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Odor Emotional Conditioning in Humans: Effects on Perception and Behavior

Rachel S. Herz, Ph.D.
Brown University

There is a substantial animal literature showing how classical conditioning of physical and emotional states to odors induces very reliable behavioral and “perceptual” effects; e.g., fear conditioning and learned taste aversions (see Otto, Cousins & Herzog, 2000). We propose that associative learning principles and classical conditioning explain human perceptual and cognitive-behavioral responses to odors. For example, a novel odor is paired with an unconditioned stimulus, such as surgical procedure in a hospital, which elicits an unconditioned emotional response, such as anxiety. The odor then becomes a conditioned stimulus and acquires the ability to elicit the conditioned response of anxiety when encountered in the future. This mechanism explains both how odors come to be perceived as pleasant or unpleasant as well as how odors can elicit emotion and influence behavior. Developmental, cross-cultural and neuroanatomical data support this proposition, but until very recently there has been no direct empirical validation of this claim. In several studies conducted in my laboratory we have shown that odors readily become associated to emotions and that through classical conditioning mechanisms the hedonics of odor perception can be changed and motivated behavior in the presence of odors can be altered. In one study, when a novel unpleasant odor was paired with an enjoyable experience, that odor was later evaluated as more acceptable than it had been previously. Similar (reverse direction) results were found when a novel, pleasant odor was paired with an unpleasant emotional experience (Herz et al., 2004). In a second series of experiments, when an odor that had been paired with an emotional experience, was later present motivated behavior was altered in accord with the associated emotion (Epple & Herz, 1999; Herz et al., 2005 in press). These findings provide strong experimental evidence to support the hypothesis that odor hedonic perception and odor-related behavior result from a learned association between an odor and the emotional context in which the odor was encountered. Further human investigations into odor classical conditioning mechanisms will have significant applied and clinical benefits.