Stimulus-hand correspondence and direct response activation: An electromyographic analysis

In a common version of the Simon task, the subjects respond by a left- or a right-hand key press to the color of a stimulus (S) presented to the left or right of a fixation point. Albeit S location is irrelevant, the incorrect response is more often activated when the required response is contralateral to the S (incongruent) than when it is ipsilateral to the S (congruent). The aim of the present study was to decipher the respective contributions of S-response key location correspondence and S-hand correspondence to such incorrect activations (IAs). The subjects were required to perform a Simon task with the arms not crossing and crossing the median plane. Present for both arm placements, the difference in the frequency of IAs between incongruent and congruent responses was smaller when the arms were crossed. IAs are (i) essentially due to S-key location correspondence and (ii) less influenced by S-hand correspondence. Latency analyses further suggest that intra-hemispheric processing mediates the influence of S-hand correspondence.