Tongue, teeth and taste

Posted on | December 14, 2005

There has been some talk on the internet of reported interviews with Professor Patrick Mac Leod and in particular two things which seemed astonishing: that the old and much quoted tongue map hypothesis (where different parts of the tongue corresponded to areas where the key elements of taste could be perceived: sweetness, bitterness, acidity etc) was not proved and that nearly half of the bulk information travelling to the brain regarding taste was transmitted through the teeth mechanoreceptors.

I spoke with Prof. Mac Leod at his office in Paris and he has helped me clarify some of these points.

Firstly, the hypothesis for tongue mapping is quite old and was written up from the notes of only one person. No scientific proof of this was ever put forward which didn’t stop it being widely adopted by almost every book on taste I have ever read. In fact, Professor Mac Leod  maintains that this will be different from person to person. It might be worth mentioning here that the work of Professor Mac Leod is based on brain imaging and neuron reception. He states that the tongue is important but  much less so than olfaction.

Prof Mac Leod suggests that using his knowledge of the nerve networks in the mouth, dental work on the teeth tends to break up this network displacing sensory perceptions around the mouth (and tongue). In other words most of us will perceive things differently.

The only part of the tongue sensory message which Mac Leod believes is fairly constant in all of us is the sensation of bitterness at the back of the tongue and which is about twice as strong as any anterior tongue sensations.

Teeth are generally recognised as being highly sensitive with densely innervated periodontal ligaments covering all parts of the roots. These are not the same nerves which provide the sensation of pain but rather send out messages on the intensity and direction of forces applied to each tooth. This therefore relates to texture and should influence wine tasting only marginally insofar as texture may be more or less of a component in wine.

When tasting something there are three elements which will go to make the whole picture:

1. olfaction which is three to four times as important as any other element.

2. the taste (from the taste buds spread around one’s mouth) and

3. the somatic perceptions of the mouth which is itself split into three parts:

• a. chemical influences such as astringency, pungency or the burning sensation caused by alcohol.
• b. mechanical - this is where the textural qualities and the influence of the teeth comes into play and
• c. the thermal properties of the wine

However, and this is really key, the tastes produced by the mouth are highly multi-sensory and it is impossible to be quantitative about their relationships thus denying us the opportunity of creating a multi-sensory map displaying percentage sensitivities.

Through his brain imagery work as well as others’ Professor Mac Leod has established that the messages which are transmitted when, say, a glass of wine is drunk, are conveyed to the  neurons of the conscious brain and that each single neuron therein receives information elements which come from all of the above. In other words, it is not possible to break down the elements making up taste as they are not transmitted to the brain separately but together.

Thus, to sum up the tongue is much less important than previously thought because of dentistry influences and because the tongue map is purely anecdotal. And, the teeth are vitally important but it is impossible to say just what their influence might be. But certainly less than olfactory stimuli. In any case, all these elements of taste are submerged when one considers sight. It has been clearly demonstrated that the colour of a wine has the greatest influence on our tasting of a glass of wine, or at least until more work is been done on the impact of other prejudices we cannot know if other elements will also affect the way we taste to a greater or lesser degree than colour.

Fabian Cobb

I asked Prof. Mac Leod to write a brief note about himself and send us a picture to put up on our site …

from Patrick Mac Leod:

 

My contribution with my co-workers was from 1995 to 1999 and can be found only in specialized journals e.g. Chemical Senses

I was born in Vendôme, France in 1932. Graduated in Paris Sorbonne in 1956. Full time basic research in Chemoreception Research (Olfaction and Taste) since that time… Elected « Professeur de l’Ecole Pratique des Hautes Etudes » and founded the « Laboratoire de Neurobiologie Sensorielle » in 1975. Headed this laboratory up to 2001. Presently retired, still doing some research in my laboratory…

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