As soon as you open your eyes, the regions of activity in the default mode network begin to decrease activity. Well, that’s exactly what happens to alpha activity on the EEG. We say alpha attenuates, which means decreases amplitude. At the same time this network turns off. Should you begin working on a task, say solve a math problem, or do whatever you do at work focusing your vision or attention at work, other relevant brain areas will become more active as are easily seen on fMRI or PET and there are changes on EEG as well.
With you focused the Default Mode Network switches off and alternative regions of brain switch on. You might refer to this as flickering patterns of activity in the brain. This shows the brain to be, as everyone is well-aware, a multi-use organ and whatever use you are putting the brain to will cause patches of cortex to flicker off and on. These networks of activity can be seen on PET/fMRI.
You and I switch, or flicker from unpurposed awareness to purposeful activity in an instant flipping back and forth many times a day. Depending on your personality, you may be more comfortable thinking of nothing or intensely working.
Below is a detailed rendering of the midline of the brain that comes from Wikipedia. Most of the structures determining mental states reside in the midline of the brain.
