A code in the nose

This talk presents a coding perspective on early olfaction. The question is, how are different smells (odorants) encoded in the olfactory epithelium? Axons from sensory neurons project into the so-called glomerular layer of the olfactory bulb, and there are say 2000 glomeruli. A sensory neuron in the nose projects on a unique glomerulus, but a given sensory neuron may have many receptor types. There about 5 × 107 sensory neurons in the rabbit; each glomerulus fed by 25,000 sensory neurons. The connection strategy, receptor specificities, and number of receptors per sensory neuron are not known, nor is the number of recognizable odorants. Coding structures are proposed which could constrain the biological parameters and be subject to experimental investigation and verification. Some of the structures involve random nearly constant weight codes, and lead to biologically plausible numbers, considering molecular and behavioral evidence.