Sustained rubber hand illusion after the end of visuotactile stimulation with a similar time course for the reduction of subjective ownership and proprioceptive drift

Abstract

The rubber hand illusion is a perceptual illusion in which participants experience an inanimate rubber hand as their own when they observe this model hand being stroked in synchrony with strokes applied to the person's real hand, which is hidden. Earlier studies have focused on the factors that determine the elicitation of this illusion, the relative contribution of vision, touch and other sensory modalities involved and the best ways to quantify this perceptual phenomenon. Questionnaires serve to assess the subjective feeling of ownership, whereas proprioceptive drift is a measure of the recalibration of hand position sense towards the rubber hand when the illusion is induced. Proprioceptive drift has been widely used and thought of as an objective measure of the illusion, although the relationship between this measure and the subjective illusion is not fully understood. Here, we examined how long the illusion is maintained after the synchronous visuotactile stimulation stops with the specific aim of clarifying the temporal relationship in the reduction of both subjective ownership and proprioceptive drift. Our results show that both the feeling of ownership and proprioceptive drift are sustained for tens of seconds after visuotactile stroking has ceased. Furthermore, our results indicate that the reduction of proprioceptive drift and the feeling of ownership follow similar time courses in their reduction, suggesting that the two phenomena are temporally correlated. Collectively, these findings help us better understand the relationships of multisensory stimulation, subjective ownership, and proprioceptive drift in the rubber hand illusion.

Keywords: Embodiment; Multisensory integration; Proprioception; Rubber hand illusion.

Fig. 1

A Photograph of the experimental setup. The experimenter, facing the photographer, is seen holding the two small paintbrushes used for the visuo-tactile stimulation. The participant’s real right hand is placed furthest to the right in the picture, hidden from the participant’s view behind the divider. The rubber hand is placed to the left of the divider in full view of the participant. The experimenter induced the rubber hand illusion by synchronously stroking the rubber hand and the participant’s real right hand with two small paintbrushes on corresponding parts and in the same direction. The metal ruler rod was placed over the participant’s wrist in this photograph illustrating the illusion induction phase, but it could easily be moved by the experimenter to a more distal position over the tip of the participant’s index finger before the participants were asked to indicate the position of their right index finger in the hand localization task used to register the proprioceptive drift. B Schematic illustration of the procedures for experiments 1 and 2. The hand localization task and illusion ratings were performed in two separate sessions in each experiment. In the proprioceptive drift trials, a baseline hand localization task was performed before the start of the 60 s of visuo-tactile stimulation. Immediately following the visuotactile stimulation, in the proprioceptive drift trials, the participants closed their eyes and performed the hand localization task again (0 s) and then again at 20 s, 40 s, 60 s, 120 s and 300 s after the end of the visuotactile stimulation. In the second part (subjective ratings) of the experiment, no hand localization task was performed; instead, the participant verbally rated the subjective feeling of the rubber hand feeling like their own hand on a seven-point Likert scale at each of the six timepoints. In experiment 1, the participants opened their eyes after they had performed the hand localization task to keep their eyes open as much as possible during the trials, whereas in experiment 2, they kept their eyes as closed throughout the session and only opened them for 1 s immediately prior to performing the hand localization task or the subjective rating at the six specified timepoints

Fig. 2

Results from experiment 1. A Evolution of the proprioceptive drift following the end of the visuo-tactile stimulation. B Evolution of the subjective ratings of the statement “it felt as if the rubber hand were my hand” after the end of the visuotactile stimulation. Timepoint 0 indicates immediately after the end of the visuo-tactile stimulation. Blue lines indicate the synchronous condition, while red lines indicate the asynchronous condition. Dots and squares indicate means, and error bars indicate the SEMs. Stars indicate significance in the comparison between Synch and Async; *indicates p < 0.05, **indicates p < 0.01, and ***indicates p < 0.001

Fig. 3

Comparison of the values of the coefficients for the fitted curves (a, b and c in the equation $y=a\times e^{\mathit{bx}}+c$) for the synchronous conditions in experiment 1. Panel A illustrates the coefficient “a” (intersect), panel B the coefficient “b” (decay) and panel C the coefficient “c” (constant). Individual data points and pairwise comparisons are shown for all participants (P1–P20). The black dotted lines illustrate mean and standard error of the mean (SEM). See Supplementary Fig. 1 for individual fitted curves and individual datapoints

Fig. 4

Panels A and B depict the mean fitted curves from experiment 1, whereas panels C and D depict the mean fitted curves from experiment 2. The exponential curves represent the mean fitted curve (averaged fitted values) and the shaded areas around them indicate the standard errors. The squares and dots indicate the mean z-scores for each of the six measured timepoints (0, 20, 40, 60, 120 and 300 s), with the error bars indicating the standard error of the mean, n = 20 for each experiment. See Supplementary Fig. 1 and Supplementary Fig. 4 for individual datapoints and illustration of the fitted curves

Fig. 5

Results from experiment 2. A Evolution of the proprioceptive drift following the end of the visuo-tactile stimulation. B Evolution of the subjective ratings of the statement “it felt as if the rubber hand were my hand” after the end of the visuotactile stimulation. Timepoint 0 indicates immediately after the end of the visuo-tactile stimulation. Blue lines indicate the synchronous condition, while red lines indicate the asynchronous condition. Dots and squares indicate means, and error bars indicate the SEMs. Stars indicate significance in the comparison between Synch and Async; *indicates p < 0.05, **indicates p < 0.01, and ***indicates p < 0.001

Fig. 6

Comparison of the values of the coefficients for the fitted curves (a, b and c in the equation $y=a\times e^{\mathit{bx}}+c$) for the synchronous conditions in experiment 2. Panel A illustrates the coefficient “a” (intersect), panel B the coefficient “b” (decay) and panel C the coefficient “c” (constant). Individual data points and pairwise comparisons are shown for all participants (P1–P20). The black dotted lines illustrate mean and standard error of the mean (SEM). See Supplementary Fig. 4 for individual fitted curves and individual datapoints

Fig. 7

A Comparison between the differences in proprioceptive drift between synchronous and asynchronous conditions in experiment 1 and experiment 2. B Comparison between the differences in the subjective illusion ratings between synchronous and asynchronous conditions in experiment 1 and experiment 2. Error bars indicate SEMs. *indicates p < 0.05. n = 20 per experiment