Posteromedial Cortex: Pathway to the Self?

PNAS Neural Connections of PMC

One of the interesting aspects of human consciousness is the sense of the "autobiographical self," which provides us with a sense of personal identity and continuity. Recent research at Stanford U. School of Medicine adds new pieces to the human consciousness puzzle, which cognitive scientists are attempting to assemble. They discovered that the posteromedial cortex was extremely active when recalling events in the person's life, but that this activity was shut down immediately when the patient was asked to do a simple arithmetic problem.

The posteromedial cortex is tucked so deeply into the brain that it is often difficult to examine its ongoing function, using traditional imaging tools such s fMRI. In the Stanford study, researchers took advantage of a previously scheduled neurosurgical procedure being done in a group of epileptic patients, using intracranial electrode monitoring of the PMC.

In 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.

...The researchers took advantage of a procedure performed on patients who were being evaluated for brain surgery at the Stanford Epilepsy Monitoring Unit, associated with Stanford University Medical Center. These patients were unresponsive to drug therapy and, as a result, suffered continuing seizures. The procedure involves temporarily removing small sections of a patient’s skull, placing a thin plastic film containing electrodes onto the surface of the brain near the suspected point of origin of that patient’s seizure (the location is unique to each patient), and then monitoring electrical activity in that region for five to seven days — all of it spent in a hospital bed. Once the epilepsy team identifies the point of origin of any seizures that occurred during that time, surgeons can precisely excise a small piece of tissue at that position, effectively breaking the vicious cycle of brain-wave amplification that is a seizure.

...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.)

...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.

Foster, Dastjerdi and Parvizi also found that the PMC circuitry activated by a recall task took close to a half-second to fire up, ruling out the possibility that this circuitry’s true role was in reading or making sense of the sentence on the screen. (These two activities are typically completed within the first one-fifth of a second or so.) Once activated, these circuits remained active for a full second.

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.” _Parvizi _via SD

As the researchers suggest, the PMC is not likely to be "the centre of self-consciousness." Rather, the PMC is one part of the human brain's mechanism of self-consciousness -- and a crucial part at that.

It is usually a mistake to claim that a specific part of the brain is "the centre" for a specific cognitive or behavioural function. Rather, different brain modules are integrally tied to various behavioural and cognitive functions, and work in concert to perform cognitive and behavioural functions and actions.

PNAS Neural Connections of PMC in Macaque

PLoS One: Functional Connections of the Human PMC

More: CUNY researchers have discovered that androgen receptors in the visual cortex appear to affect how males see the world through their eyes, as opposed to the way that females see the world. Since androgen receptors are scattered throughout the human cerebral cortex, it is likely that male brains not only see the world differently (statistically), but also perceive the world differently in all of their senses, and feel driven to react to what they perceive in a different manner (statistically) than do females.