Human odor learning

Over the centuries, humans as well as other animals have learned to associate tastes and odors, in particular, with life-threatening situations. Human excreta, for example, is a foul odor. In some fashion, humans have learned to associate such odors with unpleasant, unhealthy living conditions. The odor of smoke produces dual associations—a log burning... 

Healthy newborn human infants are endowed with a highly sensitive sense of smell. Moreover, there are documented accounts of olfactory learning during the early postpartum period. In the present chapter, we present a brief overview of the relevant research literature and suggest tentatively that olfactory learning may be facilitated by neurochemical activities associated with labor and delivery, and memory traces of odors learned shortly after birth may be retained more efficiently than early postnatal memories involving other sensory modalities (i.e., vision and audition). 

The vomeronasal organ (VNO), located in the nose, is a small chemical sensing stmcture associated with odors and behavioral effects. The vomeronasal system, which is made up of the VNO and a portion of the brain s limbic system, is stmcturaHy independent of the olfactory and nervous terminalis systems in the nose. It may, however, interact with these systems in a manner dependent on prior experience or learning, and therefore be direcdy related to the association of smells and experiences. This independent chemosensory system in the nose may prove to open doors to new learning associated with the sense of smell and human behavior. 

Odors play a much greater role in human behavior than previously thought. The sense of smell provides a direct link with the function of the brain therefore, the further study of olfaction can only advance the learning of causes and effects of stimuli to the brain. 

Adults continue to associate new odors with pleasant and unpleasant situations in social and sex life, work and recreation, and concerning food and drink. The human patterns of odor recognition and preferences do not merely involve the olfactory nerve and its central projections. Learned associations are formed and stored in memoiy. To retrieve odor information, we need affective and cognitive components, as well as verbal descriptors. Without the latter, an odor appears familiar but cannot be labeled, the tip-of-the-nose-phenomenon (Lawless and Engen, 1977). 

Odors affect human behavior more than we realize. They are now appreciated as important in human health and disease. Above all, the powerful role of learning is impressive. Odors become associated with pleasant and unpleasant experiences and can retain their hedonic value lifelong. This applies to food, to social and sexual relationships, and to environments such as houses, workplaces, or landscapes. Writers rather than scientists have described such anecdotes. In Remembrances of Things Past, Marcel Proust evoked a flood of childhood memories by the taste of a madeleine dipped in lime-blossom tea. Jean-Paul Sartre tells in his autobiography Les Mots how the halitosis of his grade-school teacher became to him the odor of authority. 

Newborn humans have few olfactory preferences most of the wiring has to be put in place during the early years of development, though we continue to learn and acquire odor preferences throughout our lives. The cultural implications of this and its relation to perfumery are of course immense, since each of us is able to learn and respond to an enormous range of odorous materials rather than only to those dictated by a genetically predetermined system of behavioral responses. 

The human being is exceptionally sensitive to some volatiles (e.g., 2-isobutyl-3-methoxypyrazine has an odor detection threshold in water of 0.002 ppb [49] and 0.015 ppb in wine [50] but insensitive to many other volatiles (e.g., ethanol has an odor threshold of 100,000 ppb in water and a taste threshold of 52,000 ppb in water) [49]. A person s ability to detect odors is also influenced by many other factors such as genetic variability, olfactory fatigue, and naturally occurring and unpredictable factors such as temperature and humidity. The complexity of food aromas and sensitivity required plus the fact that the olfactory system must be able to respond to unknown odorants (it cannot be learned response) make this a most complex phenomenon. 

In the past, cadaver training was the only way to perform VA training. Fresh cadavers may pose a hygienic issue, although newer and more expensive technology is available to prepare the human bodies with the Thiel solution, which eliminates bothersome odors. Dissection in the subcutaneous tissue is somewhat different, but the behavior of the blood vessels is quite realistic. It is even possible to prepare a torso and arms to learn how to perform more complex, so called exotic VAs in the neck and breast area. The European Society of Vascular Surgery in collaboration with the Vascular Access Society (www.esvs.org, www. vascularaccesssociety.org) and the European Vascular Course (www.evc.org) olfer such workshops. 

The comparisons of psychophysical tests in different species [29] are in direct conflict with the implications of the decline of functional olfactory receptor genes from rodents through the primate series to humans. It appears that we have a lot more to learn about the relation between counting receptor genes and testing for detection and discrimination of odors. This implies that there is not a one-to-one relation between genes and behavior. 

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