Double tactile sensations evoked by a single visual stimulus on a rubber hand.

The classical rubber hand illusion involves individuals misattributing tactile sensations 'felt' by their real hand hidden from view to a rubber prosthetic hand that they 'see' being tactilely stimulated in synchrony. However, from previous studies, it is not clear whether individuals actually feel the tactile sensation in the rubber hand, real hand, or both because the same part of the rubber and real hands were stimulated simultaneously. Here, we attempted to isolate subjects' sensations attributed to the rubber hand from those sensed in the real hand by placing the rubber and real hands in opposing orientations (e.g., palm up versus dorsum up). Interestingly, half of the subjects reported two tactile sensations for one visual stimulus, that is, one in the rubber finger stimulated visually with a light source and one in the real finger overlapping the rubber finger. This finding suggests that the tactile sensation induced by the visual stimulus is referred to the rubber hand and real hand simultaneously. Thus, both visuo-spatial and somatic codes are used in the localization of tactile sensation in the rubber hand illusion.