Intranasal trigeminal function in subjects with and without an intact sense of smell

The intranasal trigeminal system is involved in the perception of odors. To investigate the cerebral processing of sensory information from the trigeminal nerve in detail we studied subjects with and without olfactory function using functional magnetic resonance imaging. A normosmic group (n = 12) was compared with a group of anosmic subjects (n = 11). For trigeminal stimulation gaseous CO2 was used.Following right-sided stimulation with CO2 controls exhibited a stronger right-sided cerebral activation than anosmic subjects. Stronger activation was found in controls compared to anosmic subjects for the right prefrontal cortex, the right somatosensory cortex (SI), and the left parietal insula. In contrast, relatively higher activation was found in anosmic subjects for the left supplementary motor area in the frontal lobe, the right superior and middle temporal lobe, the left parahippocampal gyrus in the limbic lobe, and the sub-lobar region of the left putamen and right insula which was mostly due to a decreased BOLD signal of controls in these areas. Additional conjunction analysis revealed that activated areas common to the two groups were the cerebellum and the right premotor frontal cortex.These data suggest that the processing of the trigeminally mediated information is different in the presence or absence of an intact sense of smell, pointing towards the intimate connection between the two chemosensory systems.