The Roles of Glomerular and Granule Cell Layers in Spatial and Temporal Odor Processing

The olfactory bulb (OB) is organized in many glomerular units (GUs), which interact with each other at two levels, the glomerular and the granule cell layer. This interaction sculpts the representation of odors that occurs in the mitral cells, whose axons project to the pyriform cortex (PC). These actions subserve several crucial functions for odor recognition. However, neither the relative roles of the layers, nor how learning effects their computation, is yet understood. To address these problems, we used a realistic large scale 3D model of the OB which includes both the glomerular and the granule cell microcircuits. It was constrained to the experimental findings and used to unravel several essential features of OB organization, such as the spatially sparse and segregated organization of granule cells located below a glomerulus, which result from GU interactions during learning. Here, we show that the glomerular circuit transforms a dense and disorganized spatial representation (such as that experimentally observed for natural odors) into a sparse, normalized and contrast-enhanced one. This is essential for odor learning. Subsequently, over time, the granule cell circuit decreases the spatial representation overlaps of different odors, increasing the relative differences and improving the discrimination. This effect improves with the number of learned odors but decreases if they are too many. In the last case, the columnar organization of the granule cells is destroyed, which might be restored or avoided perhaps by neurogenesis or neuromodulation. Taken together, the results provide new insight into the fundamental role of GUs and their layered organization. The model predicts the input received at the PC, building the basis for further investigations in models that include it.