Undetectable smells influence human social behavior
A unique property of the olfactory system is its intimate anatomical association with limbic brain circuitry. Olfactory receptor neurons in the nasal epithelium are only two synapses removed from amygdala and entorhinal cortex (via the olfactory bulb), which may explain why odors appear to have a potent influence on emotion, memory, and behavior. Anecdotal evidence has long supported this idea, but recent behavioral studies have also shown that supraliminal odors (i.e., odors above the threshold of detection) can regulate human mood, cognition, and perhaps even mate selection.
Given that olfactory information has relatively direct access to cortical and subcortical emotional networks, we examined whether the hedonic content of undetectable (subliminal) odors alters social preferences of human faces (Li et al., Psychological Science 2007). Subjects participated in a novel olfactory affective priming paradigm, whereby presentation of a neutral face was preceded by a subthreshold concentration of either citral (a pleasant "lemon" odor) or valeric acid (an unpleasant "sweaty" odor). Despite the fact that subjects could not consciously perceive these smells, the subliminal pleasant odor made faces appear more likeable, while the subliminal unpleasant odor made faces appear less likable. These findings strongly suggest that the brain utilizes odor inputs to guide social behavior even in the absence of conscious odor awareness.