That key appears to be a region on the right and left sides of the brain called the insula.
The insula is thought to control conscious urges. One study showed that drug addicts who relapse have high-level insula activity during decision-making tasks.
"What happens to addictions when there's damage to the insula?" wondered Antoine Bechara, PhD, of the universities of Southern California and Iowa.
To find out, Bechara, along with Hanna Damasio of USC and other colleagues, compared people who quit smoking after injury to the insula with those who quit after suffering brain damage that did not include the insula. Not all of the patients had strokes.
Twelve of 13 patients with damage to the insula quit smoking soon after their brain damage, never started smoking again, found it easy to quit, and, after quitting, never again felt the urge to smoke. In other words, they lost their smoking addiction.
Loss of smoking addiction occurred in only four of 19 patients without insula damage.
The experience of the patient who said his body forgot the urge to smoke "suggests that the insula plays a role in the feeling that smoking is a bodily need," Bechara and colleagues conclude.
None of the insula-damaged patients who lost their smoking addiction lost their desire for food or ate less.
This suggests that insula damage does not make a person lose fundamental urges. Instead, the researchers suggest, insula damage affects only "learned pleasures."
This means the insula may also control other kinds of addiction.
Brain surgery isn't a solution to cigarette smoking or other addictions. But, Bechara says, the insula could be a target for new drug treatments.
"There is a lot of potential for pharmacological developments," he says in a news release.
Bechara and colleagues report their findings in the Jan. 26 issue of Science.