Functional MRI of Intranasal Chemosensory Trigeminal Activation

Abstract

Numerous brain imaging studies using positron emission tomography or functional magnetic resonance imaging (fMRI) have provided considerable information regarding the processing of sensory information. While many studies have also been performed on olfactory mediated sensations, cerebral activation following intranasal trigeminal stimulation has not been systematically addressed. The aim of this pilot study was to look at cerebral activation following trigeminally induced activation using CO2, a stimulant which almost exclusively activates the trigeminal system. This activation was compared to effects of stimulation with ‘pure’ olfactory stimuli [i.e. H2S and phenyl ethyl alcohol (PEA)]. Based upon the intimate connections between the trigeminal and olfactory systems (Doty et al., 1978; Hummel and Livermore, 2002), we hypothesized that there would be overlap between central activation induced by separate stimulation of the olfactory and trigeminal systems. We expected that olfactory stimulation would produce activation in, for example, the insular, piriform and orbitofrontal cortices, as well as in the cerebellum and gyrus rectus (Savic, 2002). We sought to determine which, if any, of these structures were activated by trigeminal stimulation. In terms of lateralization, stronger activation of the right hemisphere was expected (Hummel et al., 1995; Hari et al., 1997).