Mathematics or memory? Stanford study charts collision course in brain

Posted By News On September 3, 2012 - 7:30pm


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."



Nonetheless, fMRI imaging has shown that the PMC is quite active in introspective processes such as autobiographical memory processing ("I ate breakfast this morning") or daydreaming, and less so in external sensory processing ("How far away is that pedestrian?"). "Whenever you pay attention to the outside world, its activity decreases," said Parvizi.

To learn what specific parts of this region are doing during, say, recall versus arithmetic requires more-individualized anatomical resolution than an fMRI provides. Otherwise, Parvizi said, "if some nerve-cell populations become less active and others more active, it all washes out, and you see no net change." So you miss what's really going on.



Implanting these electrode packets doesn't mean piercing the brain or individual cells within it. "Each electrode picks up activity from about a half-million nerve cells," Parvizi said. "It's more like dotting the ceiling of a big room, filled with a lot of people talking, with multiple microphones. We're listening to the buzz in the room, not individual conversations. Each microphone picks up the buzz from a different bunch of partiers. Some groups are more excited and talking more loudly than others."

The experimenters found eight patients whose seizures were believed to be originating somewhere near the brain's midline and who, therefore, had had electrode packets placed in the crevasse dividing the hemispheres. (The brain's two hemispheres are spaced far enough apart to slip an electrode packet between them without incurring damage.)

The researchers got permission from these eight patients to bring in laptop computers and put the volunteers through a battery of simple tasks requiring modest intellectual effort. "It can be boring to lie in bed waiting seven days for a seizure to come," said Foster. "Our studies helped them pass the time." The sessions lasted about an hour.

On the laptop would appear a series of true/false statements falling into one of four categories. Three categories were self-referential, albeit with varying degrees of specificity. Most specific was so-called "autobiographical episodic memory," an example of which might be: "I drank coffee yesterday." The next category of statements was more generic: "I eat a lot of fruit." The most abstract category, "self-judgment," comprised sentences along the lines of: "I am honest."

A fourth category differed from the first three in that it consisted of arithmetical equations such as: 67 + 6 = 75. Evaluating such a statement's truth required no introspection but, instead, an outward, more sensory orientation.

For each item, patients were instructed to press "1" if a statement was true, "2" if it was false.

Significant portions of the PMC that were "tapped" by electrodes became activated during self-episodic memory processing, confirming the PMC's strong role in recall of one's past experiences. Interestingly, true/false statements involving less specifically narrative recall — such as, "I eat a lot of fruit" — induced relatively little activity. "Self-judgment" statements — such as, "I am attractive" — elicited none at all. Moreover, whether a volunteer judged a statement to be true or false made no difference with respect to the intensity, location or duration of electrical activity in activated PMC circuits.

This suggests, both Parvizi and Foster said, that the PMC is not the brain's "center of self-consciousness" as some have proposed, but is more specifically engaged in constructing autobiographical narrative scenes, as occurs in recall or imagination.


Yet all the electrodes that lit up during the self-episodic condition were conspicuously deactivated during arithmetic calculation. In fact, the circuits being monitored by these electrodes were not merely passively silent, but actively suppressed, said Parvizi. "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."

Post new comment

  • Lines and paragraphs break automatically.

More information about formatting options
