Memory vs. Math: Same brain areas show inverse responses to recall and arithmetic

Scientists have historically relied on neuroimaging – but not electrophysiological – data when studying the human default mode network (DMN), a group of brain regions with lower activity during externally-directed tasks and higher activity if tasks require internal focus. Recently, however, researchers at Stanford University School of Medicine recorded electrical activity directly from a core DMN component known as the posteromedial cortex (PMC) during both internally- and externally-directed waking states – specifically, autobiographical memory and arithmetic calculation, respectively. The data they recorded showed an inverse relationship – namely, the degree activation during memory retrieval predicted the degree of suppression during arithmetic calculation – which they say provides important anatomical and temporal details about DMN function at the neural population level.

You already know it’s hard to balance your checkbook while simultaneously reflecting on your past. Now, investigators at the Stanford University School of Medicine — having done the equivalent of wire-tapping a hard-to-reach region of the brain — can tell us how this impasse arises.

The researchers showed that groups of nerve cells in a structure called the posterior medial cortex, or PMC, are strongly activated during a recall task such as trying to remember whether you had coffee yesterday, but just as strongly suppressed when you’re engaged in solving a math problem.

The PMC, situated roughly where the brain’s two hemispheres meet, is of great interest to neuroscientists because of its central role in introspective activities.

“This brain region is famously well-connected with many other regions that are important for higher cognitive functions,” said Josef Parvizi, MD, PhD, associate professor of neurology and neurological sciences and director of Stanford’s Human Intracranial Cognitive Electrophysiology Program. “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.”

Ιn a study published online Sept. 3 in Proceedings of the National Academy of Sciences, Parvizi and his Stanford colleagues found a way to directly and sensitively record the output from this ordinarily anatomically inaccessible site in human subjects. By doing so, the researchers learned that particular clusters of nerve cells in the PMC that are most active when you are recalling details of your own past are strongly suppressed when you are performing mathematical calculations.