Senses in the dark

Sense of smell

The neglected sense

The acuity of our sense of smell increases from birth on to reach a plateau at about the age of eight, then declines in old age. Some researchers claim that our sensitivity to smells begins to deteriorate long before old age, perhaps even starting during the early 20s. But other scientists report that sense of smell ability depends on the person’s state of mental and physical health, with some very healthy 80-year-olds having the same olfactory prowess as young adults. Significant reduction in the acuity of this sense was recognized in persons with reduction of cerebral blood flow and in smokers.

The sense of smell is critical for almost all species since it contributes to satisfying basic  needs such as finding the right food, choosing a mate or avoiding predators.  The human olfactory system is not very different from that of goats or guinea pigs.  But the importance of olfaction in human behavior is different, and seemingly largely diminished in comparison to most mammals. A survey of college students, for example, found that olfaction was overwhelmingly classified as the least important human sense. This probably is because we, like all mammals, live in an environment packed with countless stimuli and mechanisms of selective attention have evolved to cope with this mass of information.

But in fact, humans have an extraordinary, if underappreciated, sense of smell and our odorant detection thresholds are very low.  We can even detect the scent of fear in human sweat, and select mates whose body-odor infers a favorable genetic makeup. However, only unusually high odorant concentrations spontaneously shift our conscious attention to olfaction.

Our sense of smell can also be affected by experience. On one hand, it improves with practice.  On the other hand, repeated exposure leads to decreased detection thresholds for a number of different smells.

Blind people and the sense of smell

Some studies suggest that, contrary to popular belief, blind people do not necessarily have a keener sense of smell than sighted people. In experiments on blind and sighted people, the top performers on most tests were (sighted) employees of the Philadelphia Water Department who had been trained to serve on the Department’s water quality evaluation panel. These results demonstrate that training is the factor most likely to enhance performance on smell tests.

Yet, although they do not outperform the sighted in terms of odor discrimination and odor identification, blind subjects, participating in other studies, have demonstrated a significantly lower odor detection threshold and higher odor awareness. Blind individuals pay higher attention to odorant stimuli than sighted ones, as their sense of smell helps them to build a mental representation of the external world.

Just as with tactile and auditory stimuli, smells effectively activate the visual cortex in blind people, even if a person has been deprived of sight from their birth.

Interestingly, a recent study by Rombaux et al. showed that the superior olfactory performance observed in congenitally blind subjects is associated with increased volume of the olfactory bulb which collects the sensory afferents of the olfactory receptor cells.

Smell and emotion

The perception of smell consists not only of the sensation itself but also of the experiences and emotions associated with it. Smells can evoke strong emotional reactions, probably more so than other stimuli. When subjects were presented with visual (an object), lexical (the name of an object) and olfactory (the odour of an object) stimuli, and asked to write down whatever came into their heads, the written responses to visual and lexical stimuli were much longer than those for smells, but responses to the smells were far more emotive, and all referred to memory.

It’s not clear, however, why some senses evoke positive, while others – negative emotions. Infants appear to like all classes of odours, perhaps because of the lack previous experience and their innate curiosity. Eventually, smells become good and bad with time. For example, while children younger than five years old rated sweat as pleasant, those above that age did not like it. Another thing affecting the emotional response to smell is its intensity – like and dislike of a particular odour can change with its concentration.

Smell and behavior

Experiments have shown that exposure to pleasant fragrances significantly enhances productivity on work-related tasks. For example, peppermint smell increases alertness and has been found to improve performance.

Vanilla fragrance makes you calmer. Medical experiments have shown that it reduces stress and anxiety. Interestingly, these effects have only been documented for pure vanilla fragrance – not for perfumes containing a blend of vanilla and other notes.

The mood-improving effects of pleasant smells may not always work to our advantage: by enhancing our positive perceptions and emotions, pleasant scents can cloud our judgment. In an experiment in a Las Vegas casino, the amount of money gambled in a slot machine increased by over 45% when the place was odorized with a pleasant aroma.

And just another interesting fact…

Some studies have shown that shy, introverted people are generally more sensitive to smell than sociable extroverts.

Literature

Rombaux P., Duprez T., Hummel T. (2009) Olfactory bulb volume in the clinical assessment of olfactory dysfunction. Rhinology, 47: 3-9.

Rombaux  P., Huart C., De Volder A.G., Cuevas I., Renier L., Duprez T., Grandin C., (2010) Increased olfactory bulb volume and olfactory function in early blind subjects. Neuroreport  21: 1069–1073.

Thumfart W., Plattig K.H., Schlicht N. (1980) Smell and taste thresholds in older people. Z Gerontol. 13:158-188.

The Science of Smell Part 1: Odor perception and physiological response. Johnson S.R., director, Cooperative Extension Service, Iowa State University of Science and Technology, Ames, Iowa.

The Smell Report, An overview of facts and findings, Kate Fox –Director, Social Issues Research Centre

http://www.tsbvi.edu/seehear/summer05/smell.htm