A study by researchers at the Montreal Neurological Institute gives new meaning to the expression "thick in the head."

The researchers discovered those who are tone-deaf possess more grey matter in some areas of their brain than those who are able to enjoy music.

"Specifically, we found that tone-deaf individuals had a thicker cortex in particular brain regions known to be involved in auditory and musical processing," said Krista Hyde, a research fellow at the MNI and the lead author of the study.

Congenital amusia, or tone-deafness, affects about four per cent of the general population and is a life-long condition. Those who are tone-deaf cannot comprehend melody and harmony, but they can appreciate rhythm to some extent, said professor Isabelle Peretz, one of the co-authors of the study.

Using magnetic resonance imaging, the researchers scanned the brains of those considered "musically intact" and compared them with the brain images of the tone-deaf.

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Montreal researchers have been able to pinpoint some key differences in our brains when it comes to having a knack for music.

The researchers ran melodic, rhythmic, metric and memory recognition tests on the tone-deaf. Those who scored lower on the tests exhibited a thicker cortex in musically relevant regions of the brain.

"This suggests an abnormal migration of nerve cells," Peretz explained. "It seems that these cells have proliferated in certain areas rather than migrating to the right areas."

Another way of looking at it is that the brain develops by "pruning" certain nerve cells. This process allows frequently used nerve cells and connections to strengthen, while seldom-used neuronal pathways are eliminated. But in the tone-deaf, this type of cell pruning is not done.