If you're trying to quit smoking, you may be battling not only your cravings for nicotine, but a decline in your ability to concentrate, a new study suggests.

According to the research, while abstinence increases a smoker's craving, it may harm smokers' performance on some mental tasks while leaving their performance on others unchanged.

"It is likely craving and impaired concentration, in part, function to maintain smoking in nicotine-dependent individuals and to increase the probability a smoker attempting to quit will relapse," said Stephen J. Heishman, Ph.D., of the National Institute on Drug Abuse, Baltimore, and colleagues, whose research appeared in the journal "Nicotine & Tobacco Research."

Surprisingly, the research indicated that when smokers abstain from cigarettes, their reasoning skills do not seem to decrease, but when they resume smoking they actually appear to improve.

"A more complete understanding of tobacco craving and the influence of smoking on cognitive performance should enhance our ability to treat these symptoms of nicotine withdrawal, thereby increasing the probability of successful smoking cessation efforts," the report said.

The investigation studied the effects of smoking and abstinence on 20 smokers who completed two computerized tests before and after smoking one afternoon and again after being deprived of cigarettes for 18 hours. The participants typically smoked 23 cigarettes daily and had been smoking for more than 17 years, on average.

In one test, smokers were instructed to find specific pairs of letters in a sequence of 20 other letters. The other test presented them with a series of letter pairs and statements and was designed to measure their ability to reason.

On the letter-search test, the smokers took longer to complete the task when deprived of cigarettes, but their speed improved to baseline levels after smoking. On the test of logical reasoning, however, performance did not decrease after abstinence, but actually improved after smoking. Neither smoking nor abstinence had any effect on smokers' accuracy on the two tasks.

The investigators also were surprised to find on the logical reasoning test -- the more difficult of the two -- reasoning was not impaired when smokers were deprived of their cigarettes, despite clear evidence cravings had increased. It is possible, they say, smokers' ability to maintain attention, necessary to complete the letter search task, was more compromised by tobacco abstinence than was their ability to process verbal information, which was required in the logical reasoning test.