Expiration: the moment we experience retronasal olfaction in flavor.

Respiration is essential for smell perception. Previously we found that 8-12-Hz cortical rhythms were phase-locked to inspiration onset during the presentation of odor stimuli; this is referred to as inspiration phase-locked alpha band oscillation (I-alpha). Generators of I-alpha estimated with a dipole fitting model were found in the piriform, the entorhinal cortex (ENT), the amygdala (AMG), the hippocampus (HI) and the orbitofrontal cortex (OFC). Such olfactory perception is said to occur via the orthonasal olfaction route. Another route is the retronasal olfaction route. In this study, we investigated the link between respiration phase and retronasal olfactory perception. Electroencephalograph (EEG) and respiratory flows (separately measured with mouth and nose) were simultaneously recorded during stimulation of subjects' tongues with liquids of chocolate, sucrose and water. The percentage of subjects correctly identifying the chocolate taste was higher when subjects were asked to breathe through the nose than when they were breathing through the mouth. In the averaged EEGs triggered by the onset of expiration measured from the flow through the nose, a 8-12-Hz oscillation was observed. Generators of this potential were found in the left ENT, HI, AMG and OFC in the order of milliseconds after expiration onset. Perception of retronasal olfaction is dependent on expiration, and combining retronasal olfactory information with gustatory information and somatosensation enable us to identify flavors when drinking and feeding.