Figure 1.

Sequence of the three different trial types (from top to bottom, catch trials, task change trials, and stimulus change trials). Two different tasks had to be performed; classifying the color of the letter as green or as yellow and classifying the letter as an R or an L. All trial types start with the presentation of a task cue. This task cue indicates the color task (the word “color” appears) or the letter task (the word “letter” appears). Four response buttons were used. They were allocated to the index and middle fingers of the left and right hands. One task was allocated to one hand, and the L response was always allocated to the left finger of the corresponding hand. In catch trials, the task indicated by the task cue had to be applied on the stimulus at the moment of stimulus presentation. In task change trials, we tried to elicit task errors by presenting a task change signal after stimulus presentation. This task change signal indicates a task switch with respect to the primary cue. In the figure, the correct response for the task change trial is thus green and not R. The delay between the stimulus and the task change signal was adjusted to a staircase algorithm. As a result, participants should have made ∼50% task errors in the task change trials. In stimulus change trials, we tried to elicit response errors by presenting a stimulus change after the first stimulus presentation. Participants should have tried to respond to this changed stimulus and not to the first presented stimulus. In the figure, the correct response for the stimulus change trial is thus L and not R. We also adjusted the delay between the primary and secondary stimuli to a staircase algorithm so that the percentage of response errors would be ∼50%.