Paradoxically, attempts to visualize odorant-induced functional magnetic resonance imaging (fMRI) activation in the human have yielded activations in secondary olfactory regions but not in the primary olfactory cortex-piriform cortex. We show that odorant-induced activation in primary olfactory cortex was not previously made evident with fMRI because of the unique time course of activity in this region: in primary olfactory cortex, odorants induced a strong early transient increase in signal amplitude that then habituated within 30-40 s of odorant presence. This time course of activation seen here in the primary olfactory cortex of the human is almost identical to that recorded electrophysiologically in the piriform cortex of the rat. Mapping activation with analyses that are sensitive to both this transient increase in signal amplitude, and temporal-variance, enabled us to use fMRI to consistently visualize odorant-induced activation in the human primary olfactory cortex. The combination of continued accurate odorant detection at the behavioral level despite primary olfactory cortex habituation at the physiological level suggests that the functional neuroanatomy of the olfactory response may change throughout prolonged olfactory stimulation.
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