Dissociation Between Reaction Time and Pupil Dilation in the Stroop Task

It has been suggested that the Stroop task gives rise to 2 conflicts: the information conflict (color vs. word meaning) and the task conflict (name the color vs. read the word). However, behavioral indications for task conflict (reaction time [RT] congruent condition longer than RT neutral condition) appear under very restricted conditions. We conducted Stroop experiments and measured RT and pupil dilation. The results show a clear dissociation between RT and pupil dilation. We found the regular RT pattern—large interference and small, nonsignificant facilitation. In contrast, pupil dilation showed information conflict—larger pupil dilation to incongruent than to congruent and neutral conditions—and task conflict—larger pupil dilation to the congruent than to the neutral condition. Moreover, pupil indications for task conflict appeared earlier than indications for the information conflict. These results suggest that pupil changes could indicate conflict even in the absence of behavioral indications for the conflict.