Event-related potentials (ERPs) elicited in probe-only (P) and distractor-only (N) conditions in experiment 1. A, top: group-mean ERPs at a midline frontocentral electrode (FCz) and a midline parietal electrode (Pz). Gray shading represents the time intervals of the difference between P and N conditions to reach significance. Bottom, topographies of ERP difference during the time intervals. B, left: difference in source activity between P and N conditions. Right, time series of difference in source activity, where red shading represents time intervals of the difference between P and N to reach significance. Sag, sagittal; Cor, coronal; Tra, transverse; A, anterior; P, posterior; R, right; L, left.