Temporal aspects of information processing in the first two stages of the frog olfactory system: influence of stimulus intensity

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Abstract
To understand information processing in the first two stages of the olfactory system, neural activities were examined from a temporal point of view. The extracellular unitary activity of receptor cells and bulb neurons in response to four odors delivered in a range of concentrations from 1 × 10 −6 to 5.62 × 10 −2 as a fraction of saturated vapor was recorded. For each population, unitary activities were pooled by 100-ms bins, according to the nature and/or to the concentration of the stimulus. The results provided a view of the timing of activity of cell populations and allowed a comparison of the temporal structures of the primary afferent message with that of the bulbar output message. Temporal patterns of receptor cell population were characterized by late and sustained discharges; thus, the primary input volley did not coincide with the early and brief bulbar output message. In the two cell populations, the temporal response patterns elicited depended on the nature of the stimulus. At receptor level, interstimulus differences could be explained in terms of binding interactions between odorants and receptor sites. In the bulb, while the input and output messages were not strictly synchronous, some temporal characteristics relative to the nature of the stimulus were preserved. In receptor cells, the activity occurring within 1 s following the stimulus onset specified stimulus intensity. By contrast, in the bulb, the events which seemed to be involved in intensity coding were those occurring within the first 500 ms. The convergence of primary afferences onto the bulb leads to an amplification of the earliest peripheral events setting up a sharp, earlier output bulb message; in the processing of the input information, the olfactory bulb seems to combine an amplifying role for the earliest events with a shutting action on later events.

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