You know the feeling - the shiver that shoots across your skin when you hear that song. But why do some of us feel the tingle of Barber's strings, while others are tickled by Bjork's epic voice? And why do a small proportion of us feel no shivers at all?

The glory of the iPod is lost on those who suffer from a rare condition known as amusia - a complete inability to comprehend or take pleasure from music. Where once these people would have been dismissed as "tone deaf", there is a growing recognition that amusia is a neurological condition, inherited through families.

Professor Tim Griffiths listens to the experiences of amusic patients every month in his auditory clinic in Newcastle. "Some are just indifferent to music, but for others it really sounds quite unpleasant and abrasive," he says.

Griffiths is the UK's leading expert on amusia. He was quick to recognise that those who "just don't get it" hold the key. By peering into their minds, we can begin to answer deeper mysteries - why do humans enjoy music, and what use is it to us?

Griffiths is exploring the relationships between musical processing and our mechanisms of speech, reading and learning. "It's not just about musical disorders: there are links with dyslexia, dementia and autism," he says. "These patients may give us the tools to understand the brain."

Studies have shown that the brain has distinct systems for processing pitch, melody and rhythm. A further brain region creates the emotional effect of music, and this is the area in which Griffiths is most interested. "A patient of mine was very fond of Rachmaninov, until he suffered brain damage. He could still recognise the music, but no longer felt so emotional. This suggests our brain has a separate system producing these emotions."

Interestingly, he found, the centre that produces the "shiver" also mediates our responses to cocaine and orgasms. The headiest combination is literally sex, drugs and rock'n'roll.

If all this makes you worry that you might be more than just a bad singer, there's a quick way to find out - take the online test developed by Professor Isabelle Peretz, of Montreal University, a world expert on amusia.It was Peretz who, in 2002, published the first scientific portrait of an amusia sufferer - Monica, a Canadian in her 40s who had lived her whole life without even the most basic sense of melody and tune.

Take Peretz's musical "spot the difference" test and you are likely to find you can distinguish the small variations in pitch, no matter how awful you are at karaoke. But for Monica and others like her, even leaps of an octave - for example the first two notes of Somewhere Over the Rainbow - are barely noticeable.

Peretz is trying to understand how music might be therapeutic - as an alternative to drug treatments for depression, for example. "It's not a case of reprogramming the brain - music can't do that," she explains. "It's more about changing the balance in there."