Habitual Exposure to Trigeminal Stimuli and Its Effects on the Processing of Chemosensory Stimuli.

Our objective was to compare brain responses to trigeminal and olfactory stimuli in frequent and non-frequent gum chewers in order to explore whether habitual exposure to trigeminal stimuli affects their central-nervous processing. In healthy subjects, fMRI brain scans were obtained for 20 frequent gum chewers (GC) and 20 non-frequent gum chewers (N'GC), in response to four odorous stimuli; 2 'trigeminal' (peppermint and spearmint) and 2 non-trigeminal or 'olfactory' (cherry and strawberry). During measurements, subjects reported intensity and pleasantness ratings for all stimuli. In addition, a test for general trigeminal sensitivity test (lateralization test) and an odor threshold test was performed. Brain activations in response to individual odors were investigated for the total study population followed by group wise (GC and N'GC) analysis separately for responses to trigeminal (peppermint + spearmint) and olfactory (cherry + strawberry) odors. (1) The GC group exhibited higher trigeminal sensitivity compared to the N'GC group. (2) Olfactory odors activated bilateral insular cortex and amygdala. Apart from olfactory areas (amygdala, insular cortex), trigeminal odors also produced activations in right thalamus and right substantia nigra. (3) In the GC group, olfactory odors produced higher bilateral insular cortex activation than in N'GC group, but no such differences were observed for trigeminal odors. GC subjects appeared to be more responsive to trigeminal chemosensory stimuli. However, this did not directly translate into differences in central-nervous activations to trigeminal stimuli; instead, the use of chewing gum was associated with stronger brain activation towards olfactory stimuli.