Piéron’s Law holds in conditions of response conflict

We show that the effect of stimulus colour-intensity on reaction times in the Stroop task is additive with the effect of Stroop condition. This establishes that the psychophysical relationship between stimulus intensity and response time known as Piéron’s Law holds for colour processing, and that it can hold in cognitive tasks under conditions of response conflict, not, as hitherto demonstrated, merely for simple and perceptual choice tasks. Computational modelling shows that existing single stage models of the Stroop task provide an adequate account of these results, in contradiction to the assumption that additive factors imply independent processing stages.