Even pure, simple odorant molecules tend to activate multiple high uptake foci in the olfactory bulb (Stewart et al., 1979), as should be evident from the patterns of 2DG uptake displayed in this site.  Each area of activation also tends to encompass many glomeruli (Johnson et al., 1998, 1999).  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 (Malnic et al., 1999), from the study of glomerular responses using other methods (Rubin and Katz, 1999; Uchida et al., 2000; Meister and Bonhoeffer, 2001; Wachowiak and Cohen, 2001, 2003), from the study of human olfactory perception (Polak, 1973) and from the study of action potentials in focal populations of projection neurons in the bulb (Imamura et al., 1992; Mori et al., 1992).  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.