Modulation of Conflicts in the Stroop Task: Instructions may be enough

Abstract Entel and Tzelgov (2014) showed that the proportion of congruent to incongruent trials can affect congruency proportion effects just by changing the instructions about the ratio of congruent to incongruent trials. In the present study, we manipulated instructions regarding the proportion of neutral trials. Half of the participants received general instructions that did not reveal information regarding the stimulus proportions. The other half were informed that most of the stimuli would be colour words or neutrals. This held true for the stimuli in the second part of each condition. In the first part, however, the proportion of the stimuli was equal. A decreased Stroop effect was found in the mostly colour-word condition when participants received detailed instructions. Negative facilitation was observed in the mostly neutral condition as a marker of task conflict. In our second experiment, we applied an item-specific design. Some colours were presented most often as colour words and other colours as neutrals. Participants were informed about the exact pairings in advance. Once again, this held true only for the second experimental part. Informing participants in advance resulted in an item-specific proportion effect, demonstrating that this effect is not only learning-based but can also be recruited based on mere instructions regarding the proportion of neutral stimuli. In all experiments, both task conflict (defined as the latency gap between colour words and neutrals) and informational conflict (the latency gap between congruent and incongruent trials) contributed to the performance, but only informational conflict was modulated by experience. The implications of these findings for the current models of control are discussed.