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Direct Experimental Evidence for Odor Preference Formation
Only a few studies have directly examined the relationship between odor preference formation and emotional learning. Hvastja and Zanuttini (1989) presented children between the ages of 6.5 to 10.5 with odors paired with either positive or negative slides and showed that, for the younger children in the sample, odors paired with positive pictures were evaluated more favorably than odors paired with negative pictures. Baeyens and Wrzesniewski (1996) examined naturalistic manipulations of a familiar odor paired with idiosyncratically perceived pleasant and unpleasant toilet experiences and found that, compared to a control odor, hedonic perception of the paired odor changed in accord with the individual’s emotional attribution of going to the bathroom. In another experiment assessing physiological responses to odors, Robin et al. (1998) found that the smell of eugenol (“clove” odor used in dental cement) was evaluated negatively and elicited physiological fear responses among patients who were afraid of dental procedures, but not unafraid patients. Notably, in each of these studies, changes in olfactory perception were somewhat equivocal.
Most recently, we conducted a study directly aimed at addressing the hypothesis that olfactory hedonic responses are acquired through associative learning with emotion (Herz et al., 2004). We conducted two experiments that varied with regard to whether a novel (“target”) odor was pre-experimentally pleasant or unpleasant and the emotional association that was linked to it was positive or negative. In each experiment, participants were randomly assigned to an Experimental Group (odor + emotional association) and various Control Groups. Evaluations of the target odor and several “anchor odors” that were not explicitly part of the association procedures were made several times throughout the study: (i) prior to the manipulation, (ii) post-manipulation, (iii) 24 hrs after the manipulation, and (iv) 1 week from the start date. The results from both experiments showed that evaluation of the target odor by all participants was comparable prior to the manipulations, and responses to the anchor odors were unaffected by time or experimental condition. However, in each experiment, all post-emotional manipulation ratings to the target odor were significantly altered in the Experimental Group and showed that odor perception had changed in accord with the emotional valence of the associated experience. When an “unpleasant” target odor was paired with a positive emotional experience, subsequent evaluations of that odor were more favorable, and when a “pleasant” target odor was paired with a negative emotional experience, subsequent evaluations of that odor were more unpleasant. No such effects were seen in the Control Groups. Moreover, changes in odor preferences in the Experimental Groups remained throughout the week of testing. This implies that changes in odor preferences by emotional associations can endure a long time.
Our findings showed that when a novel odor is paired with an emotional event, hedonic perception of that odor was altered in accord with the associated emotion. Although our study could not rule out the possibility of innate responding to odors, together with past empirical work and developmental and cross-cultural data (Ayabe-Kanamura et al., 1998; Hvastja & Zanuttini, 1989; Baeyens & Wrzesniewski, 1996; Mennella et al., 1995; Robin et al., 1998) it appears that emotion in conjunction with odor exposure is a powerful manipulator of odor preference formation.
The results of Herz et al. (2004) also have important implications for how odor preferences can be altered. For odors that do not yet have specific emotional associations to them, but are nevertheless not perceived as neutral, it appears that their preference evaluations can be easily altered by association to specific emotions. It has not yet been studied how to best alter preferences for odors that have already been linked to explicit emotional associations. However, indirect evidence from the clinical literature on post-traumatic stress disorder suggests that when odors have been linked to emotionally intense events they are very difficult to modify (Vermetten & Bremner, 2003), and the literature on proactive interference in odor memory substantiates this (Lawless & Engen, 1977). My hypothesis is that the effectiveness of changing odor preferences that have already been formed will depend on the strength of the original emotional association, the new emotional association and the frequency with which that odor is encountered. I predict that odors that have very powerful prior emotional associations will be most difficult to change and may require various de-conditioning methods prior to re-association with a very positive new experience. Odors that have mild-moderate prior emotional associations will be changeable with a strong new association. And odors that are encountered frequently and are very familiar (e.g., coffee) will be harder to alter than odors that are only rarely encountered because of multiply reinforced prior learning. It would be very informative for psychological research and the fragrance community to address these issues in future research.
An Evolutionary Theory
In addition to the empirical evidence, the associative learning hypothesis for odor preference learning (in contrast to the innate view) is theoretically supported by an evolutionary analysis contrasting the goals and requirements of animals that are generalists versus those that are specialists.
Specialists are animals that are restricted to specific habitats, and thus can often only eat a few foods and have particular local predators (Rozin, 1976). The eucalyptus exclusive diet of the panda bear is one extreme example. Thus, because the ecological niche of specialists is restricted, having hard-wired responses to particular odors is adaptive. Innate odor responses have been empirically demonstrated for many specialist species. For example, both lab-born and wild-reared California ground squirrels show a discriminative defensive response to their natural predator, the Pacific rattlesnake, as compared to the Pacific gopher snake (not a natural predator), when first exposed to them. And the discrimination between these two snakes is made on the basis of subtle olfactory cues that differentiate them (Coss et al., 1993; Poran & Coss, 1990). The same type of specificity in responding has also been demonstrated for food sources and in many other specialist species.
In contrast to specialists, generalists (humans, rats, cockroaches) can exploit any habitat and therefore potential prey and predators are myriad. The available resources and potential predators and dangers, however, differ widely across environments. The sea and the savannah, for example, impose very different survival challenges. Therefore, it is not adaptive to have pre-determined olfactory responses to potential prey or predators, but rather to be especially prepared to learn associations on the basis of their significance when they are encountered. The best natural example of the potency of odor learning is the case of taste aversions. Rats and humans can be made to avoid a novel flavor by being made sick after consumption. For example, presenting a rat with a sweet tasting banana scented drink and then injecting it with lithium causes avoidance of the smell of banana thereafter (Garcia & Koelling, 1966). Similarly, children who have experienced chemotherapy after ingesting a novel ice-cream flavor subsequently show avoidance to that flavor (Bernstein, 1978). Bartoshuk (1989) has made it clear that it is the smell, not the taste, of the substance that is responsible for the learned aversion response. These example show how for generalists, such as us, a scent is a blank slate before exposure in an emotional context but after association it takes on the character of that emotion. A tabula rasa olfactory system also explains the neophobia that human infants display to most new smells until they have learned their meaning. From an evolutionarily perspective it is therefore adaptive that the olfactory system of generalists not to be predisposed to like or dislike any particular odors, but rather be especially prepared to learn and remember what to approach and what to avoid based on experience.
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