News in Science

Brain 'switches between maths and memory'

Brain switching Brain cells we use to mull over our past must switch off when we do sums, say researchers, who have been spying on a previously inaccessible part of the brain.

The findings could help in the understanding of autism, ADHD and depression.

The research, reported in the Proceedings of the National Academy of Sciences, boosts our understanding of how the brain switches from being internally focused (ruminating) to focusing on a task in the outside world.

The Stanford University team, led by Professor Josef Parvizi, studied the posteromedial cortex (PMC) - a very inaccessible region of the cortex buried deep in the crack between the brain's two hemispheres.

"It's an area that's extremely active when the brain is resting", says Parvizi, who says much of the research was done by an Australian post-doc, Dr Brett Foster.

When we are not actively concentrating on something, the brain is still thinking - it ruminates or 'free-wheels', mulling over past actions and imagining the future. "The PMC is the hub of all these rumination processes," says Parvizi.

A procedure used on eight patients with severe epilepsy gave Parvizi's group a valuable opportunity to "go into the hub and spy on the cells".

Anti-epileptic drugs had failed to help these patients, so they had electrodes implanted widely over their brains in an attempt to find the seizure source - with the aim of then excising the problem region.

Some of these electrodes had been implanted in the PMC and, while the patients lay bored in bed waiting for seizures to occur, they allowed Parvizi's group to eavesdrop on their brain cells.

Memory not maths

The patients were asked to evaluate simple statements about their recent experience such as "I had a cup of coffee this morning" by pressing true or false buttons.

Dealing with these kinds of statements about their own personal history made the PMC cells extremely active - showing it is involved in processing autobiographical memory.

On the other hand, assessing whether a sum like "23 + 8 = 34" was true or false strongly reduced PMC cell activity.

"What we discovered was fascinating," says Parvizi. "The same group of neurons that are very very active during a rumination process - exactly the same neurons - have to be shut down in order to solve a maths equation."

The notion that the PMC is important in rumination and needs to be switched off to operate on tasks in the outside world is not new, says Parvizi.

But his group are the first to make detailed electrical recordings from small groups of PMC cells, showing the switch in action. "What we are offering is precision," he says.

'Gold standard' recording

Professor David Liley of Swinburne University of Technology, who was not involved in the study has welcomed the research.

"What's pretty unique here is that they've managed to record from a part of the brain that's normally completely surgically inaccessible," says Liley.

Electrophysiological recording like this is "the gold standard" of brain recording, he adds.

A spin-off has been to rule out the PMC as the site of consciousness and a feeling of 'self'. Some philosophers had suggested this might be the case, says Parvizi, but other questions the group asked their subjects made this unlikely.

"The more self-related and 'core-of-yourself' the question being judged, the less the PMC was involved," says Liley.

Parvizi says the work could help with understanding autism and ADHD where the ability of the brain to switch between internal thoughts and the outside world may be impaired. It may also shed light on depression, where patients ruminate too much.