Comparing the absolute and relative Simon effects in vision and touch

Existing evidence suggests that the stimulus location is automatically encoded even when it is completely task irrelevant. Both visual and tactile stimuli facilitate the ipsilateral response when presented on the left or right side of space or of the body (absolute or trunk-centred Simon effect, respectively). However, when more than one stimulus is presented within the same side of the space (or the body) a relative spatial code also emerges, giving rise to the relative or hand/centred Simon effect. Thus far this multiple spatial codes created by visual and tactile stimuli have been investigated under very different experimental conditions. The aim of this study was to directly compare the properties of the spatial codes activated by visual and tactile stimuli in a comparable task. On different blocks of trials, participants responded to the frequency (high/low) of a tactile or visual stimulus using the pedals (right/left) to respond. The stimulus could appear in one of four possible locations, closely matched between visual and tactile stimuli. In touch we observed both a trunk-centred and a hand-centred spatial code, in line with existing results. Surprisingly, no Simon effect emerged in vision. Responses were slower for lateral compared to central stimuli regardless of response location. These results suggest that spatial coding in vision may depend on the type of task relevant stimulus feature.