Active self-touch restores bodily proprioceptive spatial awareness following disruption by 'rubber hand illusion'

Abstract

Bodily self-awareness relies on a constant integration of visual, tactile, proprioceptive, and motor signals. In the 'rubber hand illusion' (RHI), conflicting visuo-tactile stimuli lead to changes in self-awareness. It remains unclear whether other, somatic signals could compensate for the alterations in self-awareness caused by visual information about the body. Here, we used the RHI in combination with robot-mediated self-touch to systematically investigate the role of tactile, proprioceptive and motor signals in maintaining and restoring bodily self-awareness. Participants moved the handle of a leader robot with their right hand and simultaneously received corresponding tactile feedback on their left hand from a follower robot. This self-touch stimulation was performed either before or after the induction of a classical RHI. Across three experiments, active self-touch delivered after-but not before-the RHI, significantly reduced the proprioceptive drift caused by RHI, supporting a restorative role of active self-touch on bodily self-awareness. The effect was not present during involuntary self-touch. Unimodal control conditions confirmed that both tactile and motor components of self-touch were necessary to restore bodily self-awareness. We hypothesize that active self-touch transiently boosts the precision of proprioceptive representation of the touched body part, thus counteracting the visual capture effects that underlie the RHI.

Keywords: bodily self-awareness; body ownership; rubber hand illusion; self-touch; voluntary action.

Figure 1.

Experimental set-up. Participants sat at a desk, resting both their arms on the surface. A rubber hand was placed to the right of real left hand. The participants' left arm and the robotic set-up were covered by a foamboard screen and remained unseen throughout the entire experiment. The silicone glove was instead clearly visible through an aperture in the foamboard. (a,d) Baseline proprioceptive judgement. At the beginning and the end of each trial, participants were asked to close the eyes and to use their right index finger to produce a ballistic movement to point to the location immediately above the centre of their left wrist. Pointing performance was measured using a webcam suspended above the workspace. (b) RHI induction. The experimenter sat opposite the participant and used the two identical brushes to stroke homologous points of the participants' left hand and the cosmetic glove either synchronously or asynchronously for one minute, while the participant kept their gaze on the rubber hand. (c) Self-touch stimulation. Self-touch was performed immediately after (Experiments 1 and 3) or before (Experiment 2) the RHI stimulation using two six-degrees-of-freedom robotic arms coupled in a leader-follower system. The participants were asked to close their eyes and move the leader robot with their right hand. The participants' movement was reproduced by the follower robot thus generating corresponding gentle strokes from on the back of the participants' left middle.

Figure 2.

Results from Experiment 1. Both in the no self-touch and in the passive self-touch conditions, participants showed significantly larger proprioceptive drift in the synchronous compared with the asynchronous RHI condition. By contrast, following active self-touch condition, the three visuo-tactile conditions were virtually identical, suggesting a restorative effect of self-touch on bodily awareness. The error bars represent the s.e.m.

Figure 3.

Results from Experiment 2. Self-touch delivered before the induction of the RHI had no effect on proprioceptive drift measures of RHI, ruling out the hypothesis that self-touch has a protective effect on bodily awareness. The error bars represent the s.e.m.

Figure 4.

Results from Experiment 3. Motor and tactile components of self-touch were organized according to a 2 × 2 factorial arrangement (movement absent/present; touch absent/present), and followed immediately after synchronous or asynchronous visuo-tactile stimulation. Unimodal motor and tactile stimulation conditions and no self-touch condition all showed a significant RHI, defined as the difference in proprioceptive drive between the synchronous and asynchronous visuo-tactile conditions. After active self-touch involving both movement and touch components the two visuo-tactile conditions were not statistically different. This replicates the results from Experiment 1 and confirms the role of active self-touch in restoring bodily awareness. Error bars represent s.e.m.