Underlying mechanisms of the P3a task-difficulty effect

In three‐stimulus oddball studies, even typical deviant stimuli elicited a large P3a event‐related brain potential (ERP) when target/standard discrimination was difficult. To investigate the underlying mechanisms, the effects of task difficulty on early deviant‐related ERPs were assessed. Four visual stimuli defined by an orthogonal combination of task‐relevant size (nontarget 80%, target 20%) and task‐irrelevant luminance (standard 80%, deviant 20%) were presented randomly, where two task difficulties (easy, difficult) were defined by target/nontarget discriminability. An increase in task difficulty enhanced P3a as well as a posterior negativity (change‐related negativity) and an anterior positivity (frontal positivity) elicited by deviant nontarget stimuli. These results suggest that attentional modulation of refractoriness‐based rareness detection and an attention‐triggering process underlie the P3a task‐difficulty effect.