Functional mapping of human brain in olfactory processing: a PET study

This study describes the functional anatomy of olfactory and visual naming and matching in humans, using positron emission tomography (PET). One baseline control task without olfactory or visual stimulation, one control task with simple olfactory and visual stimulation without cognition, one set of olfactory and visual naming tasks, and one set of olfactory and visual matching tasks were administered to eight normal volunteers. In the olfactory naming task (ON), odors from familiar items, associated with some verbal label, were to be named. Hence, it required long-term olfactory memory retrieval for stimulus recognition. The olfactory matching task (OM) involved differentiating a recently encoded unfamiliar odor from a sequentially presented group of unfamiliar odors. This required short-term olfactory memory retrieval for stimulus differentiation. The simple olfactory and visual stimulation resulted in activation of the left orbitofrontal region, the right piriform cortex, and the bilateral occipital cortex. During olfactory naming, activation was detected in the left cuneus, the right anterior cingulate gyrus, the left insula, and the cerebellum bilaterally. It appears that the effort to identify the origin of an odor involved semantic analysis and some degree of mental imagery. During olfactory matching, activation was observed in the left cuneus and the cerebellum bilaterally. This identified the brain areas activated during differentiation of one unlabeled odor from the others. In cross-task analysis, the region found to be specific for olfactory naming was the left cuneus. Our results show definite recruitment of the visual cortex in ON and OM tasks, most likely related to imagery component of these tasks. The cerebellar role in cognitive tasks has been recognized, but this is the first PET study that suggests that the human cerebellum may have a role in cognitive olfactory processing as well.