For decades "they" have been trying to sell us that we only use 10% of our brain. They were saying this before we could take pictures of the brain working, with MRI. So even if they were right we wouldn't know. Now we can see what parts of the brain are used. Turns out, "they" were wrong. We use 100%. This is important for stroke survivors because rewiring the brain after stroke asks more from your brain. And after stroke there is less brain to work with.
Here's a link to an article I wrote for StrokeSmart magazine about the 10% brain myth.