Posted on 
Sep 15, 2005 10:03 pm doing anything repeatedly will cause the associated brain region to grow larger. if youre a musician who can play by ear very well, whatever part of cortex that handles that is going to have a larger dedicated area than the norm.
it has not so much to do with the region having extra neurons, especially if you start late (though neurogenesis has recently been shown to be ongoing in deeper parts of the brain, throughout life, and within the past weeks studies have emerged that we even somehow missed new neural growth in cortical regions, which apparently occurs throughout life.) rather, cortical areas can expand easily in terms of dedicated area, at the expense of other areas.
some clear evidence for this: when a stroke victim loses speech ability in the speech-centered parts of the cortex, or a part of the motor cortex, adjacent areas will pick up the slack. (this has been seen on brain scans). and those people who have learned to manipulate cursors by thinking about it, thanks to new brain implants? parts of thier cortex become dedicated to that task. in fact, if you somehow rigged up a brain implant that was able to control a literal third robotic hand 9they are working on this) in a person that has two already-working hands, the brain would slowly learn to dedicate a part of the cortex to that hand, in effect mapping the imaginary hand into its cortical 'body image.'
cortical tissue can perform any processing task given to it. if it needs more space for its tasks, and if space is available, those regions learn to do the task.
so, yeah. if you have a good ear, then definitely parts of your cortex are more specialized than is normal. this is true of any sport or activity, though. i havent read much about the corpus callosum and music, but i wouldnt doubt it.