Taste and Smell: the hidden senses

By Keith Wilson It is well known that much of what we ordinarily call ‘taste’ actually comes from our sense of smell. This is why food seems very bland when you have a cold, or if you hold your nose while eating or drinking, which has the effect of removing the smell component of your experience (you can try this while eating a jellied sweet — the effect is quite striking!).

Strictly speaking, our tongues are only capable of sensing a limited number of properties that include sweetness, saltiness, sourness, bitterness, savouriness (‘umami’), metallic, and (possibly) fattiness. These combine with a much wider range of odours to generate our experience of flavour, which comes from neither taste nor smell alone. Indeed, the story doesn’t end there since our flavour experiences are affected by many other senses, including the texture and feel of food in the mouth (somatosensory), stimulation of the trigeminal nerve, which gives rise to ‘hot’ and ‘cool’ flavours, such as paprika or menthol, and even vision and hearing which have been shown to modify the perceived flavour and freshness of food and drink.

Strange as it may seem, food may actually taste better with the right kind of music playing in the background — or worse if you are on an aeroplane or other environment where there is a lot of background noise. Tasting, it would seem, is a truly multisensory experience. The role of the sense of smell in contributing to our experience of food and drink has led some psychologists and philosophers to talk of there being not one, but two senses of smell: so-called orthonasal and retronasal olfaction.

Orthonasal olfaction involves the inhalation of odours through the nose, which then reach our olfactory sensing surface, giving rise to what we normally think of as an experience of ‘smell’. In this case the odour typically seems to be located within or just in front of the nose and is typically associated with some external object or source. Retronasal olfaction, on the other hand, involves the propagation of odour molecules upwards from the back of the throat, typically as a result of chewing and swallowing food or drink.

In this case the odour reaches the same sensory surface, but via a different pathway, resulting in an experience that is typically perceived as located in, or ‘referred to’, the mouth or throat. How the brain detects this difference remains something of a mystery, though it may have something to do with the precise order and pattern of stimulation of the nerves in the olfactory epithelium — the organ that detect smells, which is shared between these two forms of olfaction.

This raises the question of whether in fact we have one or two senses of smell, and how we would go about deciding this issue. The fact that both share a single sensory surface (the olfactory epithelium) and detect the same chemical properties of odours might lead us to suppose that there is just one sense of smell, albeit one that contributes to our experience of both smell and flavour in conjunction with taste and the other senses.

Conversely, differences in the pathways by which odours are delivered (through the mouth or through the nose) and subsequent brain processing mechanisms might suggest that these are two distinct senses. Moreover, the fact that retronasal olfaction is frequently misidentified as ‘taste’ and perceived as occurring in the mouth might suggest that we should think of it as primarily a component of flavour-experience that is quite distinct from its orthonasal cousin.

A further possibility is that our common-sense notions of ‘taste’, ‘smell’, etc., do not neatly line up with a more scientifically informed view of how our senses actually operate. Even so, the scientific facts alone do not dictate a particular view of what constitutes a ‘sense’. That remains a philosophical question to be decided not only on the basis of scientific evidence, but on what the notion of distinct sensory modalities is supposed to explain in the first place.

Given that the similar considerations apply not just to smell, but to each of the other senses as well, the question of how many sensory modalities we have turns out to be much more interesting and complex than it might first appear.