The function of music has long puzzled scientists, especially those interested in human evolution. It appears to be ancient, and universal: archaeologists have unearthed bone flutes dating to at least 30,000 years ago, though nobody has the least idea what tunes they played, and music is enjoyed in every culture.
Some scientists believe that an appreciation of music is imprinted into the brain, rather like the ability to master language. Yet music, unlike speech, sex or food, seems to offer no survival advantage. So why is it so powerful and so ubiquitous?
There is no final answer, but many other mysteries of musical perception are beginning to yield to the power of modern brain scans.
These have confirmed that, while music occupies areas of the brain that overlap those of speech, they are not identical; and they have shown that there is no single area of the brain labelled music. Rather, musical appreciation calls upon diverse areas of the brain, such as the auditory cortex for tones and limbic system for emotion.
All this becomes plainer when the brains of musicians are examined. Like the muscles of an athlete, specific brain regions are larger or better developed in people who have spent years in music training.
The auditory cortex is 130 per cent larger in musicians, for example, demonstrating that learning music increases the number of brain cells used to process it. In violinists, the brain regions that receive inputs from the fingers of the left hand are significantly larger, because those fingers determine the sound the instrument makes. There is no corresponding increase in the areas devoted to the right hand, which merely holds the bow.
Keyboard players, who need to learn perfect co-ordination of the hands, show exaggerated growth in the anterior corpus callosum, the band of fibres that connect the motor areas responsible for each hand. The earlier children start to learn music, the more pronounced the changes.
There have been plenty of claims of music's ability to improve the human brain. However, the evidence is equivocal, and something almost everybody believes to be true - that listening to Mozart can raise the IQ of infants - almost certainly is not.
But there is persuasive evidence that learning music improves memory and the ability to learn other things. For instance, Canadian scientists compared a group of children aged between four and six who attended a Suzuki music school with a similar group who had no music lessons outside school. They found that the children who learnt music showed changes in brain responses and performed better in memory tests than the others.
Laurel Trainor, who led the research, said: "It suggests that musical training has an effect on how the brain gets wired for general cognitive functioning related to memory and attention."
People respond to music in a peculiar way that illustrates how it makes use of many different areas of the brain. For those affected by synaesthesia (where we experience senses simultaneously that we'd normally experience separately), musical keys trigger the sensation of particular colours. And it seems to work like a drug: among the young, the iPod generation carries its opiate of choice wherever it goes, blocking out the world while sucking in a mind-altering stream of song. There is also a whole branch of medicine called music therapy and a few studies have shown some benefit of it on depression.
But by far the most celebrated scientific study is one that gave rise to the so-called Mozart effect. Published in 1993 by Francine Rauscher and colleagues, it showed that college students who spent 10 minutes listening to Mozart's Sonata in D major for Two Pianos performed significantly better in an IQ test than those who hadn't.
Although the original experiments were done on college students, the message was quickly and inaccurately subverted to apply to infants. In 1998, the State of Georgia even passed a bill to distribute free classical CDs to new mothers. A review of 16 studies that attempted to reproduce the original experiments failed to show any significant change in IQ. Any changes that were detectable, said Christopher Chabris of Harvard, author of the review, were attributable to changes in mood or arousal and were short-lived.
Listening to Mozart never did anybody any harm and may even have given some children a taste for classical music, but claims that it enhanced their IQ are almost certainly misplaced.
So why does music play such a big part in human affairs? Biologists and psychologists have suggested, variously, that music enhances human survival by aiding courtship; promotes social cohesion in groups; and is a form of play.
But for Stephen Pinker of Harvard, an evolutionary psychologist, music is "auditory cheesecake" - a lucky by-product of human history that keeps us amused but has no evolutionary significance. He believes it is simply a side-effect of the evolution of language.
Whatever the explanation, science has made plain what music lovers have always known. It is an activity that can dominate and shape the mind, enrich the spirit, raise morale and give huge and continuing pleasure.
Nigel Hawkes is the former health editor of The Times. This is an edited version of an article that appeared in the December issue of Gramophone magazine
Trainor, L., et al (2006) One Year of Musical Training affects Development of Auditory Cortical-evoked Fields in Young Children, published online in Brain, 129:10, 2593-2608.
Rauscher, F.H., et al (1993), Music and spatial task performance, Nature, 365, 611.
Chabris, C.F. (1999) Prelude or requiem for the `Mozart effect'?, Nature, 400, 6747, 826-7.