There's no part we don't use, and there's no 'extra' part of the brain left for us to develop its potential. "Nevertheless, the brain does bear similarities to a trained muscle:" Use it or lose it. "Areas that are in frequent use grow larger, and areas that are neglected become appropriated for different purposes.
不過，大腦的確是「用進廢退」，常用的區塊會增大，久不用的地方則會被別的區域挪用。