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Consider the odor of a perspiring man, the fresh smell of cut grass, the foulness of the passing garbage truck, and the appetizing aroma wafting out of the local bakery. Odors surround you in countless forms, both positive and negative, but do you ever stop to think about them? Scientists and laymen alike have long considered humans to be “microsmatic animals,” meaning that your olfactory sense plays a minor role compared to the other senses. However, an increasing number of studies have begun to paint a different picture, one that suggests that olfactory information plays a very significant role in your everyday decisions.
Body odors carry informational cues of great importance for the individual across a wide variety of animal species. For a long time, the thought that humans could be counted among these species was dismissed outright. However, it is now known that each of you has a unique odor that carries information related to your genetic makeup and also about personal environmental variables such as diet and hygiene. And, much like your fellow animals, humans are able to extract biological and social cues from conspecific body odors, i.e. odors from your own species, that provide information and direct your behavior. For example, studies have demonstrated that human body odor conveys information that allows you to identify individuals, directs you toward a partner with an advantageous genetic makeup, and informs you of the health status of others.1–3
The percept, or mental impression, of a body odor commonly includes an emotional character that evokes a strong valence of liking or disliking. For example, the body odor from a lover may be a very pleasant percept, whereas the same percept from the person sitting next to you on the bus may be highly negative. When you hear the two words “body odor,” most think about an unpleasant percept related to heavy perspiration. This odor is consciously perceived and reflects a response to a small subset of the numerous chemicals (about 120) that comprise an individual’s body odor. In contrast, you typically are not consciously aware of perceiving the chemicals within anyone’s body odor that serve as social signals.
A recent study demonstrated that body odors are processed mostly outside of brain areas that have long been considered the main processing centers for olfaction.4 Body odors have been found to be primarily processed in areas responsible for emotional and attentional processing. What does this signify? First, in contrast to general odors not of bodily origin, body odors are processed more like emotional stimuli. More importantly, this differential processing indicates a separation between the conscious perception of body odors and the social signals they contain. Studies conducted by the Monell Chemical Senses Center indicate that this separation is automatic—when you try to fool the system by presenting “fake” body odors comprised of chemicals that do not originate from the human body, these fake body odors are processed by the brain as general odors. Remarkably, even when subjects mistakenly identify the fake body odor as real body odor, the brain processes the odor as though it is a general odor. Somehow, the brain is capable of identifying body odors, probably through recognition of specific chemicals. These results lead one to ask: Why has the brain developed these special processing networks in addition to the general olfactory pathway? What are the behavioral implications?