Nose-ology of smelling

Just how important is our sense of smell? None of us wants to lose any of our senses, but ranking them in order of importance will for many people put smell into the lower half of the list. This is certainly reflected, for example, in compensation payouts where the lost of an eye, or partial deafness, is weighed far more heavily than total smell loss. Such figures are worked out in a world in which survival no longer depends on sniffing out our daily meal. But I wonder if, in the developed world at least, old ideas about the importance of smell have not kept pace with our recent obsessions – both nutrition and flavor related – with food.

It has been obvious for some time that the key to flavour characteristics is a food’s aromas. The OED, for example, notes that the word flavour in English, thought to date from the late 14
th/early 15th century, originally referred only to a fragrance or aroma. A figure of 80% is often quoted as the contribution that odours make to food flavours. This is not based on actual data, but is rather an attempt to convey smell’s importance. Less quantitative, but more to the point, is the fact that with our sense of taste alone we are generally unable to identify foods and beverages – as we often witness when the sense of smell is disabled by nasal congestion. From the point of view of flavour, a food without an odour is no more than a nutritional supplement.

Knowing this, you might think then that our sense of taste is the one to loose (if we had a choice) and wanted to continue to enjoy food. It is fortunate, however, that total taste loss is relatively rare since when it occurs its effects are reported to be at least as, if not more, devastating to food enjoyment as is smell loss.

In contrast, major smell loss is relatively common and is associated with a wide variety of social and health problems, including loss of enjoyment of food leading to altered eating patterns, depression, and increased concerns about hygiene and household safety. Interestingly, while the acquisition of a sensory loss of any type might be expected to produce such effects, it has been reported that individuals who have never had a sense of smell also report many of these problems [1]. Compared to healthy controls, a group of such
congenital anosmics reported more household accidents, increased concerns about social interactions, and higher depression scores. Their eating behavior was, however, similar to that of the normosmic controls. This suggests that dietary changes following smell loss are primarily due to a lack of interest in foods as a result of their sudden ‘tastelessness’ or attempts to overcome this problem by, for example, boosting flavor using salt or sugar or other detectible qualities.

Smell loss is also common as we age, although it appears that this often goes unnoticed because of its gradual onset, while eating patterns are preserved due to habit. In fact, suddenly acquired total smell loss does not always produce a dramatic impact on food choices. As with aging, this may reflect the preeminent role of habit in the diet of some individuals. However, first hand reports of those suffering anosmia acquired as a result of head injury or nasal disease include examples of individuals who say that they can recall food flavours and essentially “fill in the gaps” left by the lack of food odour.

The ability to generate such olfactory imagery has been somewhat contentious, but there is now good evidence that some people can generate mental images of odours, even if they tend to be more fleeting and weaker than visual images. So-called good olfactory imagers – based on self-report – actually sniff when imaging odours, and their sniffs are larger (more air inhaled) when imaging good than bad odours, exactly what is found with sniffs in response to a physically present odour [2]. Imagined odours have recently been reported to enhance the ability of a food picture to induce salivation, and to influence the actual amount consumed of a palatable food [3]
*. An earlier study found that imagined odours enhanced a physically present congruent taste in the same way an actual odour would [4].

So, olfactory imagery can have tangible effects and this is important since olfactory cues are very powerful engagers of food wanting and craving. Variations in being able to image a food flavor may therefore be crucial to understanding why some individuals succumb to food-related cues by overeating.

In addition, though, why only some people are able to generate olfactory images and indeed why it is possible to use these to compensate for actual smell loss highlights how surprisingly little we still know about the nature of the information about smell that we store in memory. We know that odour memories seem to be more emotionally laden than other sensory memories. They are also poorly linked to language – no one has the same difficulty identifying a melon by sight as they do in naming it based only on its odour.

In a recently published theoretical review, Koster and colleagues [5] propose a model of the sense of smell that attempts to account for the apparently odd ways that smells are encoded. These authors point to the fact that, generally, we are mostly unaware of the odours around us – indeed, for most of us, most of the time, our environment is odourless. After a brief period of perception, even novel odours recede out of consciousness due to adaptation. However, this does not mean that that the odour is not encoded. Koster and co argue that odours are ignored without consequence because the encoding is implicit – odours link to events and places incidentally, without conscious effort. The meaning of the odour is supplied by the context in which it was originally experienced. They argue, too, that not only is there no point in asking questions about our ability to identify odours, but that attaching a name to an odour necessarily weakens the link between the odour and its initial context. It is only when an odour is experienced in an unusual context or when expectations about odours are violated do we need to conscious pay attention to them since this may signal something potentially important to survival.

All this may seem odd: that our sense of smell is more about ignoring odours than paying attention to them. However, as Koster
et al. note, sensory signals are meant to tell us something important about the world. Information that indicates only that the status quo is in place tells us nothing, while attempting to consciously take in and process such information would be overwhelming.

Surely though this doesn’t apply to food? We think about food enjoyment in terms of attending to the myriad odours and flavours that make up our diet. But, do we actually do this? A meal at a gastronomy temple may encourage a focus on individual flavours and odours, but did you really pay attention at breakfast today? And exactly what was the quality of peas-ness of your peas last night? On the other hand, if there was an odd flavour lurking in your greens, you can be assured that this will register and possibly help you avoid consuming something nasty.

* This article is notable for one other thing: introducing the worst neologism in recent memory ….. smellizing.

1. Croy, I., et al., Learning about the functions of the olfactory system from people without a sense of smell. PLoS One, 2012. 7(3): p. e33365.
2. Bensafi, M., S. Pouliot, and N. Sobel,
Odorant-specific patterns of sniffing during imagery distingusih 'bad' and 'good' olfactory imagers. Chem. Senses, 2005. 30: p. 521-529.
3. Krishna, A., M. Morrin, and E. Sayin,
Smellizing Cookies and Salivating: A Focus on Olfactory Imagery. J. Cons. Res., 2014. 40: p. DOI: 10.1086/674664.
4. Djordjevic, J., R.J. Zatorre, and M. Jones-Gotman,
Effects of Perceived and Imagined Odors on Taste Detection. Chem. Senses, 2004. 29: p. 199-208.
5. Koster, E.P., P. Moller, and J. Mojet,
A "Misfit" Theory of Spontaneous Conscious Odor Perception (MITSCOP): reflections on the role and function of odor memory in everyday life. Front Psychol, 2014. 5: p. 1-12.