The Simon effect: Evidence of a response processing ''functional locus''

Three factors; stimulus-response Compatibility Degree (compatible vs. incompatible), Hand Condition (between-hand vs. within-hand finger responses) and Precue Condition (valid precue, invalid precue, no-precue) served as the main factors in a choice reaction time (RT) task where response selection was determined by letter identity (stimulus location was irrelevant). Hand Condition by Compatibility Degree was the only significant interaction obtained, reflective of the fact that the RT(compatible) < RT(incompatible) inequality [i.e., the Simon effect] was greater for the within-hand than for the between-hand condition. This result supports Stoffer's (1991) contention that the ''functional locus'' of the Simon effect is positioned at the point of response inhibition. Furthermore, the data obtained indicated that a post-stimulus attention shift was not a prerequisite condition for the production of a Simon effect. Finally, the Simon effect procedure appears to prevent response factors from influencing the 'spatial precue effect' [RT(valid precue) < RT(invalid precue)] in a biased fashion.