Human olfactory perception generally consists of the integrated activation of two anatomically independent systems; the olfactory and the trigeminal system. Most chemosensory stimulants produce both an olfactory as well as a trigeminal percept (a burning, pungent, or stingy feeling) when presented at concentrations above olfactory thresholds. Sadly, the trigeminal system is often overlooked when olfactory perception of everyday odors is studied. Matching odor stimuli to the everyday odors they are meant to replace, is essential if we want to fully understand how the olfactory system functions inside, as well as outside, the laboratory. In doing so, we first need to understand the underlying mechanisms of the complete olfactory percept,–including the trigeminal component.
So far, we have discovered that men and women process both pure trigeminal stimulus and the so-called bimodal odors (odors that activate the trigeminal as well as the olfactory nerve) differently when it comes to cortical activation but do not differ substantially in behavioral measures. Why this difference in processing exists remains unknown.
Using functional magnetic resonance imaging (fMRI), a non-invasive technique, the neurological substrates of painful intranasal stimuli have only recently been explored. Functional imaging studies investigating the neurological basis of painful intranasal stimulation often use carbon dioxide (CO2), an odorless gas which almost exclusively stimulates the trigeminal system. Due to the complex sensory mechanisms involved in pain processing, these studies often yield complex neurological patterns rendering definitive conclusions impossible. However, by utilizing a new meta-analysis tool, we recently analyzed all functional imaging studies investigating CO2 stimulation of the nasal mucosa. The results of this meta-analysis imply that intranasal olfactory and trigeminal information are processed in an integrative fashion and that different sensory systems interact in the brain based on the receptor’s location rather than the sensory system to which it belongs.
These studies prove that the olfactory and trigeminal systems are both activated during perception of a pure trigeminal substance. This indicates an overlap in cerebral activation induced by separate stimulation of the olfactory and trigeminal systems. Furthermore, these interactions remind us that only when we have an understanding of the olfactory, gustatory, and trigeminal networks can we fully start to understand how we process food and flavors.