Olfaction—the sense of smell—may be the oldest and least well understood of the five senses. It alerts us to danger, food and other important matters and is closely linked with brain systems involved in emotion, mood, memory, learning and behavior. Stated in simple terms, our understanding of olfaction is as follows: an odorant activates neurons in the olfactory epithelium that project axons within the olfactory nerve to the olfactory bulb, activating mitral cells that in turn send information to the olfactory cortex and other neural processing areas.

Now, new research from scientists at the University of Colorado Anschutz Medical Campus (Aurora) and the University of Utah (Salt Lake City) suggests that the process is not unidirectional but rather includes a 'back and forth' interaction between the olfactory bulb and the olfactory cortex. “It was originally thought that the olfactory bulb filtered and the olfactory cortex made decisions on whether something is, for example, edible. Our study says it's not quite like that. You process information on reward in the olfactory bulb, send it to the cortex and there is a dialogue between the two. Then the brain will act,” explained Diego Restrepo (University of Colorado, Aurora), who led the research, in a press release.

The study specifically examined the synchronous firing of pairs of mitral cells in the olfactory bulbs of eight mice engaged in a behavioral task in which they were challenged to learn which of two new odors was associated with a reward. The scientists aimed to determine what type of information was being transmitted by synchronized firing: odor identity or odor value. Their results indicate that synchronized firing of mitral cells conveyed information regarding odor reward (Neuron 69, 1176–1187; 2011).

These data distinguish the olfactory system from other sensory systems in a new and important way: information on value or reward is integrated into an earlier stage of neural coding. This phenomenon emphasizes the importance of context in olfactory processing.

As stated by Donald B. Katz and Joost X. Maier (Brandeis University, Waltham, MA) in an accompanying discussion of the paper (Neuron 69, 1041–1042; 2011), the new results indicate that “the dividing line between sensation and perception may be found outside the brain...[W]hile receptor neurons may respond to purely physical aspects of sensory stimuli, even the earliest stages of neural processing intrinsically pertain to what that stimulus means to the organism under current contingencies.”