[The double-nail illusion revisited: commonality with the wallpaper phenomenon]

The double-nail illusion occurs when the images of two nails located at different distances from the observer fall on the corresponding retinal points, giving rise to the impression that the two nails are placed side by side at an equal distance from him. Experiment 1 showed that when two stimuli located on the mid-sagittal plane and at different distances were viewed in such a way that their images fall on the corresponding points, the perceived distance of the stimuli corresponded to the convergence distance. In Experiment 2, the convergence distance was changed gradually to get the measure of the distances at which the double-nail illusion appeared or disappeared. The results indicated the same kind of hysteresis as that previously observed in the 'dynamic' wallpaper phenomenon. Experiment 3 demonstrated the "double" wallpaper illusion, in which two wallpaper stimuli located in two different frontal-parallel planes appeared in a plane between the two and parallel to them. It showed moreover that the perceived distance of the wallpaper stimuli corresponded to the perceived distance obtained in the case of the double-nail illusion. The results of the three experiments indicated the commonality of the double-nail illusion with the wallpaper phenomenon and suggested that they share the same underlying perceptual processes. The principles of egocentric visual direction and distance were discussed with respect to the results.