Here's an article
I found the other day that is similar. It's not about piano playing per se but the parallels are obvious.
Here is an excerpt that I found particularly relevant:
A few years ago, scientist Richard Haier wanted to see if you could increase your cognitive ability by intensely training on novel mental activities for a period of several weeks. They used the video game Tetris as the novel activity, and used people who had never played the game before as subjects (I know—can you believe they exist?!). What they found, was that after training for several weeks on the game Tetris, the subjects experienced an increase in cortical thickness, as well as an increase in cortical activity, as evidenced by the increase in how much glucose was used in that area of the brain. Basically, the brain used more energy during those training times, and bulked up in thickness—which means more neural connections, or new learned expertise—after this intense training. And they became experts at Tetris. Cool, right?
Here’s the thing: After that initial explosion of cognitive growth, they noticed a decline in both cortical thickness, as well as the amount of glucose used during that task
. However, they remained just as good at Tetris; their skill did not decrease. The brain scans showed less brain activity during the game-playing, instead of more, as in the previous days. Why the drop? Their brains got more efficient. Once their brain figured out how to play Tetris, and got really good at it, it got lazy. It didn’t need to work as hard in order to play the game well, so the cognitive energy and the glucose went somewhere else instead.
Efficiency is not your friend when it comes to cognitive growth. In order to keep your brain making new connections and keeping them active, you need to keep moving on to another challenging activity as soon as you reach the point of mastery in the one you are engaging in. You want to be in a constant state of slight discomfort, struggling to barely achieve whatever it is you are trying to do.