Although numerous MIR' studies have been performed on the processing of olfactoryinformation, the intranasal trigeminal system so far has not received much attention. Ina pilot study stimulants were presented within a constantly flowing airstream birhinallyto actiNnte the olfactory (phenylethyl alcohol, H2S) or the trigeminal (CO_2) nerves. Botholfactory and trigeminal stimulation activated the ventral insular cortex. Intranasaltrigeminal stimulation additionally led to an activation of the midbrain, superior tem-poral gyrus, anterior caudate nucleus, and the dorsolateral orbitofrontal cortex. Cere-bellar activation was reduced relative to odorous stimuli. For all stimuli, right-sidedactivity was more pronounced. These results suggested that processing of intranasalactivation follows a pattern which is, at least to some degree, similar for both trigem-inal and olfactory stimulation. This and results from several other studies emphasizethe fact that there is a high degree of interaction between the different aspects of thechemical senses, also in the sense that chemosensory-induced activation in the nasalcavity is processed in similar cortical networks. Interactions between the olfactory andtrigeminal system can also be seen in patients with acquired olfactory loss, who exhibitreduced 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 theamplification of trigeminal input, which is missing in patients with olfactory loss.
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