Abstract
Focusing on a distant target and holding an object up close before our eyes leads to double vision of the object. If instead of holding an object before the eyes we hold our fingertips, double vision induces the false impression of seeing more fingertips than we normally do. By orienting our index fingers horizontally in front of our eyes and pointing them against each other, we have a clear visual impression of a disembodied finger being held between two fingertips. Similarly, by orienting our index fingers vertically, we visually perceive two pairs of fingertips touching each other. In both cases, double vision creates a conflict between vision and proprioception about the position and numerosity of our fingertips. Here, we asked how does this visuo-proprioceptive conflict impact the localization of tactile sensations felt by the index fingertips. In two studies, participants reported their agreement with different tactile interpretations, ranging from an interpretation based on proprioception only, to interpretations involving different effects of visual capture on tactile mislocalization, or even supernumerary tactile sensations (feeling more touches than actual). Participants agreed most strongly with interpretations in which the tactile localizations were constrained by their compatibility with visual signals, despite visual signals were evidently not valid, but also by their compatibility with the proprioceptively sensed positions of the hands and knowledge about the arrangement of body parts. Participants strongly agreed also with the interpretation in which the tactile sensations were localized on all four fingertips, showing that the tactile interpretations were not simply the result of combining visual and proprioceptive position estimates. Our results show that a brief self-induced conflict between proprioception and vision instantly modifies the localization of tactile sensations, leads to multistable percepts, and produces the illusion of supernumerary tactile sensations.