i do not believe that we only use a portion of our brains, i think these other functioning parts of the brain are just working in a way that we cannot measure just yet. this is just my opinion based on other theories by scientists i have read about and things that exist that cannot be recorded... things we have yet to discover ,, and hypothesis. 

 one example forces and dimensions ,, many of these are recently discovered and some not even yet.  these forces and dimensions are operational even before they were discovered by human. just as our brain operates in such complex ways so does our universe. not only the big universe but the smallest of small in mater and space. so as i see it ,, i think our full brains are fully operational and functioning, but not fully discovered by science.   yet.