Human brain function during odor encoding and recognition. A PET activation study.

In previous positron emission tomography (PET) studies we have shown significant regional cerebral blood flow (rCBF) increases during olfactory stimulation: unilaterally in the right orbitofrontal cortex, and bilaterally in the inferior frontal and temporal lobes (piriform cortex). In the present study we investigated brain function during different stages of olfactory memory processing. Subjects were scanned during four tasks: odor encoding, long-term odor recognition, short-term odor recognition and a no-odor sensorimotor control task. Subjects were 12 right-handed healthy volunteers (6 men, 6 women). Each subject underwent a training session four days prior to their PET scan to learn the six odors required for the long-term memory scan. PET scans were obtained with a Siemens Exact ECAT HR+ 3D system using H2(15)O methodology and 60-sec scanning intervals. PET images were coregistered with each subject's magnetic resonance imaging scan, averaged, and transformed into standard stereotaxic space. Paired image subtractions were analyzed for rCBF changes. Preliminary analyses have revealed significant activation of the right orbitofrontal region and bilateral piriform cortices during the long-term odor recognition task compared with the control task. Activation of the right piriform cortex was present during the short-term recognition task. Brain activity during encoding and retrieval tasks also involved prefrontal cortices. PET activation studies of memory in other modalities have led to hypotheses of a hemispheric encoding/retrieval asymmetry in frontal cortex; the generalizability of this theory to olfactory memory will be discussed.