Perspectives on olfactory processing, conscious perception, and orbitofrontal cortex.

The orbitofrontal cortex receives inputs from all the major sensory pathways, but olfaction is the only pathway that projects directly to it. We discuss several unique properties with which this is associated. Olfactory stimuli are converted into spatial images, varying in time, in the olfactory bulb, which are processed by the olfactory cortex for input to orbitofrontal cortex. The input from olfactory cortex to orbitofrontal cortex is mostly direct, though some fibers project through mediodorsal thalamus in some species. Studies are needed to determine the specific contributions of olfactory cortex and orbitofrontal cortex to conscious smell perception. A major challenge to the field is accounting for how conscious perception of this sense is coordinated with conscious perceptions of the other major senses, which are known to depend on thalamocortical circuits. The fact that the primary olfactory area at the neocortical level is embedded in the multisensory region of the orbitofrontal cortex indicates that at this level smell perception is heavily influenced by other senses, particularly related to food flavors through retronasal smell, which is being documented in experimental studies in rodents, nonhuman primates, and humans. Also requiring clarification is how behavioral modulation at each step of processing of the odor images is coordinated. In sum, the orbitofrontal cortex is emerging as the next frontier in understanding the neural basis of smell.