The size of the Simon effect depends on the nature of the relevant task.

Four experiments were conducted to investigate contextual modulations of the Simon effect. The results showed that the Simon effect was quantitatively different depending on which kind of task needed to be performed. Importantly, this effect did not depend on the relative processing time of the relevant dimension, nor on a direct or indirect overlap between the relevant and irrelevant stimulus part. To account for the data, we refer to the neural overlap hypothesis, which extends the definition of dimensional overlap (Kornblum, Hasbroucq, & Osman, 1990) with similarity of processing regions as the key factor for the interaction between relevant and irrelevant information processing.