Investigating the effect of a transparent barrier on the crossmodal congruency effect.

A transparent barrier, such as a window, protects us from approaching objects (such as flies), although we can still see the objects coming toward us through the occluder. In the present study, we examined whether this potential dissociation between tactile and visual experience introduced by the presence of a transparent barrier also affects visual-tactile interactions in normal participants, as indexed by performance in the crossmodal congruency task. Participants discriminated the elevation of vibrotactile target stimuli (upper vs lower) presented to the left or right hand while trying to ignore visual distractor lights that could independently be presented from upper or lower locations on either the same or the opposite side. A transparent occluder was placed between the vibrotactile targets and the visual stimuli (the barrier occluded the vibrotactile targets in Experiment 1 and the visual distractors in Experiment 2). Vibrotactile elevation discrimination performance was slower and less accurate when the distractor lights were presented from incongruent locations relative to the target (e.g., lower tactile target with upper distractor light). However, there were no significant differences between performance with and without the transparent occluder present. This pattern of results was replicated in Experiment 3 under conditions where the participants were periodically required to reach around the transparent occluder in order to press buttons placed near to the visual distractors. Taken together, these results support recent neuropsychological evidence [Farne et al. (2003) Int J Psychophysiol 50:51-61] suggesting that visual-tactile interactions in peripersonal space are not necessarily modulated by conscious awareness of the impossibility of our hand being touched by the visual stimuli.