ASYMMETRIC SIGMOID NONLINEARITY IN THE RAT OLFACTORY SYSTEM

The statistical relationship between multi-unit spike activity and simultaneously recorded local dendritic field potentials in the olfactory system of the waking rat was studied with chronically placed electrodes. The relationship had the form of a sigmoid increase in axonal firing probability conditional on the amplitude of dendritic potentials. These data were fitted with an asymmetric sigmoid curve previously derived from the Hodgkin-Huxley equations. The curve was fitted using non-linear regression to optimize its parameter: the maximal firing rate. The maximal rate also gave the steepness of the slope of the sigmoid. Pulse trains were recorded from excitatory and inhibitory neurons in the olfactory cortex (including the anterior olfactory nucleus, the prepyriform cortex and the lateral entorhinal area) as identified by the phase relations of the pulse probability and the dendritic potentials, and from the excitatory neurons in the bulb (the inhibitory granule cells do not give extracellularly detectable action potentials). All these neurons are known to interact in disynaptic negative feedback loops giving rise to oscillations. The same sigmoid function fit the data from both types of neurons in all locations. The curves for neurons in all parts of the olfactory cortex had a 3-fold higher slope and maximal value than the curves from bulbar neurons. The significances of this difference and of the asymmetric sigmoid are discussed in terms of models for olfactory oscillatory dynamics and pattern recognition.