Feeling Tastes

When we learn about the senses, it is usually as if we are studying a set of five or six separate devices, each responsible for a distinct function: sight, hearing, touch, smell, taste, and perhaps balance. But sensing – and in particular, perception - is not really about what the sensors (eyes, eardrum, tastebuds, and so on) do, anymore than computing is mostly about the keyboard or mouse. Perception is a function of what our brains do with the signals arriving from our peripheral sensory receptors.

 There has recently been a distinct shift in emphasis in both behavioural and neuroscience research to examining the ways in which information from different sensory system interacts to provide information about the world. In particular, there has been increasing interest in how odours and tastes combine to generate perception of food flavours [1]. The idea of sensory integration in the perception of flavour is not new. Writing in the early 19^th century, the gastronomic pioneer, Brillat-Savarin was “tempted to believe that smell and taste are in fact but a single sense, whose laboratory is the mouth and whose chimney is the nose” [2].

 An emphasis on the importance of senses working together has been termed an “ecological approach” to perception, an approach especially associated with the psychologist J.J. Gibson. Gibson [3] argued that the primary purpose of perception is to seek out objects in our environment that are biologically important. As such, the physiological origin of sensory information is less relevant than that the information can be used in object identification. Effectively, then, the key to successful perception is that sensory information is interpreted as qualities that belong to the object itself. Viewed this way, flavour is a functionally distinct sense that is “constructed” from the integration of distinct sensory systems (smell, taste) in order to identify and respond to objects that are important to our survival, namely foods.

 Even what we think of as distinct senses may in fact be multisensory. Many of the “odours” that we encounter everyday in fact stimulate both our sense of smell and the touch, pain and temperature receptors in the nose that belong to the trigeminal nerves. So, the coolness of menthol is a tactile sensation, rather that a smell. The role of both olfactory and trigeminal receptors in producing smells is well-known. The role of the sense of touch in our perception of taste is less well understood, but may be vital in our taste experiences.

 Under most circumstances, taste and tactile sensations in the mouth are so well integrated that we cannot begin to disentangle them. After all, foods and drinks simultaneously produce both tastes and tactile sensations. However there is growing evidence that our tastes experiences may themselves be multisensory. From a physiological point of view, this is not too surprising since taste buds contain fibres that respond to touch and temperature. This explains why, for example, a moving tactile stimulus, e.g. a cotton bud moved along the side of the tongue, has been shown to “capture” a taste placed on the tongue, with the location of the taste following the movement of the cotton bud.

 Recently, sweet and sour/salty tastes have been shown to be elicited by, respectively, heated and cooled probes placed on areas of the tongue that contain taste buds. This research, by Barry Green and colleagues at the John. B. Pierce Laboratory at Yale University in the USA, has now been followed by another study from this laboratory showing the importance of temperature in taste perception. Green & Nachtigal [4] observed a difficulty in perceiving sweet tastes – for example, while licking a lollipop – when the tongue remained outside of the mouth. Once the tongue was retracted into the mouth, however, the sweetness was obvious. These researchers examined that possibility that the higher internal mouth temperature was responsible for this effect. Prior research had shown that temperature could influence sweetness, an effect that is evident when we compare the sweetness of ice-cream straight from the freezer with the same, but now much sweeter, ice-cream that has been allowed to melt at room temperature.

 By comparing the rate of adaption to sweetness – the way in which the sweetness declines with continued exposure to the taste – at different temperatures, Green & Nachtigal were able to show that warming the tongue outside of the mouth by dipping it into a solution with the same temperature as inside the mouth (37^oC) produced the same effect on sweetness as withdrawing the tongue into the mouth.

 An intriguing question though is why the same effects did not occur when the study was carried out using the bitter taste of quinine. Why there are particular interactions between sweetness and temperature and not other tastes and temperature is unknown, but is again consistent with earlier studies that have been unable to consistently show that tastes at different temperatures vary in intensity.

 Another theoretical contribution by J.J. Gibson was to make the distinction between the active gathering of information via the senses (sniffing, listening) versus a more passive process (smelling, hearing). Implied in this distinction is a process by which we attend to the sensory information, but also involves movement, and tactile feedback from that movement. For example, if I want to gather more information about something that occurs in my peripheral vision, I’ll turn towards it.

 Active perception seems to be a hallmark of eating. We sniff the aromas coming off the food, we sip and reflect on the flavour of the wine, and manipulate the food in our mouth to maximise the tastes and flavours. Surprisingly, however, there has been very little research on what effects such active perception has on our perceptions of foods and beverages. In the case of eating, though, one effect of active perception is to stimulate our sense of touch, once again producing a multisensory experience. One well-known example of this is the sensation of astringency. Eating a green banana, or walnuts, or drinking black tea, cranberry juice or red wine all produce sensation of drying or roughness on the tongue and palate ….. but only if we move the tongue so that the two surfaces connect with one another.

 Green and Nachtigal, in another study of how tastes and touch sensations interact, compared the effects of passive to active tasting of sweet, salty, sour and umami (mono-sodium glutamate, or MSG taste) tastes.  In the former condition, tastes were applied with a swab to different parts of tongue, but tongue remained immobile. In the active condition, the participants were asked to touch the tongue to the roof of the mouth and also swallow. Hence, this condition more closely resembled normal eating.

 This study did indeed find that active tasting had a strong impact on taste intensity …… but only for the taste of MSG. MSG taste intensity was much higher during active tasting but that of the other tastes was largely unaffected. Of all the tastes, umami tastes have seemed to have a “mouthfilling” quality. Moreover, as these researchers point out, active food manipulation including chewing is needed to break down the food’s physical structure, releasing the glutamate and other amino acids responsible for umami tastes. This process may also be behind the fact that in this study, MSG intensity in either condition was highest at the rear of the tongue. No such ‘tongue geography’ effects were seen however for the other tastes.

 The more taste perception is studied, the less it appears to be a simple interaction of soluble molecules with receptors in taste buds. This study further reinforces an emerging view that, in the mouth, what we touch is what we taste, and vice versa.

                                                                                                                                               

1.  Prescott, J., Psychological processes in flavour perception, in Flavour Perception, A.J. Taylor and D. Roberts, Editors. 2004, Blackwell Publishing: London. p. 256-277.

2.  Brillat-Savarin, J.-A., The Physiology of Taste. 1994 ed. (1825), London: Penguin Books.

3.  Gibson, J.J., The Senses Consideered as Perceptual Systems. 1966, Boston: Houghton Mifflin Company.

4.  Green, B.G. and D. Nachtigal, Somatosensory factors in taste perception: Effects of active tasting and solution temperature. Physiol. Behav., 2012. in press.