Less than a third of smokers are able to go a full year without a cigarette, even when they're getting help from a nicotine patch or gum. But why is giving up the habit so tough?

Research published in the journal JAMA Psychiatry by neuroscientists at the University of Pennsylvania suggests that it has to do with how well certain brain networks interact during withdrawal.

"The brain is very active at rest, and so we were able to look at changes in that activity at rest in the brain and, importantly, the connections between large-scale brain networks and how those connections become stronger or weaker when people quit smoking," said Caryn Lerman, director of the Brain and Behavior Change Program at the University of Pennsylvania. Lead author of the latest study, she used functional magnetic resonance imaging to detect brain activity in daily smokers.

Of the three large-scale networks, Lerman said, the salience network helps direct our attention. 

"When the salience network is doing its job, it may be increasing activity in the networks important for focusing on goal-directed activities [and] simultaneously suppressing activity in parts of the brain that may be involved in inward focused or daydreaming," she said.

But for smokers in withdrawal, the researchers found, that that type of connectivity is reduced. The severity of the lost connectivity also predicted which smokers in withdrawal would have the most severe cravings for a cigarette.

Eventually, Lerman hopes to use the brain scans as a biomarker to find out which smokers will struggle the most with quitting, and identify those who might be good candidates for alternative therapies.

"Certain types of cognitive exercise training may be effective in helping people exert more top-down control from the executive control network and be able to disengage more easily from the types of ruminations that are characteristic of default mode network activity," Lerman said.

One of these exercises, she said, could be a computer game that asks you to hold a particular string of letters in your mind.