Schematic representation of the task-switching paradigm. A: Example trial. During a block of trials, a rectangular frame with the labels "shape" and "filling" was visible. On each trial, a different imperative stimulus (i.e., a stimulus that requires an immediate response) was presented in the top or bottom part of this frame. The location (i.e., in top or bottom part of frame) determined whether the participant had to apply the shape or filling task rules to it. B: There were four different imperative stimuli, which needed to be responded to as follows. In the shape task, a "diamond" required a left-button response, and a rectangle a right-button response. In the filling task, a filling of two circles required a left-button response, and a filling of three circles a right-button response. Congruent stimuli are those that required the same response in both tasks, whereas incongruent stimuli required opposite responses in the two tasks. Thus, the imperative stimulus in panel A is incongruent: It appears in the top of the frame, thus is should be responded to in accordance to the shape task, and because it is a diamond (the filling of three circles is irrelevant in the shape task) it should be responded to with a left-button response (see Additional file 1 for demonstration).