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 18th 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.
________________________________________________________________________
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.
No comments:
Post a Comment