Spatially mediated interactions between singletons in tactile search

In visual search tasks competitive interactions between target and distractor typically result in degraded target selection processes which increase as the target-distractor distance decreases. Here, we investigated for the first time whether analogous selective mechanisms operate within the somatosensory system. Specifically, we investigated whether the N140cc, an ERP component known to be associated with target selection in touch, changes as function of the target-singleton distractor distance in an additional singleton search task. Six simultaneous vibro-tactile stimuli were presented on each trial to three fingers of the left and right hand. Participants were asked to localise the singleton target (high frequency stimulus) while ignoring the singleton distractor (middle frequency stimulus) delivered to a different finger of the same hand as well as the homogenous distractors (low frequency stimuli) presented to the remaining fingers. The spatial separation between target and singleton distractor was varied according to both somatotopic space (singletons to contiguous or non-contiguous fingers) and external space (fingers open or closed). Behavioural results demonstrated that accuracy improved when target and distractor were close together than far apart with respect to both somatotopic and external distance. Thus, unlike vision, competition between singletons was reduced when their distance decreased. ERP results showed that the N140cc amplitude was enhanced when the somatotopic distance between target and singleton distractor increased (non-contiguous singletons), while it was reduced when their external distance increased (fingers open). These findings confirm the N140cc as an index of target selection and further suggest that this component is able to capture and distinguish the effects of different body representations (somatosensory and external) on tactile selective attention.