Co-occurrence of sequential and practice effects in the Simon task: Evidence for two independent mechanisms affecting response selection

The Simon effect refers to the observation that responses to a relevant stimulus dimension are faster and moreaccurate when the stimulus and response spatially correspond than when they do not, even though stimulusposition is irrelevant. Recent findings have suggested that the Simon effect can be strongly modulated by priorpractice with a spatially incompatible mapping and by correspondence sequence. Although practice is thought toinfluence conditional stimulus–response (S–R) processing, leaving response priming through the unconditionalroute unaffected, sequential effects are thought to represent trial-by-trial adaptations that selectively involveunconditional S–R processing. In the present study, we tested this assumption by assessing the effects of correspondencesequence both when the Simon task alone was performed and when it was preceded by a spatialcompatibility task with either incompatible (Experiments 1–2) or compatible (Experiment 2) instructions. Theobservation that practice and correspondence sequence co-occur and exert additive effects strongly demonstratesthat the two factors affect different processing routes.