What's Happening in the Brain
The control of attention and concentration is a complicated process. In part, it is linked to the process by which we stay awake. It depends on brain pathways that go from deep parts of the brain in the brainstem and activate the brain's two hemispheres (the right and left sides of the brain) and vice versa. This is a continual activity, which even goes on to some extent while we are asleep. However, the activation back and forth from the brainstem to the two hemispheres can become disrupted. Each of us has had the experience of having trouble concentrating, when we are sleepy, or worried, or even when we have a bad 'flu'.
In Alzheimer's disease , the activation back and forth from the brainstem to the two hemispheres is more prone to be disrupted. Because of this, people with Alzheimer's disease feel it is harder to filter out disruptions in the environment. When they become ill - say with a cold, or from too much medication - the problem can become much worse. When attention is very disrupted, the problem can be severe and is called delirium.