The Smell Of Spring: More Than Just Cherry Trees And Rain

Ability To Recognize An Odor Goes Beyond Recognizing Its Components

By Amy Pletcher

New research in olfaction is showing that there are neurons in the brain that respond not just to individual scents, but to combinations of specific scents. "Clove" and "rose" for example, activate entirely different responses in the brain than their combination which is perceived as "carnation."

Linda Buck and Zhihua Zou of the Fred Hutchinson Cancer Research Center in Seattle, Wash. published these findings in the March edition of Science. Their article helps explain how the 300 distinct human odorant receptors can together distinguish up to thousands of different odors.

In 2004, Buck and another colleague, Richard Axel of Columbia University, received the Nobel Prize in Physiology/Medicine for clarifying how the sense of smell works. They began by identifying the family of genes that create the actual odorant receptors on the walls of the nasal cavity. From there, Buck and Axel were able to map the interactions of the receptors in the nose onto specific regions of the brain.

What they found was that when an odorant drifts into the nasal cavity, it is recognized by specific receptors. From there, the receptor sends a signal to the olfactory bulb of the brain, which then sends the signal to the olfactory cortex in the brain, and from there on to the higher cortex which is where odors are perceived and discriminated. The information is also being relayed to the limbic system at the moment of perception, which is responsible for the emotional and physical responses to odors. Buck and Axel were able to trace these reactions into the higher cortex where related receptors and neurons converge on specific spots that are the same across various individuals.

Buck has continued to tease apart the workings of these neurons and receptors. She has shown that a single odorant molecule is recognized by several receptors, but each molecule triggers a unique combination of those receptors. For example, while the smells of sweat and orange are actually very closely related and share most of their odorant receptors, it is the few that differ that make those smells distinguishable. The olfactory system then functions very much like the English alphabet: a limited number of characters combine to form an almost infinite number of words.

It is how these "words" are perceived in the brain that is the subject of Buck and Zou's research, and evidence does seem to suggest that there are cortical neurons in the brain that recognize individual combinations. However, there are some alternative explanations that would cause the same patterns.

The other most probable explanation is that the combinatory effect is actually due to inputs from other cortical neurons that respond to the different odorants. So instead of the neuron recognizing the combination as a primary response, the combination is actually a secondary response to neurons that have already registered the separate components. In the English language, it would be as if instead of recognizing the word "discomfort", that neuron could only be triggered indirectly by the primary recognitions of "dis" and "comfort."

Buck's research has implications for research outside of simple odor recognition. Donald Wilson of the Department of Zoology at the University of Oklahoma in Norman, Okla. has spent most of his career looking into the role of memory in olfactory perception. His research demonstrates that analytical processing and odor recognition doesn't occur at the initial behavioral level, but rather that complex odors are encoded and discriminated at higher levels in the cortex, results that coincide with the work of Buck. However, Wilson's research focuses much more on how this affects and incorporates the emotional system. There is evidence that the olfactory cortex has access to other parts of the brain. Once it learns that A and B occur together to make AB, it starts creating a mental context that also associates emotions and tastes.

What Wilson has found is that since odors aren't recognized before the higher levels in the brain, the entire odor combination must be present in roughly the same proportions as before in order for a memory to be activated. For a smell to activate the emotional memory of "home," the smell of mom's cooking isn't enough. To activate that specific memory, the odor combination of cooking, laundry, and mom herself must be present. The original memory serves as a template, says Wilson, and the new smell is tested against that template. If it's close enough, there's a match: feelings of home and mom's kitchen.

With smell as one of the most effective cues for memory, the more we can learn about smell the better. Any research done has implications not only for humans, but also for all of the technologies that are being developed that rely on smell, like artificial noses that sniff for bombs in airports. The more we know about how smells are configured and recognized, the more accurate those technologies may become.

Amy Pletcher is pursuing a master's degree in technical communication at the University of Washington.