Odor maps of aldehydes and esters revealed by functional MRI in the glomerular layer of the mouse olfactory bulb.

Odorant identity is believed to be encoded in the olfactory bulb (OB) by glomerular activity patterns. It has not yet been possible to visualize and compare entire patterns for different odorants in the same animal because of technical limitations. For this purpose we used high-resolution functional MRI at 7 T, combined with glomerular-layer flat maps, to reveal responses to aliphatic homologues in the mouse OB. These odorants elicited reproducible patterns in the OB, with the medial and lateral regions containing the most intense signals. Unexpectedly, in view of the symmetrical projections of olfactory receptor neurons to medial and lateral glomeruli, the activity patterns in these regions were asymmetrical. The highly activated medial and lateral areas were shared by homologous members, generating a conserved "family signature" for a homologous series. The moderately active areas, including the dorsal region that has been extensively studied by optical imaging, were more sensitive to the length of the carbon chain, producing more subtle features of individual members and different changing trends among homologues. The global mapping with functional MRI not only extended previous studies but also revealed additional rules for representation of homologues in the OB. Insights into possible relations between the functional patterns, molecular projections, and odor perception may now be obtained based on the global from the olfactory epithelium to the OB glomerular activity patterns.