Central Processing of Trigeminal Activation in Humans

Although numerous fMRI studies have been performed on the processing of olfactory information, the intranasal trigeminal system so far has not received much attention. In a pilot study stimulants were presented within a constantly flowing airstream birhinally to activate the olfactory (phenylethyl alcohol, H2S) or the trigeminal (CO2) nerves. Both olfactory and trigeminal stimulation activated the ventral insular cortex. Intranasal trigeminal stimulation additionally led to an activation of the midbrain, superior temporal gyrus, anterior caudate nucleus, and the dorsolateral orbitofrontal cortex. Cerebellar activation was reduced relative to odorous stimuli. For all stimuli, right-sided activity was more pronounced. These results suggested that processing of intranasal activation follows a pattern which is, at least to some degree, similar for both trigeminal and olfactory stimulation. This and results from several other studies emphasize the fact that there is a high degree of interaction between the different aspects of the chemical senses, also in the sense that chemosensory-induced activation in the nasal cavity is processed in similar cortical networks. Interactions between the olfactory and trigeminal system can also be seen in patients with acquired olfactory loss, who exhibit reduced trigeminal sensitivity, possibly due to the lack of a central-nervous interaction. Both the orbitofrontal cortex and the rostral insula appear to be of significance in the amplification of trigeminal input, which is missing in patients with olfactory loss.