Long-lasting capture of tactile attention by body shadows

Four experiments addressed the role of cast shadows of the body in orienting tactile spatial attention to the body itself. We used a modified spatial-cueing paradigm to examine whether viewing of the cast shadow of a hand can elicit spatial shifts of tactile attention to that very same hand. Participants performed a speeded tactile-discrimination task (thumb versus index finger, regardless of touched hand), while viewing the shadow of either the touched or untouched hand cast in front of them by a lateral light-source. The hand casting the shadow changed either between blocks (expt 1) or unpredictably within each block (expts 2-4). In experiments 1 and 2 tactile targets were preceded by central non-informative visual cues delivered near the shadow of the index finger and thumb. Despite the fact that cast shadows were always task-irrelevant and non-predictive of which hand was stimulated, tactile discrimination was consistently faster at the hand casting the shadow than at the other hand. This effect was not modulated by the duration of cue-target asynchrony, nor did it depend on whether the visual cue was present or not (expt 3). In addition, it was still reliable when vision of the hands was precluded, whereas it became inconsistent when the cast shadow of the hand was replaced by the cast shadow of an object (expt 4). Our results suggest that body shadows can induce a long-lasting capture of tactile attention for stimuli at the body itself.