Suppression of irrelevant activation in the horizontal and vertical Simon task differs quantitatively not qualitatively

The Simon effect is usually explained by the assumption that the irrelevant stimulus location automatically activates the corresponding response. In the case of incongruent stimulus–response assignments automatically activated responses therefore have to be suppressed to ensure correct responses. This account, however, has been called into question for other than horizontally arranged visual Simon tasks. We investigated whether there is a qualitative or quantitative difference in suppression of irrelevant activation between horizontally and vertically arranged Simon tasks, using delta-function analyses. Sequential analyses revealed suppression after incongruent trials in both tasks, supporting the idea of a quantitative rather than a qualitative difference between the tasks. We conclude that automatic response activation is weaker in vertical tasks resulting in lower inhibitory demands as compared to horizontal tasks.