Gamma activity in the piriform cortex and behavioral thresholds for electrical stimulation in the olfactory bulb.

The olfactory bulb was stimulated by trains of electrical pulses in freely moving rats. Evoked responses resembling damped oscillations at the gamma frequency of 30-60 Hz were recorded in the anterior and posterior piriform cortex. Different types of unconditioned sniffing were induced by stimulation of the olfactory bulb. They were similar to those evoked by an odorant (amylacetate) but differed from the behavioral patterns evoked by non-olfactory (auditory) stimulation. Rats were trained to avoid foot-shocks following electrical pulses into the olfactory bulb as conditioned stimulus in a two way shuttle-box paradigm. Threshold electrical intensities for inducing evoked responses in piriform cortex, unconditioned behavior, and learned avoidance were compared. Thresholds for unconditioned and conditioned behavior were significantly higher in comparison with those for evoking gamma discharges in anterior and posterior piriform cortex. The results suggest that fast time-locked synchronization in the gamma range in the piriform cortex induced by synaptic input from theolfactory bulb is not sufficient for inducing corresponding behavior. Thus behavioral detection and probably also olfactory recognition do not seem to be direct consequences of this fast time-locked neural synchronization. Additional neuronal processes that are connected with further elevation of stimulation intensity seem to be necessary for that.