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 19th 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 (37oC) 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.
