As a high school English teacher for 35 years, I saw everyday how avid readers were fundamentally different from the kids that read only what they had to, if that. Avid readers read better, wrote better, concentrated better, and did better across the board in all of their classes.
Do you think avid-reading behavior also causes brain changes. That would be my guess.
It was only when I started letting my high school students choose most of their reading, that their love of reading, and their reading scores, soared. It was amazing to me to see how kids could fairly easily be brought to a love of reading simply by allowing them to choose most of the books they read.