Coding of odor intensity.

A model for coding of odor intensity in the first two neuronal layers of olfactory systems is proposed. First, the occupation and activation by odorant molecules of receptor proteins of different types borne by the first order neurons are described as birth and death processes. The occupation (birth) rate depends on the concentration of the odorant, whereas the probability of activation of an occupied receptor depends on the type of the odorant. Second, the spike generation mechanism proposed for the first order neuron depends on the level of the generator potential evoked by the activated receptors and on a time-decaying threshold which is reset to infinity after each spike. The various resulting stochastic regimes of firing activity at different concentrations are described. Third, each second order neuron is influenced by excitation coming from numerous first order neurons, lateral inhibition from other second order neurons, and self-inhibition. All these incoming signals are integrated at the second order neuron. The firing activity of the first and second order neurons is modeled by a first passage time scheme. For both types of neuron the shapes of the curves predicted by the model for the mean firing frequency as a function of stimulus concentration are shown to be in accordance with available experimental results.