Topographic coding of olfactory quality: odorant-specific patterns of epithelial responsivity in the salamander.

1. Electrophysiological recordings were made on the ventral olfactory epithelium of the salamander Ambystoma tigrinum in order to investigate whether individual odorants can elicit unique patterns of receptor neuron responses. 2. Slow transepithelial voltage transients, Veog(-), were recorded from 30 sites on each epithelium. For each odorant a topographic pattern was derived from the Veog(-) amplitudes across the 30 recording sites. 3. Nine odorants were tested, each in seven animals, and topographic patterns of Veog(-) amplitudes were drawn for each animal. Due to the morphological variability among animals, the electrode sites for each animal were assigned to six epithelial regions for which responses were then compared by analysis of variance. 4. Odorant-specific regional differences in responsivity were observed. The odorants can be grouped according to the similarity of the topographic distributions of responses elicited by them. We observed that no two odorants elicited exactly the same response patterns. This suggests that olfactory receptor neurons with similar responses are grouped together in the same region of the epithelium. 5. Dramatic differences in responsivity among the various epithelial regions, irrespective of the test odorant, were also noted. This observation may be due to regional differences in receptor neuron density or overall sensitivity. 6. It is concluded that differences between the topographic distributions of receptor cell responses, elicited by the nine test odorants, permits these responses to be discriminated as unique patterns of information at the olfactory bulb.