For a long time, scientists believed two things about the brain.
The first was the idea that you're born with a set number of neurons -- "brain cells." Then, you lose them all your life.
The second idea was that there are "hardwired" parts of the brain. Various experiments showed that one region was involved in visual processing, another in movement (controlling the arms or legs, for instance). If you went blind, the part of the brain set up to handle information from the eye just sat there, dark and quiet.
Both of these ideas, we now know, were wrong. And that's very good news.