Abstract
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.
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Notes
To examine whether validity effect changed across time during the object-shadow experimental blocks, we analysed the first and the second halves of each block separately. For simplicity (and also in consideration of a potential speed-accuracy trade-off in this data set) we calculated a combined measure of RTs and accuracy known as inverse efficiency (IE) as the mean RT divided by the proportion of trials correct (Spence et al. 2001; Townsend and Ashby 1983). Mean IE scores for each participant were submitted to an ANOVA with two within-participants factors: trials (first versus second halves) and Validity (valid versus invalid). This ANOVA revealed a two-way interaction [F(1,15)=5.506, MSe=896.229, P=0.03], caused by larger (and positive) validity effect for the trials belonging to second (mean=19, SE=9) than first halves of the object-shadow blocks (mean=−16, SE=10). This may constitute preliminary evidence that a validity effect for object-shadow can build up over time.
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Acknowledgements
We are grateful to Giuliano Miotto for technical support, to Maurizio Dal Bosco and Claudia Bonfiglioli for help with artwork, and to Gianluca Godino for help with data collection. We also thank Nicholas Holmes, Salvador Soto-Faraco, and an anonymous reviewer for their excellent comments on a previous draft.
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Preliminary results of this study were reported at the Fifth Annual Meeting of the International Multisensory Research Forum (Sitges, Spain), June 2004.
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Galfano, G., Pavani, F. Long-lasting capture of tactile attention by body shadows. Exp Brain Res 166, 518–527 (2005). https://doi.org/10.1007/s00221-005-2392-9
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DOI: https://doi.org/10.1007/s00221-005-2392-9