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Bilateral computations in odor localization and its relation to behavior

Final Report Summary - BILATERALCOMPUTATION (Bilateral computations in odor localization and its relation to behavior)

The neural computation underlying odor localization and its relation to behavior
Tracking odor sources is crucial for foraging, courtship and reproductive success for many animals. Animals, including humans, can track odor sources. Odor localization is generally believed to relay on the small bilateral difference of the odor arrival times or intensities, together with sequentially comparing odor concentrations at two different locations. The role of each of these possible mechanisms, the neurons that are involved in this task, and the neural mechanism underlying it are unknown. In this stud y, we characterized odor localization behavior ability in mice.
As a first step we trained water deprived mice to respond with licking to an odor arriving from the right side but not the left side. We found that mice can discriminate between an odor arriving from left or right. We next tried to uncover the neural mechanism underlying this ability. For this end, we recorded the activity of M/T cells while we delivered odors from the left or right sides. As might be expected wee found that a subset of M/T cells responded differently to left or right odors and this response modulation depended on the odor identity, air flow and distance from the odor source. Strikingly, many odor directional (OD) neurons responded at different phases to odors arriving from one side or the other. To find out if this modulation was related to inter-bulb interactions we occluded the contra nostril and examined the neural response to left and right odors. We found out the occluding the contra-nostril almost always abolished the phase response difference. This suggest that inter-bulb interactions underlay these phase coding. Occluding the ipsi-nostril revealed that OD neurons respond to odors from the two nostrils while non-OD neurons respond only to ipsi-odors. This result indicates that inter-bulb interactions are utilized to encode odor direction by shifting the neural response phase.
We next examined the connectivity between the two olfactory bulbs. We recorded M/T cells in one bulb while light-stimulating the glomeruli in the other bulb. We used mice that express Channelrhodopsin2 in their M/T cells. We found that in contrast to what might be expected from anatomy, around one third of the M/T cells receive excitatory inputs from the contra-bul. These interconnected neurons were symmetrically organized.
To summarize, this study provide insight on inter-hemispheric connectivity and how this might be used to calculate the odor locations.

The implications of this study are:
1. It reveal a novel circuit that inter-connects the two hemispheres which may enable us to better understand how the two hemispheres are communicating. The brain has two hemispheres and yet there is only little research on how the two hemispheres work together, although it has long been suspected that the disruption of inter-hemispheric connections may underlie some of the severe psychiatric illnesses such as schizophrenia and epilepsy.
2. It suggest a mechanism by which mice can detect where the odor source is. How animals find the odor source is unknown. Understand the neural mechanism of odor localization can help us build odor sources detectors.