Finding an interaction between Stroop congruency and flanker congruency requires a large congruency effect: A within-trial combination of conflict tasks.

Responding to a conflict is assumed to trigger attentional-control processes-that is, processes that enable us to activate goal-relevant information and to inhibit irrelevant information. Typically, conflict is induced in tasks, such as the Stroop task (which requires identifying the color of color words) or the flanker task (which requires identifying a central character among flankers). Combining the conflicts within the same trial has been found to result in an interaction in reaction times (RTs), suggesting a generalization of attentional control. However, this interaction was observed when the congruency effect was substantial-that is, when the RT difference between incongruent trials (e.g., the word "green" printed in red for the Stroop task) and congruent trials (e.g., the word "red" printed in red) was large. The purpose of the present study was to investigate whether a large congruency effect is the necessary condition for observing the interaction. To this end, Stroop and flanker tasks were combined, and participants were asked to respond to the color of the central letter/word while ignoring the flanking letters/words. The magnitude of the congruency effect was increased: (a) by testing older adults (Experiment 1), (b) by manipulating the proportion of trials in which participants were asked to respond to the word meaning (Experiment 2), and (c) by using vocal responses (Experiment 3). The results showed an interaction when the Stroop congruency effect was large. Therefore, such interactions can be used to validate or invalidate theoretical explanations only when the precondition-a large congruency effect-is fulfilled.