Stop being neutral: Simon takes control!

The Simon task is one of the well known tasks that recruit cognitive control. The Simon effect, the reaction time (RT) difference between congruent and incongruent stimuli, has been commonly discussed as interference based. Nevertheless, in recent years some studies have referred to the facilitation component of the task. In the current research we measured effects of cognitive control by conducting sequential analysis and adding neutral conditions. Two neutral stimuli were employed in order to examine their effect and their validity as neutrals. It was found that presentation of Simon stimuli on the central meridian at the top or bottom of a screen (but not at the centre of the screen) created a valid neutral condition. Facilitation as well as interference effects were found. Adding a nonconflict condition modulated cognitive control independently of sequential effects. Namely, the Simon effect increased by adding both types of neutrals, but the decrease in the effect after incongruent trials was only present for the vertical neutral block, in which the Simon effect appeared after congruent but not after incongruent trials. We suggest the possibility of two different mechanisms of cognitive control.