Proactive control adaptation in Stroop tasks reflects adjustments in the strength of distractor suppression

Conflict tasks often yield smaller mean congruency effects when relevant (target) and irrelevant (distractor) information is mostly incongruent rather than mostly congruent. While this proportion congruency effect may reflect proactive control adaptation, only a few previous studies have provided convincing evidence for proactive control adaptation when ruling out contingency learning and reactive (item-specific) control adaptation. In this study, we present further evidence for proactive control adaptation (as reflected in proportion congruency effects and asymmetrical list-shifting effects in contingency-free transfer items) across three experiments (each N = 100 participants) using manual counting Stroop tasks (Experiment 1: number words as distractors; Experiment 2: Arabic numerals as distractors; Experiment 3: number words and Arabic numerals as inducer and transfer items, or vice versa). To better understand the processes underlying proactive control adaptation, we conducted fine-grained distributional analyses (delta functions) and model-based analyses (diffusion model for conflict tasks). These analyses suggest that proactive control adaptation in manual counting Stroop tasks mainly reflects adjustments in the strength of distractor suppression rather than in the timing of distractor suppression, the strength of target amplification, or response caution. Additional distributional and diffusion model re-analyses of the data by Spinelli and Lupker (2023, Experiments 1–3) revealed a similar pattern in vocal color Stroop tasks. In conclusion, the present study provides new evidence for proactive control adaptation in manual counting Stroop tasks, and indicates that proactive control adaptation mainly reflects adjustments in the strength of distractor suppression in both manual and vocal Stroop tasks.