A critical function of the human brain is to monitor behavior and prevent undesirable actions. Evidence suggests that the medial frontal cortex, particularly the anterior cingulate cortex (ACC), is involved in this action monitoring. Studies of the error-related negativity (ERN or Ne), a medial frontal negative component of the event-related scalp potential recorded in humans have contributed to this evidence. The error-related potentials, which peak about 100 ms after an erroneous response, are maximal at frontal-central recording sites. Single unit recordings in SEF and ACC during the countermanding task, a task which manipulates subjects' ability to withhold planned saccades, have revealed distinct populations of neurons that modulate their firing rate in response to errors during the same interval as error-related potentials originating in medial frontal cortex (Stuphorn et al., Nature 408:857; Ito et al., SfN Abstract 28:464.6, 2000). Local field potentials were recorded in SEF and ACC simultaneously ...