It is impossible to daydream and solve a math problem simultaneously. This is probably because complex math problems suppress the daydreaming region of the brain as both the activities are processed by the same area.

Researchers from Stanford University say that the brain shuts off the region of the brain that helps in recalling information or daydreaming while doing tasks that require close attention. Stanford researchers studied the posterior medial cortex, or PMC, a highly inaccessible region of the brain. This region shows an increase in activity when a person is resting. It is located where the brain's two hemispheres meet.

Previous research on this structure is based on functional magnetic resonance (fMRI) studies which, according to the researchers, do not provide complete information about its role in brain function.

"This brain region is famously well-connected with many other regions that are important for higher cognitive functions. But it's very hard to reach. It's so deep in the brain that the most commonly used electrophysiological methods can't access it," said Josef Parvizi, MD, PhD, director of Stanford's Human Intracranial Cognitive Electrophysiology Program.

In the present study, researchers used a different and highly specific technique. The study involved epilepsy patients who weren't responding to any treatment. Doctors were trying to pinpoint the exact location of the seizures in these patients by using a procedure where a small piece of skull is removed and an electrode in placed in the brain. The electrode is placed at the site where the doctors feel the seizures might be originating.

The placement of electrode is followed by a period of 5 to 7 days when the patient has to wait for a seizure to occur. When a seizure occurs, doctors will know its location and then will operate on that particular part of the brain and remove some tissue without disturbing the other parts of the brain.

Researchers found eight such patients whose seizures were believed to be occurring from somewhere near the brain's midline. Researchers sought permission from these patients to carry out a few simple tests.

These patients were asked to answer few easy questions; some related to recall tasks like "I drank coffee yesterday", and others were based on mathematical ability. Researchers found that activity at the PMC increased when people recalled specific information about the past. The activity was suppressed when people performed arithmetical tasks

"The more a circuit is activated during autobiographical recall, the more it is suppressed during math. It's essentially impossible to do both at once," said Parvizi.

Previous research has suggested that the PMC might be associated with conscious states because the activity of this region is low in people who are under anesthesia or in coma and the activity in the PMC activity when the person reaches conscious state.

Researchers in the present study were able to establish that the PMC is not the seat of self-consciousness but is important in recall about self and imagination, a press release from Stanford University said.

The study was published in the Proceedings of the National Academy of Science.