Conway, M. W.; Oncul, M.; Johnston, J.

The mechanisms through which the brain encodes odour identity and maintains the stability of the resultant olfactory percept are not well understood. The combination of activated receptors is thought to encode identity (1, 2), while a cascade of mechanisms is thought to give rise to odour constancy (3). Yet some odours evoke different percepts at low and higher concentrations (4). Here we show that a change in odour percept corresponds to rapid and complete adaptation in a single olfactory receptor channel that is most sensitive to the odour. This concentration-dependent shift to fast adaptation is not a property of circuit interactions within the olfactory bulb, rather it arises from depolarisation block of olfactory receptor neurons within the nose. It therefore seems that mechanisms to promote odour constancy are unable to operate when rapid peripheral adaption has occurred. Furthermore, our data imply that odour identity relies on a sparse code, as perceptual constancy depends on the activity of only a few olfactory receptor channels.