Prefrontal Cortex Underlies Slips of the Tongue

by Tom Valeo

September, 2009

“Don’t think of a white bear.”

To obey that command you must periodically remind yourself not to think about a white bear, thereby thinking about it.

That paradox represents an ironic process that bedevils the human mind, argues Daniel Wegner, a Harvard University psychology professor and author of White Bears and Other Unwanted Thoughts. In the July 3 Science, Wegner writes that in order to avoid committing an error—mispronouncing a name, for example, or avoiding inadvertent sexual innuendo—we must keep the dreaded error in mind, thereby rehearsing it. The danger of making the mistake therefore increases, especially if we’re distracted, anxious or otherwise mentally burdened.

Wegner wrote his first paper on thought suppression in 1987 and since then has found many ways in which efforts to inhibit unwanted thoughts and actions may backfire. Memories we try to forget—of an unhappy love affair, for example—tend to be more easily remembered. Suppressing thoughts of sex tends to increase arousal. People told to keep a secret become more likely to reveal it. And golfers who concentrate on not missing an easy putt often develop what they call “the yips,” and miss it.

What makes suppression so difficult?

Blame the prefrontal cortex, located at the very front of the brain, says Jordan Grafman, chief of the Cognitive Neuroscience Section at the National Institute of Neurological Disorders and Stroke.

That part of the frontal lobes enables us to develop plans and carry them out—a process that often involves suppressing distractions and inappropriate impulses, Grafman says. Disrupting this energy-intensive process or placing greater demands on this brain area will make suppression more difficult.

Imaging studies by Grafman and others, such as Steven W. Anderson and his colleagues at the University of Iowa, provide evidence that dysfunction in the prefrontal cortex produces an array of problems linked to poor impulse control, such as aggression, lack of planning before acting and even a tendency to hoard worthless objects.

“If you have prefrontal cortex dysfunction, you may demonstrate inappropriate behaviors,” Grafman says. “Patients with frontal lobe lesions often violate social norms. They’re often not aware of violating them, but normal people are aware of violating them in just the way Wegner describes ironic errors. If you interfere with this executive control, often these errors occur more frequently.”

Simple fatigue or stress may be enough to disrupt the prefrontal cortex, Grafman says, making the suppression of inappropriate thoughts and impulses more difficult.

Matthew T. Gailliot, a senior fellow at the Center for Human Science in Chapel Hill, N.C., found that people who watched a video while trying to ignore words that flashed on the screen—an act of self-control that requires significant effort from the prefrontal cortex—performed less well at subsequent self-control tasks than subjects who drank a glucose drink after performing the first task, thereby replenishing the fuel the brain needs to function.

“Suppressing thoughts can deplete glucose, which could later reduce the ability to suppress,” Gailliot says.

In the same way, thoughts of death, which humans routinely suppress, seem to increase when glucose is low or people are hungry, Gailliot says. And one study he conducted showed that participants primed to think about death gave up sooner on a task that required mental persistence.

“There are also links between interracial interactions, depleted glucose, and impaired self-control, which might affect the suppression of stereotypes and prejudice,” Gailliot says.

While suppressing unwanted thoughts appears to require significant effort from the frontal lobes, relaxing the suppression seems to relieve stress significantly, says James Pennebaker, a psychologist at the University of Texas. In his book Opening Up, he reported that student volunteers who “confessed” their secrets through writing experienced improvements in blood pressure, immune function and other measures of health.

Wegner agrees that disclosing problems rather than keeping them secret may help. Also, the small percentage of people who are susceptible to hypnosis appear to excel at suppressing unwanted thoughts and impulses.

For the rest of us, however, the best hope involves keeping our prefrontal cortex strong and vigorous by not overburdening that part of the brain with distractions, multitasking or other demands, Wegner says. It’s good to have lots of spare mental capacity, he adds, because stress and exhaustion will only compromise the ability of the brain to exercise conscious control over “taboo” thoughts and impulses.