Electro-cortical signatures of performance errors are thought to indicate the need for top-down control. Mid-frontal Theta oscillatory activity (4-8Hz) is a well-established marker of committed or observed errors. By combining EEG and immersive virtual reality, we reported that observing errors in reach-to-grasp actions of an avatar seen from a first-person perspective elicited greater theta oscillations over fronto-central electrodes (Pavone et al., 2016). Previous studies on committed or observed errors used sequences of trials were erroneous actions were less frequent than correct actions (e.g. 30% vs 70%). Therefore, it was not possible to disentangle whether the activation of the performance system was due to error per se or to surprise/novelty effect associated with rare and less predictable events. To address this issue, we recorded the EEG signal of 20 participants observing correct or erroneous actions performed by an avatar. Importantly, at variance with Pavone et al, (2016) the proportion of erroneous vs correct actions was 70% vs 30%. The results show that observation of erroneous actions enhanced Theta power compared to correct actions. Our data suggest that error per se, and not its percentage of occurrence, triggered the activity of the performance monitoring system, likely with the aim of flexibly adapting actions to the challenges of the external environment
