The findings of the study, by researchers at the Brookhaven National Laboratory, were presented Wednesday at the annual meeting of the Society for Neuroscience in San Diego.
Their "altered sensitivity to reward" explains why some cocaine users are unable to stop despite the "negative consequences or positive incentives for behavioral change", said Rita Goldstein, who led the study.
In studying the brain activity of cocaine users, the researchers were specifically interested in the P300 component of the brain waves.
The P300 -- a positive charge on being offered an incentive - was seen to be blunted in cocaine users, even when the reward was monetary.
Earlier studies have shown that P300 is blunted in individuals addicted to alcohol as well as their offspring.
The study found that, in healthy control subjects, the P300 response was significantly higher when a monetary reward was offered compared with when the reward was absent.
In contrast, the responses to money were reduced in cocaine addicts. Also, more frequent users were the least able to improve their behavioral performance in response to monetary rewards.
The study, however, found that cocaine users were not disinterested in the tasks assigned to them and were, in fact, more interested than non-users.