External coding and salience in the tactile Simon effect

Previous studies have demonstrated a tactile Simon effect in which stimulus codes are generated based on the stimulated hand, not on limb position in external space (the somatotopic Simon effect). However, given evidence from visual Simon effect studies demonstrating that multiple stimulus codes can be generated for a single stimulus, we examined whether multiple stimulus codes can be generated for tactile stimuli as well. In our first experiment using four stimulators (two on each side of the hand), we found novel evidence for a hand-centered Simon effect, along with the typical somatotopic Simon effect. Next, we examined whether the potential salience of these somatotopic codes could be reduced, by testing only one hand with two stimulators attached. In Experiments 2-4, we found a strong hand-centered Simon effect with a diminished somatotopic Simon effect, providing evidence that stimulus salience can change the weighting of somatosensory stimulus coding. Finally, we also found novel evidence that the hand-centered Simon effect is coded in external, not somatotopic, coordinates. Furthermore, the diminished somatotopic Simon effect when testing on one hand only provides evidence that salience is an important factor in modulating the tactile Simon effect.