral, indicating minimal coupling between the two MBs at the input level. In contrast, modulatory MB input neurons (MBINs) responded symmetrically to unilateral stimulation, suggesting broad pooling of reinforcement signals across hemispheres. Nevertheless, some MB output neurons (MBONs) preserved stimulus laterality up to 5 synapses from the sensory periphery, and asymmetric activation of these MBONs biased turning behavior. We also demonstrate that larvae can exploit instantaneous spatial comparisons for navigation in certain sensory contexts. These findings suggest that the deeply lateralized architecture of the larval olfactory system balances the need for interhemispheric integration with the advantages of parallel sensory processing.