Researchers at Duke University Medical Center in Durham, N.C., took brain scans during normal smoking activity and 24 hours after quitting. The researchers found a marked increase in a particular kind of brain activity when quitters see photographs of people smoking.

Joseph McClernon, an associate professor in the department of psychiatry and behavioral sciences at Duke University Medical Center, said the finding sheds light on why it's so hard for some people to quit smoking, and why they relapse so quickly.

"Quitting smoking dramatically increased brain activity in response to seeing the smoking cues, which seems to indicate that quitting smoking is actually sensitizing the brain to these smoking cues," McClernon said in a statement.

The study, published online in Psychopharmacology, found activation in the dorsal striatum -- an area involved in learning habits or things people do by rote -- like riding a bike or brushing teeth.

The research shows that when smokers encounter these cues after quitting, it activates the area of the brain responsible for automatic responses, McClernon said.