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Chemical Reaction: Learning to Smell
By: Steve Herman
Posted: June 9, 2008, from the June 2008 issue of GCI Magazine.
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There are analogies in sound, as well. In a noisy room, you pick out the words of your friend. You hear a dog bark outside. It is not a muddle of sound because your brain has filtered out the background noise and given you the information you need. In both hearing and seeing, the brain’s ability to filter out noises in order to provide us the information we need to understand the world is a critical tool for our survival. But what about smell? How do we pick out a flower in a swirl of other odors?
The role of the brain in vision and hearing has been extensively investigated, and an invaluable new book, Learning to Smell: Olfactory Perception from Neurobiology to Behavior, written by neurobiologist Donald A. Wilson and psychologist Richard J. Stevenson, does the same for olfaction—illuminating how odors are processed in the brain. Just as understanding how the lens in the eye works does not truly explain vision, the understanding of the role of the hair-like cilia in the nose does not provide the whole story of olfaction. The current emphasis on how molecules react at the receptor sites has done little to inform our understanding of odor recognition and discrimination, and even less to explain aromas’ emotional power.
The basic anatomy of olfaction is well understood. A molecule enters the nasal cavity, reaches the olfactory cilia, and a signal is sent to the olfactory bulb and then into the brain. Actually, the cilia are part of the brain. The signal goes to the piriform cortex and then to the orbitofrontal cortex, where it can converge with gustatory and other sensory input—thus the possibility of strong interactions between the senses.
Learning to Smell presents a new theory of olfactory perception, and the authors provide an alternative to the receptor-centered view of olfaction—maintaining that olfaction is not a simple physiochemical process but is strongly tied to memory. Where the traditional approach concentrates on identifying how the particular features of a chemical stimulate the olfactory system, the primary emphasis in the book is how past knowledge and current experience inform our olfactory recognition.
Hundreds of molecules contribute to the smell of coffee, but we inhale and think “coffee.” Our brain puts it together and compares it to other patterns stored in our olfactive memory. Wilson considers it essential to separate the physical stimulus from the psychological effect to understand how we respond to odors.