Feeling touch through a mirror: The role of vision and body ownership in generating non-veridical tactile experiences

Tactile experience can be non-veridical, i.e., not related to the actual stimulation of one's body. Recently, using a mirror box procedure in healthy subjects, we found that during bilateral asymmetrical touches, the vision of the right-hand being stimulated, reflected in the mirror, elicited on the real left-hand the feeling of being touched in the same position as the right-hand. Because these errors resemble synchiria, we called these false feelings 'synchiric errors' (SEs). Here, we investigated both the role of top-down feeling of body ownership (BO) over the mirrored hand-using explicit (BO questionnaires) and implicit (Electrodermal Activity) measures of BO- and bottom-up visual processing-by manipulating the presence/absence of visual feedback-in generating SEs during the Tactile Quadrant Stimulation Test (TQS). In TQS, subjects had to indicate the position of a tactile stimulus, applied in asymmetrical quadrants on the dorsum of the two hands, under three conditions: Baseline (no vision), Mirror Condition Vision (MCV; full visual feedback), and Mirror Condition Blind (MCB; visual feedback occluded). We tested 35 healthy individuals. First, measures of BO indicated that most subjects felt the right reflected hand as their own left hand. Moreover, we found a significant presence of SEs in the MCV. Crucially, SEs were significantly higher for the left hand in MCV compared to Baseline and MCB, confirming the critical role of vision in inducing the non-veridical tactile experience. Moreover, the absence of a correlation between SEs and body ownership measures, and a double dissociation between them, indicate that SEs are driven by vision and not by an alteration of BO.