Neuroimaging studies suggest that the anterior cingulate cortex (ACC) is involved in the monitoring of response conflict. A frontocentral electrophysiological component, the N2, is often enhanced in incompatible trials in S-R-compatibility tasks where conflict is assumed to be large. Here we investigated whether N2 effects are also seen in cuing tasks, where the validity of the cues and hence response conflict is manipulated. We hypothesized that the N2-effect would disappear in a condition in which compatible and incompatible trials were equally probable, and reverse in a condition in which the incompatible trials were more common. Moreover, if the N2 reflected conflict monitoring exclusively, it should occur in incompatible trials only.
We used a response priming paradigm in which 14 subjects had to respond quickly to centrally positioned target letters X or O. These were preceded by cues which predicted the target correctly in 80%, 50% and 20% in the first, second and third block of trials, respectively. Results showed a significant interaction between trial compatibility and validity for RTs and error rates as well as for the amplitudes of the frontocentral N2, P3a and the parietal P3b. While the amplitude of the P3a correlated negatively with RTs, the latency of the N2 showed a positive correlation with this behavioural measure. The findings that compatible and incompatible trials both evoked an N2, and that its latency correlated with RTs in compatible trials as well, suggest that this component not only reflects response conflict. In line with roles suggested for the ACC, the N2 may also be related to response selection.