Alzheimer's disease may be linked to processing problems in part of the brain that triggers daydreams.
Distress may cause changes in the brain
Activity in this area normally reduces or shuts down when somebody has to concentrate on a task at hand.
But scientists at Washington University in St Louis have found evidence from brain scans that this may not be the case in people with Alzheimer's.
The work, published in the Proceedings of the National Academy of Sciences, may lead to new diagnostic tests.
The researchers found there are parts of the brain in young people that are very active when they are day dreaming.
But when they are asked to do something, they seem to have the ability to shut down activity in these areas.
The researchers found that activity in these parts of the brains is not so pronounced as people get older.
But it appears that when people with Alzheimer's disease are asked to concentrate, activity in these areas is actually stimulated, rather than suppressed.
Lead researcher Dr Cindy Lustig said: "What we found in our study is that rather than turning these regions off when asked to concentrate, as young adults do, people with Alzheimer's seem to turn them on.
"This might reflect a 'broken brain' in Alzheimer's, making it hard for people to turn these brain regions on or off appropriately.
"Or, more optimistically, it might be an attempt to compensate for the memory problems that come with Alzheimer's."
It is known that the brain has the capacity to reduce activity in one region so that resources can be shifted to other areas where more challenging mental tasks are currently being processed.
Problems with this ability have been linked other neurological illnesses, such as schizophrenia and amnesia.
There is also mounting evidence to support the existence of a "default network" in the brain - a set of interconnected areas responsible for routine, passive mental processes.
The latest research suggests that Alzheimer's may be linked to problems with switching this "default network" on and off effectively.
Areas of the brain thought to be included in this network include medial frontal, the lateral parietal and the posterior cingulate regions of the cortex.
The researchers found the biggest differences in activity in the posterior cingulate cortex.
Dr Lustig said: "In the long run, this quirk may help us understand what's going wrong with fundamental cognitive processes that underlie mental declines associated with aging and Alzheimer's.
"In the meantime, we're very interested in whether these changes can be used to identify older adults in the earliest stages of Alzheimer's disease so that they can begin treatment as soon as possible."
Rebecca Wood, chief executive of the Alzheimer's Research Trust told BBC News Online the finding was potentially significant as the region of the brain on which the researchers focused was known to play a role in memory.
It was also known to be damaged in people with Alzheimer's.
She said: "We need to do much more work to develop these results, but this exciting research could lead to a greater understanding of Alzheimer's, including improved diagnosis in the early stages and even prevention of this devastating disease."