Brain networks work together to help us decide what is most important to pay attention to. We generally focus first on what is salient – that is, what is of immediate importance required, or urgent. Even when we study intently, we shift our focus of attention if we see or smell smoke. The possibility of FIRE becomes most salient. People with ADHD often have difficulty determining what is salient, because their focus tend to shift – often randomly.
Published research out of Stanford University, sponsored by NIH,  advances our knowledge of salience networks and the use of fMRI.
http://med.stanford.edu/news/all-news/2015/12/attention-networks-different-in-kids-with-adhd.html