Just my imagination

Every first year psychology student learns the difference between sensation and perception, although whether or not this distinction has any long-term impact is open to debate. Psychology students are probably like the rest of us. We tend to imagine that our perceptions are somehow a true picture of the outside world, and that our cognitions - memories, thoughts, and imaginings - belong to another part of the brain which doesn't really interfere much with what we perceive.

And yet it is our cognitions (“top-down processes” as the psychologists say) that give the meaning to what we perceive. If we taste something, for example, the tasting experience will be examined in the light of a mental model of that taste that we already have – essentially a prediction or expectation of the taste based on our memory of similar experiences or other sources of information (perhaps from advertising or packaging, and so on). This tells us two things. The first of these is that we can’t really understand what we experience and enjoy (or not) without knowing something about these top-down processes. 

More specifically, it tells us that we carry around mental models of our experiences, and this raises the question of just how “real” these internal representations of the stuff that’s out there really are. This, it must be said, is not a novel question, having been regularly asked by John Locke and others, as they shared pints at their local pub (The Empiricist & Bishop) in 18^th century Britain. But more recently, and particularly in relation to tastes and smells, we have had some actual evidence about “imagined” sensory experiences.

In the early to mid 1990s, there were a number of studies published in what was known as memory psychophysics. The idea was to ask what would happen if you held mental image of sensations (memories), combined them mentally with actual physically-present sensations, and then rated the intensity of the combination. While, on the surface, this sounds like the ultimate waste of time, in fact it was one way of asking whether or not people explicitly or implicitly knew how different sensations interact. Dick Stevenson and I explored this idea in both mixtures of sweet and sour tastes and the hot sensations of capsaicin (from chilli) with these tastes [1]. Mixing an imagined taste (e.g., sourness) with a physically present sweet taste showed the same pattern of suppression (less sourness, less sweetness) as found when the two tastes are actually mixed together and tasted. But then, we consciously know this anyway - how do you make lemon juice less sour? More interesting was the fact that when participants imagined adding a burning sensation to sweet or sour tastes, only sweetness was rated as less intense. This is exactly what happens in actual mixtures – capsaicin only reliably reduces sweetness. But most importantly, the participants, when debriefed about their knowledge of such interactions, felt that any taste would be suppressed by the burn. In other words, they carried implicit information about the interactions, of which they were not consciously unaware.

A few years later, in an interesting exploration of the interactions of tastes and congruent odours – e.g., sweetness and vanilla – a group at McGill University in Canada [2] found that imagined odours could enhance the detectability of a sweet taste, in the same way that a physically present odour could. And in exactly the same way as a physically present odour, the imagined odour had to be congruent. So, imagined strawberry odour was effective in increasing sweet taste detectability (because strawberry odour carries with it the quality of sweetness, learned from previous experiences). An imagined ham odour, which is incongruent with sweetness, was not.

So, it appears that we can conjure effectively real sensory stimuli from imagination. Most fascinating of all though is how these cognitively-based stimuli can influence behaviour, including potentially our food preferences and choices. Again, to some extent, we know how influential thoughts can be.  Imagine sucking a lemon and saliva will start to flow; similarly, think about food close to dinner time to get the gastric juices flowing. This latter effect is consistent with data showing that thinking about food is associated with craving and, especially if you are restricting intake by dieting, succumbing to the temptation to eat. It seems that exposure to food as a mental image acts just like an initial bite, which tends to elicit an appetiser effect (“you can’t eat just one!”) [3].

A demonstration of the potential impact of mental imagery on actual food consumption was published in the prestigious journal Science in 2010 [4]. Noting that both sensory input and mental images can elicit similar responses across a range of different behaviours, these researchers examined whether repeatedly thinking about a food actually decreased desire to consume the food. In other words, can thinking act just like an actual bite or sip, in producing a type of sensory-specific satiety (SSS) effect? We know that the first bite or first sip is always the best, with pleasure decreasing after that [5]. In fact, losing pleasure in eating or drinking is just as much a reason that people stop eating as being full. A series of experiments using those much-loved research foods M&Ms and cheese cubes showed exactly this effect. For either food, multiple repeats (x 30) of thinking about eating led to less actual later ad-lib consumption than did fewer (x 3) thinking episodes. Important to the interpretation of these results was the fact they controlled for simple exposure to the image of the food – thus, imagining placing M&Ms into a bowl, as opposed to imaging eating them, had no impact on subsequent consumption.

How did these researchers reconcile the two potential effects of imagining food – the appetiser effect (increased desire to eat) and the SSS (reduced pleasure in eating) effect? A final study in their paper measured two separate reasons for eating, namely liking and wanting (see: Learning to want). Repeatedly imagining a food was found not to produce a decrease in liking for the food. So, in fact, the effect is unlike SSS which is based on reduced liking with repeat exposure. In addition, the researchers asked participants to perform a reinforcement computer game in which points in the game could be exchanged for consumption of cheese cubes. Exactly as would be predicted if the imagining effects were due to reduced motivation to consume (reduced wanting), those previously asked to imagine eating for the high multiple occasions (30 vs. 3) were less motivated to respond to obtain the cheesy reward.

The practical implications of research such as this for reducing overeating are pretty obvious. But, more generally, the studies tell us that the more we know about cognitive “top-down” processes that feed into our perceptions, emotions and motivations, the more we will understand food choices. What’s in the brain is just as important as what’s in the mouth.

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1. Stevenson, R.J. and J. Prescott, Judgments of chemosensory mixtures in memory. Acta Psychologica, 1997. 95(2): p. 195-214.

2. 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.

3. Yeomans, M.R., Palatability and the micro-structure of feeding in humans: the appetizer effect. Appetite, 1996. 27(2): p. 119-33.

4. Morewedge, C.K., Y.E. Huh, and J. Vosgerau, Thought for Food: Imagined Consumption Reduces Actual Consumption. Science, 2010. 330: p. 1530-1533.

5.    Hetherington, M., B.J. Rolls, and V.J. Burley, The time course of sensory-specific satiety. Appetite, 1989. 12: p. 57-68.

Remembering to like

Most of us have no trouble naming the foods we like most, or indeed saying how much we like them. We may not use a rating scale to do this, but can still convey that we like chocolate cake “a lot” or even – perhaps to confuse non-English speakers – “an awful lot”. But making a general statement about liking for foods (or anything else) carries with it a couple of important assumptions. The first of these relates to context (doesn’t everything?): chocolate cake is great in the evening as a dessert, but perhaps less enjoyable first thing in the morning. This is why we can say that we like chocolate cake more than we do cornflakes, but at the same time also say that we prefer the cornflakes if we are talking about breakfast.

Secondly, though, and much less obviously, I could ask you about your love of chocolate cake in relation to past or current consumption of it. Thus, being hungry and faced with the prospect of your first bite of the slice in front of you, your answer is pretty obvious. Ok, then what about after your first bite? After your second bite? Third? Tenth? There is a well-known phenomenon called sensory-specific satiety (SSS) that kicks in once we start to eat. This refers to the fact that simple exposure to the sensory properties of a food – its taste, smell, texture – causes the food to become less liked. 

The effect of SSS isn’t enormous – we see no more than about a 20% reduction in liking if we measure it using rating scales. But it seems to be enough to motivate us to want to eat other foods that are available instead. Hence, the theory behind SSS is that it is an adaptation that helps maintain a varied diet. It is apparently also important in telling us when to stop eating. In studies that allowed participants to consume as much food as they wanted, the reason ‘I just got tired of eating that food’ was found to be just as common a reason for stopping eating as ‘feeling full’.

The distinction between these reasons is important: SSS is not about being full. Even chewing a food and not swallowing it will produce a decrease in liking for that food. Nor is it about nutrients – SSS occurs between foods that are similar in their flavours, but not between foods that are similar in nutrient content, but not flavour. So, we get bored with exposure to flavours and, indeed, pretty much any aspect of the food. Repeatedly eat red M&Ms and you’ll soon find the blue ones becoming strangely more appealing. Chew on something quite elastic for a while (say, a caramel) and you’ll start to long for a little bit of crispness. 

Becoming bored with eating the same food repeatedly is not news, of course. With the exception of breakfast, where many people trade off boredom for convenience, having the same meal over and over again on successive days leads to both loss of pleasure and reduced consumption. It is not a surprise that many diets use boredom as a way pushing the dieter to eat less and hence loose weight. Whether over the short term, within a meal (SSS) or over days or weeks (boredom/monotony), we eat more when confronted with foods that are varied in the flavours or textures.

But what if you didn’t recall what you’d recently eaten? In an earlier post (February, 2012: Remembrance of foods past), I wrote about the dramatic case of a severely amnesic man who ate three full lunches over the space of little more than an hour with apparent gusto. Clearly, in his case, the inability to recall what he’d eaten allowed him to maintain an interest in eating.

A recent study suggests that, for all of us, memory and attention play a crucial role in determining the degree of pleasure we derive from food, and hence how much we are likely to consume. Larson and colleagues [1] asked study participants to view pictures of either sweet or salty foods and rate how appetizing they thought those foods were. The idea was that in making these ratings, the participants were implicitly retrieving experience of those foods from memory. All participants were then asked to eat some salty peanuts and rate how enjoyable these were. If the participants had rated only 20 food pictures, then there was no effect on enjoyment of whether the pictures were of sweet or salty foods. However, after 60 pictures, those who had previously evaluated salty food pictures enjoyed the peanuts much less than those who evaluated sweet food pictures. What was producing this effect? In a subsequent study, the researchers included a condition in which participants were asked only to evaluate the brightness of the food picture and not how appetizing the food in the picture was, and this produced no such effect. As a result, the authors concluded that attending to the food sensory qualities (sweetness or saltiness) and retrieving information from memory was the source of this effect.

This study has a number of crucial implications. The first of these is that it provides evidence of the cross-modal nature of SSS, in that a picture, when allowed to activate prior experiences, could reduce enjoyment of an actual food flavour. We know that different aspects of foods (tastes, odours, textures) are bound together in memory as flavours, and that experiencing one aspect such as an odour can elicit the food’s taste qualities and any positive (sweet) or negative (bitter) qualities the taste may have [2]. This study points to a similar integration of visual and taste/odour food properties, also with hedonic consequences.

Secondly, these authors suggest that such effects may be in operation when we view advertising or other cues to foods, if these cues cause us to imagine the experience of the food based on our memory. That is, rehearsing the experience of a food in response to a cue will cause us to experience less enjoyment of the food itself and potentially to reduce the amount eaten. If true, this runs contrary to what we know about the effect of environmental cues, which are known to trigger eating [3]. This apparent paradox may be due to the fact that those affected by cues most are those suffering from hedonic hunger – that is, those who have deprived themselves of highly palatable foods. In their case, relative lack of recent memory for such foods may be the issue.

One final implication of this study concerns the phenomenon of mindless eating (see February, 2012:  Unaware eating), in which attention diverted from eating – for example, by watching TV – leads to higher consumption. The Larson study suggests that it is possible that paying attention to food while we are eating is necessary for SSS to occur because it requires access to prior memories. Perhaps we therefore experience less SSS with novel foods, for which we have no memories?

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1. Larson, J.S., J.P. Redden, and R.S. Elder, Satiation from sensory simulation: Evaluating foods decreases enjoyment of similar foods. Journal of Consumer Psychology, 2014. 24(2): p. 188-194.

2. Prescott, J. and R.J. Stevenson, Chemosensory Integration and the Perception of Flavor., in Handbook of Olfaction & Gustation: Modern Perspectives., R.L. Doty, Editor. 2015. p. 1008-1028.

3. Fedoroff, I., J. Polivy, and C.P. Herman, The specificity of restrained versus unrestrained eaters’ responses to food cues: general desire to eat, or craving for the cued food? Appetite, 2003. 41: p. 7-13.

A taste of emotion

If you study perception, you tend not to be particularly concerned with the “secondary” emotions that sensory experiences might evoke (for a fuller discussion of this, see: http://www.expo.rai.it/eng/2014/07/08/taste-neuroscience-prescott/). Except if you study taste, in which case you can hardly avoid it. Emotional responses to tastes are, we can say with a reasonable degree of certainty, built into the fabric of the taste experience. Jacob Steiner’s famous photographs of newborns smiling or grimacing in response to sucrose and quinine [1], respectively, are if not exactly proof of this, then nevertheless compelling. The adaptive argument, too, is simple: tastes evoke emotions because this activates motivations to consume (calories, salt) or avoid (toxins) and hence promote survival. 

Our everyday language provides us with some hints that taste may find links with our emotions more broadly. Thus, unpleasant emotional experiences will leave a bitter taste in your mouth; we say that we can taste the fear, and of course sweetness is synonymous with all sorts of pleasant emotions.  The usual interpretation of such figures of speech is that we have co-opted tastes to help us describe emotions, perhaps because we tend to be poor at this.

But increasingly this does not seem to be the whole picture. A recently published study by Herbert and colleagues [2] examined the way in which autonomic nervous system responses - eye-blinks and changes in pupil diameter in response to startling bursts of white noise - are modified by emotion-inducing pictures. Importantly, though, they compared these responses in tasters and non-tasters of the bitter compound 6-n-propylthiouracil (PROP). This study was prompted by an earlier study in which PROP supertasters showed more intense self-reported emotional responses (anger, tension, sadness and fear; decreased mood and joy) than non-tasters or medium tasters after an anger-inducing film clip.

Herbert and colleagues found that startle responses were increased for pictures meant to elicit fear, anger, disgust, or pictures that were pleasant in content, relative to neutral pictures – but in the PROP-taster group only, and not the non-tasters. This suggests that the PROP tasters are hyper-responsive to both negative and positive emotions generally, not just to tastes.

Why would broad emotional responsiveness be linked to the ability to taste a particular bitter compound? Certainly, facial expressions to bitterness – the familiar grimace, nose-wrinkle and gape – has previously been thought to be the basis of the classic disgust facial response and perhaps even the origin of this basic emotion itself. Indeed, PROP super-tasters has been shown to be more responsive to the visceral – but not moral – aspects of disgust than are (medium) tasters and non-tasters, with taste sensitivity being positively correlated with degree of disgust [3].

But a common origin for disgust and bitterness dislike suggests something broader than the involvement of one particular bitter compound linked to a specific taste receptor (T2R38), one of perhaps 30 or more such bitter receptors. Two possibilities suggest themselves. The first is that responses to bitterness in general are reflective of an underlying overall emotional responsiveness, a relationship yet to be tested. In this sense, bitter substances are simply effective at eliciting emotions, in the same way as pictures of disgusting scenes or objects are effective. In addition, though, ratings of PROP intensity are often used as a proxy measure for taste responsiveness in general. PROP tasters also find sweet, salty sour and bitter tastes more intense than do PROP non-tasters (see for example, [4]).  This is more than likely due to the fact that increased numbers of taste buds underlie overall taste responsiveness, including (at least for those not ‘blind’ to it), PROP bitterness.

It has been hypothesized for some time that the pleasure given by sweetness is really no different than pleasure of any other sort, and that a common reward system in the brain underlies the popularity of sex, drugs and rock ‘n roll ….. and cake. But yet another broad link between taste and more general responsiveness to reward has been reported [5]. I have discussed the phenomenon of sweet liking and disliking previously (How sweet it is ... or is it?). The origins of this hedonic dimension are not well understood, but this study suggests that it may be linked to variations in overall disinhibition. Here, disinhibition was defined in terms of delay discounting – the idea of how much we value a given reward given now versus one that may be much larger, if we are willing to wait for it. So, if I say that I would rather have $10 now rather than $20 in a month, I am effectively discounting the later amount. In food terms, I can have one piece of chocolate now or three pieces in an hour. If you opt for the smaller reward now, you are unable to inhibit your need to satisfy your immediate desire. It turns out that such “discounters” are more likely to be sweet likers, that is, to prefer higher levels of sweetness. This then is another case of taste responses being seen in terms of more general responsiveness to reward.

An obvious interpretation is that, as mentioned above, pleasant tastes are merely part of an overall reward system that has adaptive value. However, there is an intriguing alternative explanation of why tastes might be so tied into other aspects of emotional life. If we start with the premise that tastes are responsible for some of the earliest emotions that we experience, it becomes possible to think about these early experiences having a large impact in shaping our broader emotional landscape. So, in this scenario, the fact you respond more positively to sweetness than I do becomes a major factor in whether or not you delay discount, not just for chocolates but in general for all reward. Similarly, sensitivity to bitterness – present at birth – has such a strong emotional impact that it influences the way you subsequently respond to all emotional stimuli. As a corollary, individual variations in taste sensitivity or hedonic value becomes a primary determinant of variations in our emotional responsiveness generally. 

Admittedly, this is a “chicken/egg” argument that may be impossible to determine but ….. the chronological primacy of taste hedonics does mean that we ought to consider the possibility that the taste of fear is not a way of helping us describe emotions but rather a clue to where our emotions originate.

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1. Steiner, J.E., et al., Comparative expression of hedonic impact: affective reactions to taste by human infants and other primates. . Neurosci. Biobehav. Rev., 2001. 25(1): p. 53-74.

2. Herbert, C., et al., Supertaster, super reactive: Oral sensitivity for bitter taste modulates emotional approach and avoidance behavior in the affective startle paradigm. Physiol Behav, 2014. 135C: p. 198-207.

3. Herz, R.S., PROP Taste Sensitivity is Related to Visceral but Not Moral Disgust. Chem. Percept., 2011. 4: p. 72-79.

4. Prescott, J., N. Ripandelli, and I. Wakeling, Intensity of tastes in binary mixtures in PROP non-tasters, medium-tasters and super-tasters. . Chem. Senses, 2001. 26: p. 993-1003.

5.    Weafer, J., A. Burkhardt, and H. de Wit, Sweet taste liking is associated with impulsive behaviors in humans. Front Behav Neurosci, 2014. 8: p. 228.

Absolutely love it! … relatively speaking

You can never go back. I had an opportunity recently to take a sip of a wine that I used to happily drink some … ok, many ….years ago. A sip was more than enough! The shame of it was that the amount of pleasure I can get from an excellent wine now is probably no greater than the pleasure that Chateau Chunder (ok, not that bad) gave me all those years ago. 

Pleasure is relative … not only to our current state of mind, but to recent experiences as well. In an excellent discussion of this phenomenon, Parducci [1] analysed the impact of recent experience on how happy we feel. How happy would you be to find $10 on the street? Let me ask you another question before you answer: are you just stepping out of your Ferrari or on your way to the unemployment office? Alternatively, imagine you find $20 on the street every day for a month ….. how do you feel if one day there’s only $10? In both cases, the context (your financial state or your recent experiences) provides the background that determines the degree of pleasure that you are likely to experience.

Our sensory experiences are not exempt from context effects either. Another balmy 19 C (66 F) day. Now, depending on where you live, you might agree with me or alternatively consider that it’s me who’s balmy. The fact is, I have given you a piece of information and an interpretation, but what is missing is to know where I am at present, and hence the season*. These extra details allow you to place both the information and my interpretation in context. Most of us at some time have eaten a dish that is just too hot (spicy). Part of this may be due to the physiological impact of recent chilli eating, or lack thereof. But mostly, it will be a mental comparison of the heat of the spicy dishes that you have had recently with what you are eating now [2].

Each of these examples show how any given sensory or emotional experiences occurs against a background of other similar recent experiences. From the perspective of wanting to measure sensory or hedonic experiences, context effects are not merely an interesting quirk – they are the main game. All human measurement is relative. Even a simple question of deciding whether or not you like a beverage is really a case of liking it “compared to what”. In evaluating consumer preferences for foods or beverages, the common scenario is that multiple samples – different brands or different versions of a product – are compared at each tasting session. In the less common situation, though, that a single product was rated for liking, we would expect that the point of comparison was the consumer’s prior, and especially recent, experiences with that class of product [3]. 

When multiple products are compared within the same evaluation session, the products are compared relative to one another. One consequence of the influence of context in ratings is that a given product can be given a low or a high liking rating entirely as a function of what other products are in the evaluation [4]. To take an obvious (and unrealistic) example, imagine that you are asked to rate your liking for different flavours of jelly-beans: orange, lime, blueberry, banana, and … tomato. The latter is obviously an odd sample (despite being a fruit) and is likely to be rated lower. Next, however, I ask you to rate your liking for the following jelly-bean flavours: spinach, brussel sprouts, potato, cabbage, and …. tomato. Here, I imagine that the tomato flavour would be a winner. 

It is crucial to understand the implications of context effects if consumer data are not to be misused. Many food companies have in the past, and to a lesser extent still today, relied on fixed acceptability ratings as criteria for action. For example, on the standard 9-point hedonic scale, a mean rating of 7 (representing “like moderately”) in consumer testing may be required to launch a product. Since any given number on a rating scale can only really be understood in relation to the ratings given other products, there is no validity to adopting such a value. Ironically, dogmatic reliance on such cutoff values is a key reason why there is resistance in industry to adopting new approaches to measurement. 

A key theoretical question about context effects is whether they reflect actual subjective experience or occur due to the way we use rating scales. One of the main explanations for context effects is Adaptation Level Theory which suggests that we adapt to the changing environment, establishing a reference level against which intensity/pleasure is judged [5]. This approach has many parallels to sensory adaptation (e.g, adapting to be able to see in a dark room) and leaves open the possibility that context alters experience. The Range-Frequency Theory on the other hand is a theory of how we use rating scales [6]. This theory suggests that when faced with the task of rating the intensity or pleasantness of a stimulus range (e.g., products varying along some dimension), we mentally divide the scale into equal intervals, and distribute the ratings over these intervals. Thus, context effects are due to the range of stimulus values and the frequency of each value.   

There are certainly data to support range-frequency theory but clearly I do not need a rating scale to judge whether or not I am happy – to greater or lesser degrees - at finding $20 on the street. That’s an unusual, and positive event. Once this has happened 20 times though, it becomes the norm against which daily events are judged. Tomorrow’s $20 may not even evoke a smile, and $10 in its place may even leave me a little annoyed.

Food consumers, too, experience the same phenomenon. And this may give rise to an interesting paradox in which favourite foods – as reflected in recent, repeat consumption – do not evoke particularly strong positive emotions. A relatively new field in consumer research is the measurement of a wider range of emotions beyond liking. Understanding the impact of context effects in such measures is clearly an important step before their widespread application. Moreover, while sensory acceptability is crucial, there are other reasons why we consume. These include feeling of comfort or security. Perhaps these motivations are exempt from the effects of context, but as yet we don’t know.

[ * The missing bit is ‘Sydney in winter’ and, though making you envious is far from my mind, I need to point out that, yes, it’s often 19 C]

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1. Parducci, A., Happiness, Pleasure and Judgment, 1995, New Jersey: Lawrence Erlbaum.

2. Stevenson, R.J. and J. Prescott, The Effects of Prior Experience with Capsaicin on Ratings of Its Burn. Chem. Senses, 1994. 19(6): p. 651-656.

3. Walter, F. and R.A. Boakes, Long-term range effects in hedonic ratings. Food Qual. Pref., 2009. 20: p. 440-449.

4. Schifferstein, H.N.J., Contextual shifts in hedonic judgments. J. Sens. Stud., 1995. 10: p. 381-392.

5. Helson, H., Adaptation-level as frame of reference for prediction of psychophysical data. Am J Psychol, 1947. 60(1): p. 1-29.

6. Parducci, A., Contextual effects: A range-frequency analysis, in Handbook of Perception Vol. II. Psychophysical Judgment and Measurement, E.C. Carterette and M.P. Friedman, Editors. 1974, Academic Press: New York. p. 127-141.

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.