Imaging of brain activation by odorants in humans.

Application of positron emission tomography and magnetic resonance imaging has provided several new insights into various olfactory functions. One is that sniffing and smelling engage separate subsystems in the human olfactory cortex. Another is that perception of odorous compounds (odorants) is mediated by a set of core regions, which are partly different for pure olfactory than for olfactory plus trigeminal odorants. Depending on the task associated with odor perception, the core regions are recruited together with other circuits, in a parallel and hierarchical manner. The sense of smell seems, therefore, to be organized similarly to other sensory modalities, and the specific psychophysical characteristics of olfaction should be attributed to an early involvement of the limbic system rather than to a conceptually different mode of processing.