The nonvisual illusion of self-touch: Misaligned hands and anatomical implausibility

The self-touch illusion is elicited when the participant (with eyes closed) administers brushstrokes to a prosthetic hand while the examiner administers synchronous brushstrokes to the participant’s other (receptive) hand. In three experiments we investigated the effects of misalignment on the self-touch illusion. In experiment 1 we manipulated alignment (0°, 45°, 90°, 135°, 180°) of the prosthetic hand relative to the participant’s receptive hand. The illusion was equally strong at 0° and 45°: the two conditions in which the prosthetic hand was in an anatomically plausible orientation. To investigate whether the illusion was diminished at 90° (and beyond) by anatomical implausibility rather than by misalignment, in experiment 2 hand positioning was changed. The illusion was equally strong at 0°, 45°, and 90°, but diminished at 135° despite the prosthetic hand now being in an anatomically plausible orientation. Thus the illusion is diminished with misalignment of 135°, irrespective of anatomical plausibility. Having demonstrated that the illusion was equally strong with the hands aligned (0°) or misaligned by 45°, in experiment 3 we demonstrated that participants did not detect a 45° misalignment. Large degrees of misalignment prevent a compelling experience of the self-touch illusion, and the self-touch illusion prevents detection of small degrees of misalignment.