I would assume that it is simply the amount of thought and focus that is required to memorize thousands of streets that would cause these increases. I wish there was a control or a group of people that were studying something besides memorization to see if there was truly a difference. The time frame here is 3 to 4 years.
Here you go: "Musical training-induced functional reorganization of the adult brain: functional magnetic resonance imaging and transcranial magnetic stimulation study on amateur string players."[1]
