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 degrees, 45 degrees, 90 degrees, 135 degrees, 180 degrees) of the prosthetic hand relative to the participant's receptive hand. The illusion was equally strong at 0 degrees and 45 degrees: the two conditions in which the prosthetic hand was in an anatomically plausible orientation. To investigate whether the illusion was diminished at 90 degrees (and beyond) by anatomical implausibility rather than by misalignment, in experiment 2 hand positioning was changed. The illusion was equally strong at 0 degrees, 45 degrees, and 90 degrees, but diminished at 135 degrees despite the prosthetic hand now being in an anatomically plausible orientation. Thus the illusion is diminished with misalignment of 135 degrees, irrespective of anatomical plausibility. Having demonstrated that the illusion was equally strong with the hands aligned (0 degrees) or misaligned by 45 degrees, in experiment 3 we demonstrated that participants did not detect a 45 degrees 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.