The finding may help scientists better understand the underlying mechanisms of attention deficit disorder (ADD), addiction and other personality disorders that stem from an inability to control impulses.
"Our study is only a first step to investigate a cognitive mechanism that might be crucial for impulsive behavior," says lead study author Marcel Brass, an experimental psychologist at the Ghent University in Belgium. "The question is whether we can gain a better understanding of the brain areas involved in personality traits such as impulsivity. One necessary step in this direction would be to develop experimental paradigms that are more closely related to everyday situations."
In the current study, Brass and co-author Patrick Haggard, a professor of cognitive neuroscience and psychology at University College London, asked 15 subjects to push a button on a keyboard while undergoing functional magnetic resonance imaging (fMRI) to monitor brain activity; participants were instructed to occasionally skip the action. Subjects would indicate to researchers when they were planning the movement, so the researchers could see what part of the cerebral cortex—the outermost region of the brain, which plays a key role in functions from movement to attention to learning—responded if a button-push was or was not executed.
According to the fMRI data, plans and intentions for movement coincide with activity in the supplementary motor area, a region of the brain near the top of the head. If the action takes place, the nearby premotor and primary motor cortexes take over. In the cases where the decision to push the button is reversed, the fMRIs indicate that the inhibitory activity takes place in the dorsal fronto-medial cortex located just above the eyes.
"The location of the activation makes sense from a functional neuroanatomical perspective," notes Brass. "The area is located anterior to [(in front of)] brain regions that are involved in forming the intention to act." This way, it may operate to feed a signal back to the motor areas, telling them no movement will be taking place.
Unfortunately, he says, the fMRIs failed to reveal the inhibitory circuitry involved; to try to get to the bottom of that, he plans to repeat the study using electroencephalography (EEG), which involves placing many electrodes on a skintight rubber cap placed over a subject's head that allows researchers to observe communication between different parts of the brain. "The inhibition process might directly influence the motor cortex," the cortical area involved in executing movement, Brass says. "Alternatively, it might modulate areas that are involved in forming motor intentions."
Martha Farah, director of the Center for Cognitive Neuroscience at the University of Pennsylvania, says it is crucial to figure out what neural circuitry is behind "free won't," as she refers the ability to control impulses, because it is one "of the many psychiatric disorders for which self-control problems figure prominently."