Salamander olfactory bulb neuronal activity observed by video rate, voltage-sensitive dye imaging. III. Spatial and temporal properties of responses evoked by odorant stimulation.

1. Activity patterns across and within the laminae of the olfactory bulb were analyzed by imaging voltage-sensitive dye responses during odorant stimulation of all or part of the ventral olfactory mucosa. 2. The time course of the signals was generally characterized by a brief, small hyperpolarization, followed by a period of depolarization, and then a longer-lasting hyperpolarization similar to that seen with electric stimulation but with longer durations. 3. The activity was distributed nonhomogeneously across the bulbar laminae in the form of spatially segregated clusters having bandlike appearances. Clusters were observed with three monomolecular odorants, amyl acetate, ethyl-n-butyrate, and limonene, and with the complex odor of meal worms. Although response patterns to different odorants overlapped, they also showed differences in overall distribution. 4. Delivery of high odorant concentrations increased the size of the activated areas and accentuated the degree of response pattern overlap among different odorants. The general properties of the response patterns generated by each odorant were, however, similar at different odorant concentrations and in each of the animals tested. 5. The spatial and temporal distributions of the bulbar responses were somewhat similar regardless of whether the odorants were applied to local epithelial regions via punctate stimulation or to the entire mucosa. Certain regions did, however, have lower thresholds than others for eliciting bulbar activity in response to particular odorants. 6. Odorants applied to regions of the epithelium outside the areas of maximum sensitivity elicited odorant-related activity patterns with depolarizing and hyperpolarizing components similar to those seen with overall stimulation, but only if higher concentrations were used. Activation of distributed odorant sensitivities presumably gave rise to these patterns. 7. These data suggest that subsets of odorant receptor types are found in different areas of the olfactory epithelium, and demonstrate that there is widespread distribution across the epithelium of receptors sensitive to particular odorants. On the basis of the structure of these epithelial fields and the bulb response patterns that they relate to, these findings also provide evidence for complex spatial relationships between the olfactory epithelium and bulb. 8. The findings from this study suggest that representation of odor information in the salamander olfactory bulb does not occur by activation of a few selective bulbar regions, each related to a different odorant species. Instead, large regions of bulbar circuitry are involved in which molecular epitopes may be the unit of representation. Incorporation of new data presented here into a hypothesis of odor coding is discussed.