The error-related negativity (ERN) represents a neural response, recorded from scalp electrodes, that is associated with monitoring activities. It is most likely generated in the anterior cingulate cortex (ACC). Measures of the ERN, and of behavioral and perceived accuracy, were obtained from participants while they performed a visual 2-choice reaction time task under degraded stimulus conditions. Irrespective of behavioral accuracy, the amplitude of the ERN (measured at the time of the response) covaried with the perceived inaccuracy of the behavior (measured at the end of the trial). Errors due to premature responding (errors perceived as errors) were associated with large ERNs. Errors due to data limitations (errors about which there was uncertainty) were associated with smaller ERNs. These and other results are consistent with the proposal that performance monitoring, as manifested by the ERN, involves a comparison between representations of the appropriate response and the response actually made.