The correlation between physiological and psychological responses to odour stimulation in human subjects.

Objectives: To correlate physiological and psychological responses to odour stimulation.

Methods: Olfactory event-related potentials (OERP) were recorded from human subjects in response to different odour pulse protocols. Pulse duration and interstimulus interval (ISI) were varied while the subjects recorded pulse detection by a button press.

Results: There was a correlation between odour strength and OERP. The amplitude of the OERP (peak positive-peak negative) declined by 48% with repetitive stimulation for all stimulus strengths. The time constant for this decline (adaptation) was concentration dependent and varied from 10 to 4 s for increasing odorant strength (from 35 to 200 ms pulse of saturated amyl acetate vapour diluted 1:3 at 3 l min(-1) at 24 degrees C). The psychometric test score (cognitive odour perception) was also concentration dependent and increased with increasing stimulus strength at all ISIs except the lowest value; 2.5 s. At this ISI adaptation/habituation interfered with the dose-response. The decline of the psychometric test score with increasing stimulus frequency (decreasing ISI) was more rapid (tau approximately 2.5 s) than that of the OERP. The psychometric test score declined to zero at a constant rate and was not dependent upon stimulus strength.

Conclusions: Continuous olfactory stimulation results in a shut-down of cognitive perception with a time constant of around 2.5 s while the response of the physiological system (receptors, transduction and relay system) declines by about 50% with a slower time course (> or = 4 s). This former process defines habituation in the olfactory system while the latter describes adaptation. Since the adaptation process is concentration dependent the rate of adaptation contains information about odour strength, thus both the amplitude of the OERP and the rate of adaptation could encode stimulus strength.