Arrows placed at the ends of a line may affect its estimated length. These illusions (Müller-Lyer and its variants) are equally powerful in 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, a manual bisection task of the Judd variant of the Müller-Lyer figure was administered to 24 undergraduate participants 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. We then replicated the bimodal task of Experiment 1 in another group of 20 undergraduate participants, manipulating the spatial position of the horizontal shaft on the backside of the board (Experiment 2).
Illusory effects were comparable in touch and vision, and were present, albeit reduced, in the bimodal visuo-haptic condition. The visual arrowheads affected haptic bisection, only when the shaft was aligned with them. In conclusion, illusory effects transfer cross-modally from vision to haptics depending on the spatial coincidence of the visual and haptic components of the figure.