That feedback helps explain why we're so pleasantly surprised at unexpected rewards, so disappointed at unforeseen penalties, and relatively blase about "predictable" outcomes.

"The brain learns from it - whether you should go ahead with this experience the next time or you should stay away from it," said the study's lead researcher, Muhammad Parvaz, a cognitive neuroscientist at the Icahn School of Medicine at Mount Sinai in New York.

Among chronic users, said Parvaz, "the worse-than-expected response was not there".

Researchers compared a range of EEG signals among people who did not use cocaine with those from two chronic-use groups: one that had used cocaine within the past 72 hours and one that had not.

All 75 participants played a game in which they guessed which of four doors hid prizes. The trials offered randomly different odds of winning, from 1 in 4 up to 3 in 4, flashed onscreen before subjects made their choice.

Subjects had to report whether they thought they had made the right or wrong prediction before they could see the result. A correct choice earned 60 cents, an incorrect one lost 30 cents.

As expected, the feedback signal from nonusers was greater for unpredicted outcomes than for predicted ones.

Across both groups of chronic users, however, EEG readings showed no significant difference between expected and unexpected losses. Their management of negative reward error prediction was impaired.

This could explain why addicts will return to drugs despite the negative impacts. "They don't learn from it," Parvaz said.

Those who tested positive for recent cocaine use had relatively normal feedback for positive reward prediction error. Users who had not taken the drug in the past 72 hours showed the worst of both worlds: impaired positive and negative feedback. That's consistent with the chemistry of cocaine use - it tamps down production of dopamine, leaving the neurons unresponsive to just about everything but the drug.