The function of the anterior cingulate cortex (ACC) remains controversial, yet many theories suggest a role in behavioral adaptation, partly because a robust event-related potential, the feedback-related negativity (FN), is evoked over the ACC whenever expectations are violated. We recorded from the ACC as rats performed a task identical to one that reliably evokes an FN in humans. A subset of neurons was found that encoded expected outcomes as abstract outcome representations. The degree to which a reward/non-reward outcome representation emerged during a trial depended on the history of outcomes that preceded it. A prediction error was generated on incongruent trials as the ensembles shifted from representing the expected to the actual outcome, at the same time point we have previously reported an FN in the local field potential. The results describe a novel mode of prediction error signaling by ACC neurons that is associated with the generation of an FN.