Non-topographical contrast enhancement in the olfactory bulb

We describe a novel neural circuit mechanism, non-topographical contrast enhancement (NTCE), which enables contrast enhancement among high-dimensional odor representations exhibiting unpredictable patterns of similarity. The NTCE algorithm relies solely on local intraglomerular computations and broad feedback inhibition, and is consistent with known properties of the olfactory bulb input layer. Unlike mechanisms based upon lateral projections, NTCE does not require a built-in foreknowledge of the similarities in molecular receptive ranges expressed by different olfactory bulb glomeruli, and is independent of the physical location of glomeruli within the olfactory bulb.Non-topographical contrast enhancement demonstrates how intrinsically high-dimensional sensory data can be represented and processed within a physically two-dimensional neural cortex while retaining the capacity to represent stimulus similarity. In a biophysically constrained computational model of the olfactory bulb, NTCE successfully mediates contrast enhancement among odorant representations in the natural, high-dimensional similarity space defined by the olfactory receptor complement and underlies the concentration-independence of odor quality representations.Primary olfactory sensory neurons (OSNs) line the nasal epithelium and respond to the presence of odors that diffuse through the nasal mucus layer and bind to olfactory receptors expressed on OSN cilia. Each OSN expresses only one or a few species of olfactory receptor, which define the molecular receptive range [1], or chemical receptive field, of that OSN. The axons of several thousand OSNs expressing the same receptors converge onto a few discrete glomeruli in the olfactory bulb, within which they form glutamatergic synapses with the dendrites of mitral cells, periglomerular cells, and external tufted cells. Both mitral and external tufted cells also form excitatory synapses onto periglomerular cells within the same glomerulus, whereas the