Effects of attention and stimulus probability on ERPs in a Go/Nogo task.

Event-related potentials were measured to letters presented either to the left or the right of a fixation point that were preceded by a precue that indicated the position of the upcoming letter either correctly (valid trials) or incorrectly (invalid trials). One letter required a response (Go stimulus), while the other letter required no response (Nogo stimulus). In experiment I, Nogo letters occurred only on 25% of the trials, while in experiment II, Go and Nogo letters were equiprobable. In both experiments, Nogo stimuli elicited larger N2 components and P3s with a more anterior topography than did Go stimuli. The N2 enhancement elicited by Nogo stimuli showed a frontal maximum (most markedly when Go and Nogo stimuli were equiprobable) and was significantly influenced by precue validity. These results are discussed in terms of "Response Mismatch" and action inhibition processes.