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

To understand information processing in the first two stagesof the olfactory system, neural activities were examined froma temporal point of view. The extracellular unitary activityof receptor cells and bulb neurons in response to four odorsdelivered in a range of concentrations from 1 x 10^–6 to5.62 x 10^–2 as a fraction of saturated vapor was recorded.For each population, unitary activities were pooled by 100-msbins, according to the nature and/or to the concentration ofthe stimulus. The results provided a view of the timing of activityof cell populations and allowed a comparison of the temporalstructures of the primary afferent message with that of thebulbar output message. Temporal patterns of receptor cell populationwere characterized by late and sustained discharges; thus, theprimary input volley did not coincide with the early and briefbulbar output message. In the two cell populations, the temporalresponse patterns elicited depended on the nature of the stimulus.At receptor level, interstimulus differences could be explainedin terms of binding interactions between odorants and receptorsites. In the bulb, while the input and output messages werenot strictly synchronous, some temporal characteristics relativeto the nature of the stimulus were preserved. In receptor cells,the activity occurring within 1 s following the stimulus onsetspecified stimulus intensity. By contrast, in the bulb, theevents which seemed to be involved in intensity coding werethose occurring within the first 500 ms. The convergence ofprimary afferences onto the bulb leads to an amplification ofthe earliest peripheral events setting up a sharp, earlier outputbulb message; in the processing of the input information, theolfactory bulb seems to combine an amplifying role for the earliestevents with a shutting action on later events.