Smokers who anticipate lighting up a cigarette are more stimulated by images of other people smoking than by craving or the length of time they have gone without smoking, according to a new study.

Researchers broke 20 healthy smokers into two study groups: "expectant" smokers, who were told they could smoke immediately after the test, and "non-expectant" smokers, who were told that they had to wait four hours after the test before smoking. Using MRI scans, the researchers found that the expectant smokers were aroused by images of people smoking, while there was almost no reaction from the non-expectant group.

"Although the effect of exposure to drug-associated cues has been studied with various drugs of abuse, this is the first study to show the link between expectancy levels and smoking cues," said researcher Alain Dagher, who led a team from the Montreal Neurological Institute and the Department of Psychology at McGill University. "Our findings confirm the importance of expectancy in the neural response to these cues, and lend support to the theory that these cues act on brain areas involved in arousal and attention, particularly the dorsolateral prefrontal cortex, which is involved in the regulation and planning of drug-seeking or drug-avoiding behavior."

Dagher added that the findings had important implications for treatment and recovery. "People who quit smoking can expect to be exposed to other smokers, or to situations in which they previously smoked cigarettes," he said. "These exposures have an activating effect on the brain, which can lead to relapse."

The study appears online in the journal Neuropsychopharmacology.