The double-nail illusion: experiments on binocular vision with nails, needles, and pins

When two nails or similar slender objects are held straight ahead at reading distance, one a few centimetres behind the other and aligned at the same eye level, they are seen side by side rather than one behind the other. A quantitative study of this 'double-nail' illusion shows that the objects are judged to be at the positions of the so-called apparent or ghost images known from fusional theories of stereopsis. Most recent fusional theories assume that apparent images are suppressed by neuronal interactions, and the usual absence of percepts corresponding with apparent images is often quoted as an argument against 'projection' theories of stereopsis. The double-nail illusion shows, however, that percepts which correspond with apparent images do occur. The results are interpreted in terms of a neuronal-network type of fusional theory, in which the interpretation that corresponds with the minimum overall disparity is assumed to dominate. The following parameters were varied in the experiments: length, width, colour, and contrast for each of the nails; and fixation point position; and the orientation of the nail carrier. The results show that identity of the stimuli is not a necessary condition for the illusion. A stable vergence can be enforced by the double-nail illusion, and with additional nails multistable states of vergence can be obtained.