Olfactory perception is initiated by the recognition of odorants by a large repertoire of receptors in the sensory epithelium. A dispersed pattern of neural activity in the nose is converted into a segregated map in the olfactory bulb. In mice, as well as flies, projections from the olfactory bulb (antennal lobe) extend to multiple, higher order processing centers in the brain.

We have identified a spatially invariant, dimorphic circuit responsive to pheromones that dictates components of sexually dimorphic courtship behavior in Drosophila. We have also asked how the representation in the bulb is transformed at the next processing center for olfactory information in the mouse, the piriform cortex.

Optical imaging of odor responses in the cortex of the mouse reveals that the piriform discards spatial segregation as well as chemotopy and returns to a highly distributed organization in which different odors activate unique but dispersed ensembles of cortical neurons. Neurons in piriform cortex, responsive to a given odorant, are not only distributed without apparent spatial preference but exhibit discontinuous receptive fields.

This representation suggests organizational principles that differ from those in neocortical sensory areas where cells responsive to similar stimulus features are clustered and response properties vary smoothly across the cortex.