Does response interference depend on the subjective visibility of flanker distractors?

Response interference (or response conflict) refers to the phenomenon whereby response times to a target stimulus are longer in the presence of distractor stimuli that indicate contrary motor responses. Response interference has been observed even when the distractor stimuli cannot be discriminated above chance levels. These results raise the question of whether response interference might be driven automatically by the physical distractor stimuli, independently of one's subjective perception of the distractors. Using a modified version of the Eriksen flanker task, we applied metacontrast masks to the flanker stimuli and measured their subjective visibility after each trial. We found converging lines of evidence that the subjective perception of flankers contributed to response interference, over and above the contribution of automatic processing of the stimulus itself. A factorial analysis revealed that the objective, physical congruency of target and flankers and the subjective, perceptual congruency of target and flankers make additive, noninteracting contributions to target response interference, suggesting that the two interference effects originate from independent levels or stages of cognitive processing.