STIMULUS PARAMETERS AND TEMPORAL EVOLUTION OF THE OLFACTORY EVOKED-POTENTIAL IN RATS

Evoked potentials were recorded from olfactory bulb, piriform cortex and scalp in urethane anesthetized rats in response to brief odorant stimuli (amyl acetate, phenylethyl alcohol, eugenol) presented through a nasal cannula by means of a constant flow olfactometer. The effects of stimulus duration, nasal cannula position, flow rate, concentration and interstimulus interval were examined. The highest amplitude potentials were evoked by 10% amyl acetate at 20 ms duration, 1000 ml/min flow rate and a 60-s interstimulus interval with the stimulus delivered at the nares. Odorant evoked potentials from deep within the olfactory bulb consisted of a triphasic wave with major components at 60 ms (P60), 90 ms (N90) and 140 ms (P140) with the latter two reversing polarity close to the surface of the bulb. Potentials recorded from layer I of piriform cortex were of similar amplitude, but opposite in polarity to the deep olfactory bulb potentials. Recordings from the skin over the nose elicited waveforms of similar morphology to the deep olfactory bulb potentials, but one-quarter the amplitude and of opposite polarity. The evoked potentials changed with repetitive stimulation. The N90 component was not present initially and only appeared after several stimuli. The appearance of the N90 component depended on the integrity of the olfactory peduncle. Thus, olfactory evoked potentials to odorant stimuli reflect dynamic aspects of the encoding of olfactory information dependent on connections between olfactory bulb and piriform cortex.