Olfactory bulb responses to odor stimulation: analysis of response pattern and intensity relationships

Extracellular recordings were made from mitral cells, tufted cells, and presumed glomerular layer and external plexiform layer interneurons of the olfactory bulb of anesthetized rats during odor stimulation. Intensity responses of these cells were studied by presenting a series of six or seven concentrations, spanning a range greater than two log units, in a cyclic artificial sniff paradigm, which produced repeated response measures at each concentration. Experiments focused on obtaining a complete intensity series, including interspersed unstimulated spontaneous activity records, for a single odorant (usually amyl acetate), but concentration responses to other odorants were tested when possible. Odor responses of 46 cells were studied with two approaches. Response form was examined in an attempt to define response classes based on qualitative characteristics of the temporal pattern of response. Assessment of response magnitude was attempted, in order to construct stimulus-response functions for each cell, independent of response form. As previously reported for olfactory bulb cells, the cells in our sample responded to odor stimulation with spike trains of a variety of temporal patterns, consisting of excitatory and inhibitory components that were frequently recognizable in the responses of a cell across a range of concentrations. However, response patterns usually changed significantly with concentration, such that response form across the concentration range could not be predicted from the response at any one concentration. Responses of different cells were sometimes similar to each other in form at one concentration and quite different from each other in the rest of their concentration-response profiles. Classification of response profiles into discrete types, based on consistency of response form throughout the profile, was therefore not feasible. In agreement with other reports, response of a single cell to different odorants sometimes showed similar forms and sometimes showed very different forms across the concentration-response profiles. Since the response form depends on the stimulus intensity as well as the stimulus quality, characterization of response magnitude and of the pattern of response to different odors require testing with a series of stimulus concentrations. Because odor responses consisted of temporally patterned spike trains, whose components changed in complex ways with stimulus intensity, it was not possible to quantify response magnitude by measuring characteristics of particular response components or counting mean frequency.(ABSTRACT TRUNCATED AT 400 WORDS)