Interhemispheric connections between olfactory bulbs improve odor detection

Interhemispheric connections enable interaction and integration of sensory information in bilaterian nervous systems and are thought to optimize sensory computations. However, the cellular and spatial organization of interhemispheric networks as well as the computational properties they mediate in vertebrates are still poorly understood. Thus, it remains unclear to which extent the connectivity between left and right brain hemispheres participates in sensory processing. Here, we show that the zebrafish olfactory bulbs (OBs) receive direct interhemispheric projections from their contralateral counterparts in addition to top-down inputs from the contralateral zebrafish homolog of olfactory cortex. The direct interhemispheric projections between the OBs reach peripheral layers of the contralateral OB and retain a fine-grained topographic organization, which directly connects similarly tuned olfactory glomeruli across hemispheres. In contrast, interhemispheric top-down inputs consist of diffuse projections that broadly innervate the inhibitory granule cell layer. Jointly, these interhemispheric connections elicit a balance of topographically organized excitation and non-topographic inhibition on the contralateral OB and modulate odor responses. We show that the interhemispheric connections in the olfactory system enable the modulation of odor response and improve the detection of a reproductive pheromone, when presented together with competing complex olfactory cues, by boosting the response of the pheromone selective neurons. Taken together, our data shows a previously unknown function for an interhemispheric connection between chemosensory maps of the olfactory system.