Unlocking the secrets of taste

As thousands of neuroscientists gather in Milan for the FENS forum, psychologist John Prescott recounts the scientific history of the least understood among our senses

Brain-food-listFrom 5 to 9 July 2014 Milan, the city where Expo2015 will take place, is home to thousands of neuroscientists gathering for the 9th FENS Forum, the most important scientific meeting in Europe for brain studies. The conference also includes some cool side events on food and taste. RaiExpo will be there, so stay tuned. 

Still, compared to the other senses (vision, hearing, touch and even smell) taste is surprisingly under-represented in the FENS scientific programme. Thing is, taste is possibly the least understood sense for psychologists and neuroscientists. But things are changing, says John Prescott, a psychologist who specialises in taste perception and author of the book “Taste Matters“. 

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When I was a psychology undergraduate in the 1970s, neuroscience was mostly what brain anatomists and physiologists did. We did study brain structure and neurochemistry, but there is little comparison to neuroscience today – a multi-discipline area seeking to answer fundamental questions regarding human behavior, thought and action. Nowadays, understanding the senses is part of understanding the brain. However, in my initial training, we had two separate but very serious and technical courses: Perception and Learning & Motivation. Perception was, of course, about the way our senses worked – or at least certain aspects of their functions. It was generally recognized that psychologists had provides some very worthwhile contributions to our understanding of visual phenomena and, to a lesser extent, to the processing of sound. Touch was mostly left to the physiologists, but we did have one professor who worked in the strange world of the sense of balance. I don’t think other professors talked much to him. So that was Perception.

rat-foodIn the Learning and Motivation classes, we spent an awful lot of time playing with rats and in particular rewarding them with food that they enjoyed. At other times, we made them sick so that they would avoid the taste of the food. Out of this type of experiment, decades of psychological theory about the rules of learning and why some activities were preferred over others had been built. It never occurred to us – or to me at least – that we were somehow also studying the role of two vital senses, smell and taste, most often experienced together as flavours. I did not know then that these senses, although absolutely vital to survival to most animals, were considered to be relatively unimportant areas of interest for anyone wishing to understand human behavior or cognitive function. In what seems now to be a 19th century hangover, these were treated as base senses that played little role in “higher” functions like language or intellect.

Even in the 1970s, visual and hearing sciences held conferences in large halls that held hundreds if not thousands of delegates. At that time, the number of scientists worldwide and of any discipline working on taste or smell could have met in any local bar without it seeming too crowded. But fast-forward a few decades and there are now regular meetings in which new discoveries in the chemical senses (taste, smell) are presented to audiences of many hundreds of physiologists, psychologists, molecular and neuro-biologists, geneticists, and even food scientists.

There are three obvious reasons for this growth.

1) This first of these was a recognition that the chemical senses share some of the same molecular receptor processes as vision, which meant that technologies existed to study such basic processes in taste.

2) Secondly, and in contrast, the dearth of knowledge about chemical senses, and especially about our sense of smell, was embarrassing if we wished to be comprehensive in our understanding of the senses. We had no idea, for example, about the basic units of smell perception. In vision, we had light wavelengths; in hearing, sound frequencies; in touch, vibrations. Even in taste, there was obviously a way in which a limited number of qualities (sweet, sour, salt, bitter) were produced by certain chemicals – we just had to find how sweet taste receptors attached to these sweet molecules, and so on [Note that this is merely a contrast with smell – the taste system itself turns out to be very complex and remains only moderately well understood]. But smell? Were there basic smells, as some scientists suggested? If so, how do we go about finding what they are? What if there aren’t basic qualities – this means that the sense of smell is almost unimaginably complex. And how do we understand the fact that the odour of substances such as chocolate or coffee consists of hundreds of different chemical compounds and yet are interpreted by the brain as smelling just like one thing.

Axel_BuckWhile some of these issues are yet to be solved, there has been remarkable progress in recent years. If anything, the existence of such basic, yet complex, questions, addressed much earlier in vision and hearing science, acted as an incentive for scientists to “make their mark”. A notable success was the awarding of the 2004 Nobel Prize for Physiology & Medicine to Linda Buck and Richard Axel for their work on identifying a family of genes that encoded odour receptors, and interpreting how these receptors functioned to bind to odour molecules. But far from solving the problem of odour complexity, their work emphasized it. It appears that there are hundreds of such genes in humans, each encoding a different receptor type. If we consider that a smell could result from the activation of many receptors, some of which might also be activated by quite different smells, then the sense turns out to be very complex indeed.

3) Another reason for an increased emphasis on taste these days is that there is now a large health problem facing all developed countries, namely obesity, and it is clear that taste is a hugely relevant factor. We eat those foods that taste good, and unfortunately very many of them are high in sugar and fat.

But to understand this problem, we need to move beyond just taste perception and into the slightly less comfortable realm of taste hedonics – in other words, the pleasures associated with taste. Strangely enough, the intrinsic hedonic qualities of tastes (e.g., sweet tastes are always liked at birth in almost all mammals, including man) are both a reason that taste was ignored by neuroscience in the past and a reason that this sense is now hugely relevant. Emotions of all kinds have seemed a sort of difficult overlay on top of the more important issue of how senses function, partly because many scientists have felt that such “subjective” experiences were outside the realm of neuroscience. However, many neuroscientists now recognize that emotions are an integral part of many other brain functions. Thus, we now know that attempting to understanding how decisions are made or attention is allocated to stimuli in the world is impossible without taking into account the emotional aspects of these cognitive processes. Even more so, our responses to tastes and odours are essentially about emotions and it is those emotions that underlie our motivations to consume or not consume foods, with all the health consequences that that entails.

If I went back to my university classes now, whether Perception, Learning & Motivation, or Physiological Psychology, I would be asking my professors firstly, why these were separate subjects at all. Aren’t the senses our way of providing information about the world, and isn’t this information essential to learning? Moreover, aren’t our motivations about understanding our behavior that arises from both learning and genetics? And perhaps more to the point, from an evolutionary perspective, what behaviour has been more important than the search for food that finds acceptance with our sense of taste?

John_Prescott

John Prescott

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