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Neural Processing of Body Odor

By: Johan Lundström
Posted: August 11, 2009, from the August 2009 issue of GCI Magazine.

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When trying to understand a phenomenon in one sensory modality, it is often instructive to examine how similar phenomena have been handled by other modalities, such as vision or audition. Indeed, the differential brain processing demonstrated for body odors vs. general odors is mirrored in the visual system—visual stimuli having high survival value, such as images of snakes or spiders, receive heightened attention and prioritized access to brain processing areas compared to less threatening visual stimuli. Thus, critical information is transmitted through a specialized pathway that is faster and capable of accessing action centers; in contrast, general information is sent through a separate sensory pathway that is slower but more accurate.

Imagine a scenario in which you walk through a garden. Suddenly, you catch a glimpse of a snake lying in the grass. Your visual system alerts you, and your body, now controlled by non-conscious processes triggered by the fast pathway, starts to turn away from the snake as your fear network is activated. Meanwhile, the slower but more accurate visual pathway has had time to process the “snake” and reports that what you’re seeing is just the garden hose. The arrival of this information, however, cannot prevent the evasive action set into motion by the high-priority pathway, and after the outward physical reaction, you are left with a rapid heartbeat and a slight feeling of embarrassment.

This prioritized system and its effects are commonly referred to as pre-attentive processing. You have all experienced misfires, courtesy of a processing system that operates under the principle that errors are better than omissions. It’s safer to react fearfully to a non-threat 10 times in error than to miss the danger once.

This knowledge about the visual system’s special processing of biologically important stimuli leads to speculation that body odors similarly possess a high level of inherent relevance for the perceiver. To test the hypothesis that body odors are processed differentially, the speed at which the brain processes a body odor compound relative to a general odor of similar valence and intensity was measured. It was discovered that the brain processed the body odor compound up to 20% faster than the general odor, indicating that the olfactory system functions similarly to the visual system for biologically relevant stimuli.5

In the visual system, these biologically important stimuli have prioritized access to the brain’s fear network. To investigate whether body odors also activate the fear network, subjects were exposed to body odors from strangers while measuring their brain activity. Body odor from a stranger elicited activation in the brain’s fear network (amygdala and insular cortex; Figure 1), demonstrating that the mere smell of a stranger elicits cerebral processing patterns similar to those responding to visual images of a snake. Together, these studies indicate that body odors are processed in a pre-attentive manner, similar to the prioritized processing of visual images that are important for survival. Recent research suggests that exposure to the body odors of friends or lovers can produce a soothing effect. Exactly what this means for everyday interactions is not known and is currently under investigation.

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