Simon in action: the effect of spatial congruency on grasping trajectories.

The Simon effect, one of the well-known stimulus-response compatibility effects, is usually explained as an expression of a conflict that occurs at the response selection stage. Here, we extended previous findings to provide evidence for post-response selection expression of the Simon effect. Following a presentation of a visual stimulus, participants grasped one of two objects that differed slightly in size. The results showed that visual stimulus congruency modulated grasping trajectories. Particularly, movements were more lateralized in congruent trials. This lateralization decreased as reaction time (RT) increased and therefore this effect could not be fully dissociated from the response selection stage. However, size sensitivity, as measured by the time taken to reach the maximum grip aperture between the fingers, was decreased for incongruent trials, unrelated to RT. This finding provides novel evidence for an independent expression of the Simon effect in post-response selection stages. Overall, our findings extend previous studies and demonstrate that the spatial conflict evoked by the Simon task encompasses several components and independently affects response selection stages as well as other components of motor execution.