Figure 1. Anatomical stages and signal-processing primitives in the early olfactory pathway.

(1) odorants are detected by a large population of sensory neurons in the olfactory epithelium (vertebrates)/antenna (invertebrates) that convert the chemical information in to combinatorial activity patterns (2) sensory neuron of the same type (expressing the same receptor gene) converge onto the same location (spherical structure of neurophil called glomeruli) in the next anatomical region (olfactory bulb in vertebrates; antennal lobe in invertebrates). This massive convergence of redundant input serves two computational functions: (i) it allows the system to average out uncorrelated noise to allow reliable detection, and (ii) it enhances the sensitivity of the system over that of individual sensory neurons (3) divisive pattern normalization circuits at the input of the olfactory bulb/antennal lobe allows the system to compress the concentration information (4) neural circuits at the output of the olfactory bulb/antennal lobe decorrelate odor response patterns such that both coarse clustering and fine discrimination can be achieved (5) the output patterns from the olfactory bulb/ antennal lobe is sent to olfactory cortex (vertebrates)/ mushroom body(invertebrates) for further reformatting, storage and association with other sensory modalities. Typically, these projections are both convergent and divergent (many-to-many) allowing detection of combinations of co-occurring molecular features of the odorant (or “coincidence detection”) (6) in vertebrates system cortical feedback modulates the activity in the olfactory bulb. Adapted from (97)