Right-nostril dominance in discrimination of unfamiliar, but not familiar, odours.

In a recent PET study on processing of unfamiliar odours we observed that odour discrimination performance was superior during right compared with left nostril presentations, and that mainly the right cerebral hemisphere was activated. In the present study we investigated whether the asymmetric performance is present also during the processing of familiar odours. Seventy-one right-handed healthy subjects (age 21-49 years, 40 females) with normal nasal anatomy and olfactory thresholds participated. Forty pairs of odours (20 familiar and 20 unfamiliar) were presented in the same/different paradigm, alternating nostrils and balancing the order. The number of errors during the discrimination task was compared with respect to nostril and odour familiarity. The overall odour discrimination performance was superior on the right side. However, this difference was valid only for unfamiliar odours, whereas the performance for familiar odours was symmetrical. Familiar odours were easier to discriminate than unfamiliar ones. The present data are congruent with the idea of a semantic influence on odour processing. Odours seem to be processed with a right sided preponderance when not clearly familiar, and symmetrically when language becomes involved. Future studies on odour processing should therefore take into account odour familiarity and side of presentation.