Electro-Olfactograms in Humans in Response to Ortho- and Retronasal Chemosensory Stimulation

Ortho- and retronasal olfaction represent two aspects of a shared sensory system yet evoke different sensations. The differences between ortho- and retronasal olfaction have triggered a number of studies during the past years, which pointed towards a decreased sensitivity to odors presented through the retronasal olfactory pathway. Especially intensity was reported to be lower after retronasal olfactory stimulation. The aim of this study was to investigate how this compares to activation at the level of the olfactory epithelium in humans. Trigeminal (CO2) and olfactory (H2S, phenylethyl alcohol) stimuli were presented ortho- and retronasally. Electro-olfactograms (EOG) in response to chemosensory stimulation were recorded in 10 participants (6 women, 4 men, mean age 23.4 years). Typical EOGs were demonstrable after either orthonasal or retronasal stimulation across the stimulus qualities. Overall, EOG amplitudes to retronasal stimulation were smaller when compared to those to orthonasal stimulation, but a significant difference was obtained only in phenylethyl alcohol (p = 0.048). The present data indicate that the perceptual differences between ortho- and retronasal olfaction may start at the level of the olfactory epithelium. The data support the idea that the intensity of physically identical stimuli is lower after retronasal stimulation compared to orthonasal stimulus presentation for both olfactory and trigeminal stimuli. The current electrophysiological results are in line with reported differences in psychophysical properties of retro- and orthonasal stimulations meaning that different sensations are elicit through these two channels although the same molecule is presented.