Ortho- and Retronasal Presentation of Olfactory Stimuli Modulates Odor Percepts Johannes Frasnelli & Mary Ungermann & Thomas Hummel Received: 29 September 2007 / Accepted: 20 October 2007 / Published online: 13 December 2007 # 2008 Springer Science + Business Media, LLC Abstract Retronasal olfaction gives us information impor- tant not only for the well-being by detecting dangerous substances but also provides a basis for the pleasures of eating and drinking. However, odors presented via the retronasal route appear to evoke different sensations compared to orthonasal presentation. In this study, we differentially stimulated anterior and posterior parts of the subjects’ nasal cavity with odors. Subjects were able to tell retronasal and orthonasal smelling apart; the ability of doing so seemed to be linked to but could not completely be explained by the degree to which the odorant stimulated the trigeminal nerve. Furthermore, we examined whether food and nonfood odors were perceived as differently pleasant, depending on the presentation site. In fact, some but not all nonfood odors were described as less pleasant when they were delivered retronasally. In conclusion, the present results clearly suggest that there are differences between sensations produced by presenting odors through the retronasal or the orthonasal route, which bears signif- icance for both basic and applied research. Keywords Retronasal . Flavor . Localization . Lateralization Introduction Retronasal olfaction represents a peculiar and fascinating aspect of the olfactory system. Contrary to the orthonasal pathway, retronasal olfaction is analyzing the interior of the body rather than the external world, thereby, giving information important not only for the well-being of the individual by detecting dangerous substances but also providing a basis for the pleasures of eating (Halpern 2004). Certain foods in daily life seem to be sensed differently depending on the olfactory presentation mode. Hence, retronasal aroma presentation seems to evoke different sensations compared to the orthonasal olfactory stimulation with the same compound (Rozin 1982). A number of studies have been performed to investigate differences between ortho- and retronasal olfaction. Olfactory thresh- olds were found to be lower when stimuli were presented orthonasally than when presented retronasally (Heilmann and Hummel 2004; Voirol and Daget 1986). Consequently, intensity ratings of orthonasally presented odors were higher than those of retronasally presented ones (Heilmann and Hummel 2004; Small et al. 2005). Similarly, retronasal odor identification appears to be more difficult than orthonasal (Pierce and Halpern 1996), although this difference was not always significant (Sun and Halpern 2005). Thus, retronasal olfactory stimulation appears to lead to a lower degree of activation of the olfactory system than orthonasal stimulation. This was confirmed in electro- physiological experiments; orthonasal stimulation led to larger amplitudes than retronasal stimulation in the electro- olfactogram, which is measured directly on the olfactory mucosa (Hummel et al. 2006). Thus, all studies indicated lower performance in retronasal olfaction. Chem. Percept. (2008) 1:9–15 DOI 10.1007/s12078-007-9000-2 J. Frasnelli : M. Ungermann : T. Hummel (*) Smell and Taste Clinic, Department of Otorhinolaryngology, University of Dresden Medical School, Fetscherstrasse 74, 01307, Dresden, Germany e-mail: thummel@mail.zih.tu-dresden.de J. Frasnelli Cognitive Neuroscience Unit, Montreal Neurological Institute, McGill University, Montreal, Quebec, Canada