Sensory input reaching the brain from bilateral and offset channels is nonetheless perceived as unified. This unity could be explained by simultaneous projections to both hemispheres, or inter-hemispheric information transfer between sensory cortical maps. Odor input, however, is not topographically organized, nor does it project bilaterally, making olfactory perceptual unity enigmatic. Here we report a circuit that interconnects mirror-symmetric isofunctional mitral/tufted cells between the mouse olfactory bulbs. Connected neurons respond to similar odors from ipsi- and contra-nostrils, whereas unconnected neurons do not respond to odors from the contralateral nostril. This connectivity is likely mediated through a one-to-one mapping from mitral/tufted neurons to the ipsilateral anterior olfactory nucleus pars externa, which activates the mirror-symmetric isofunctional mitral/tufted neurons glutamatergically. This circuit enables sharing of odor information across hemispheres in the absence of a cortical topographical organization, suggesting that olfactory glomerular maps are the equivalent of cortical sensory maps found in other senses.

Imagine smelling a novel perfume with only one nostril and then smelling it again with the other nostril. Clearly, you can tell that it is the same perfume both times. This simple experiment demonstrates that odor information is shared across both hemispheres to enable perceptual unity. In many sensory systems, perceptual unity is believed to be mediated by inter-hemispheric connections between iso-functional cortical regions. However, in the olfactory system, the underlying neural mechanisms that enable this coordination are unclear because the two olfactory cortices are not topographically organized and do not seem to have homotypic inter-hemispheric mapping. This review presents recent advances in determining which aspects of odor information are processed unilaterally or bilaterally, and how odor information is shared across the two hemispheres. We argue that understanding the mechanisms of inter-hemispheric coordination can provide valuable insights that are hard to achieve when focusing on one hemisphere alone. I n the early 1940s, patients suffering from severe untreatable epileptic seizures underwent a pioneering operation in which their corpus callosum was severed. While this mitigated the epilepsy syndromes, it generated strange phenomena: each hemisphere seemed to have its own separate perception, concepts, and even actions. When such split-brain patients were shown an image of an object in their left visual field, they could not vocally name what they had seen. This is because information about the image seen in the left visual field is sent only to the right side of the brain, while the speech-control center is usually on the left side of the brain. Nonetheless, patients could pick up the correct object with their left hand because it is controlled by the right hemisphere 1. Some of the surgery-induced deficits improved over time, but they illustrate one important principle: the fibers that interconnect the hemispheres are responsible for transferring information from one side to the other. These connecting fibers may be required to achieve perceptual unity, at least in some sensory modalities. But how is perceptual unity achieved? There are several possible ways to achieve perceptual unity in the healthy brain. One way is to ensure that sensory information is transmitted to both hemispheres directly from the sensory organs. This is how auditory information is unified across hemispheres: information from both ears converges to the left and the right Superior Olivary

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