To examine the time course of effects of working memory (WM) load on interference control, ERPs were measured in a combined WM and Stroop task. A WM load of 0, 2, or 4 letters was imposed, and during the maintenance-interval Stroop trials were presented that required participants to classify names of famous people while ignoring faces that were either congruent or incongruent with the names.
Behavioral interference was not modulated by WM load, but WM load led to an overall reduction of Stroop stimulus encoding as reflected by reduced N170 and N250 amplitudes independent of congruency. Incongruent distractor faces induced interference as shown by a delayed and reduced positivity between 480–600 ms (N450) and an enhanced positivity between 760–1000 ms (P600), indicating longer stimulus evaluation, conflict detection, and conflict resolution, respectively. WM load led to an increase of the P600 at frontal and parietal sites, possibly reflecting PFC-driven top-down control of posterior sites, necessary for conflict resolution.