Even pure, simple odorant molecules tend to activate multiple high uptake foci in the olfactory bulb (63), as should be evident from the patterns of 2DG uptake displayed on this site.  Each area of activation also tends to encompass many glomeruli (27,28).  These findings and others suggest that the perception of an odor probably arises from a combination of activated odorant receptors.  Similar conclusions have been drawn from the study of sensory neurons known to express distinct receptor genes (41), from the study of glomerular responses using other methods (44,56,70,73,74), from the study of human olfactory perception (51) and from the study of action potentials in focal populations of projection neurons in the bulb (20,47).  Indeed, there are far more unique odor perceptions than there are odorant receptors, a situation that logically necessitates such a combinatorial code. 

Both the activation of multiple glomerular clusters in different parts of the olfactory bulb (left) and the presence of multiple glomeruli within certain response modules (right) are suggestive of a combinatorial odor code involving multiple odorant receptors and multiple glomeruli underlying the perception of even single, pure odorants. Illustrated above are responses to the odorant valeric acid.