The phrase "functioning brain" refers to a brain that works properly and performs all its necessary tasks, such as thinking, remembering, and making decisions. Full definition
In a normally functioning brain, dopamine is released from the neurons in response to pleasurable or life - sustaining activities, such as eating or sex. (sciencedaily.com)
You're not just a bunch of cells; you're an organism with a higher functioning brain. (religion.blogs.cnn.com)
So one really important function our brain provides us is the ability to not act on all the possibilities that it's generating. (discovermagazine.com)