Distributed and concentration-invariant spatial representations of odorants by receptor neuron input to the turtle olfactory bulb

We sought to characterize how odorants are represented at the level of afferent input to the olfactory bulb of the box turtle, a terrestrial reptile that, like mammals, detects airborne odorants. Using methods developed first in zebrafish, we selectively labeled olfactory receptor neurons with Calcium Green-1 dextran and imaged odorant-evoked input to glomeruli in vivo. Odorant representations were imaged at a glomerular level of resolution over a portion of the dorsal olfactory bulb and at a regional level of resolution over the entire dorsal surface. We report two new findings. First, even at low concentrations, odorants typically elicited input to a large fraction of all imaged glomeruli. Second, while the amplitude of the odorant-evoked input to glomeruli was concentration dependent, the relative pattern of input to the bulb changed only slightly over a concentration range of up to three log units. These results suggest the hypothesis that odorant representations in the turtle involve differential levels of input to many glomeruli, and that detecting relative patterns of distributed glomerular activation may be an important strategy for encoding odor quality independent of intensity.