Multisensory integration in the Muller-Lyer illusion: from vision to haptics.

The Muller-Lyer figure is a powerful illusion in both vision and touch, although it is still unclear whether the visual and haptic illusory effects are due to modality-specific or shared processes. In order to investigate the hypothesis of a common underlying mechanism, the performance of 24 undergraduates in a manual bisection task of the Judd variant of the Muller-Lyer figure was assessed under visual, haptic, and visuo-haptic presentations (Experiment 1). In the bimodal condition participants saw the arrowheads on the front and touched the shaft on the back of a stimulus-supporting board. Illusory effects were comparable in touch and vision and were present, albeit reduced, in the bimodal visuo-haptic condition. In Experiment 2 we used the bimodal task of Experiment 1, manipulating the spatial position of the horizontal shaft on the backside of the board. The visual arrowheads affected haptic bisection, bringing about the illusory effects, only when the shaft was aligned with them. In conclusion, illusory effects transfer cross-modally from vision to haptics only in the presence of spatial coincidence, which appears to be a crucial factor for cross-modal integration. Implications for multisensory processes are discussed.