In fact, the authors report in Science this week that damage to that region is 136 times more likely to result in a "disruption of smoking addiction"—defined as the ability to quit easily without relapse—than injury to other parts of the brain.
"Finding the insula may not be surprising in a way, because there was knowledge that preceded that and it makes sense," says senior study author Antoine Bechara, a neuroscientist at the University of Southern California and the University of Iowa's Carver College of Medicine. "But, it's surprising in another way because nobody was looking there."
The insula has long been associated with conscious urges and visceral sensations. According to Steven Grant, a researcher at the National Institutes of Health's National Institute on Drug Abuse, it is also believed to act as a sensory receptor for the internal organs, perhaps remembering the way something tastes or if it upsets the stomach. During imaging studies probing the causes of addiction, the insula often was activated when drug abusers were shown movies of others taking drugs or shown pictures of cocaine, heroin or nicotine.
But until now researchers had ignored the silver dollar–size region because it was not implicated in reinforcement pathways and did not affect the signaling of dopamine, the neurotransmitter associated with motivation and pleasure.
Bechara and his team studied 69 stroke patients culled from a registry at the University of Iowa that was assembled to study the effect of brain damage on cognition, memory and motion; they selected subjects who, at the time of their strokes, had been smoking at least five cigarettes daily for more than two years. Of this group, 19 patients had suffered damage to either their right or left insula.
Twelve of the 19 patients with insula damage reported a sudden end to their smoking habit, and none had relapsed over the past year. While many other stroke victims had also quit smoking, most of them cited health reasons such as fear of another stroke for doing so. "This is not a 100 percent proof, not every single insula damaged caused the disruption and vice versa," Bechara points out. "There was damage in things that did not involve the insula and there was a disruption of addiction" as well as people who suffered insula damage but did not suddenly stop smoking.
Bechara speculates that these disparities may be caused due to some gender differences in the way the brain is wired. He points to studies that found diminished decision-making capacity in women with damage to their left prefrontal cortex and men with injuries to their right side, but not vice versa. He notes, however, that the sample size in this study was not enough to draw those sorts of conclusions.
Still, Grant believes the study points to the insula as key to addiction. "It really now calls upon the research field to start focusing on this area," he says, "to look at drug effects in this area, to look at the types of changes that go on during addiction in this area, to understand what the basic function of this area is and why it would be important in this kind of context."
Bechara also believes that the insula may also play a role in other addictions such as alcoholism, drug abuse and compulsive eating. While inducing a stroke clearly is not the answer, Bechara says that safe therapies designed to disrupt the insula's activity may eventually be able to help people at least quit smoking.