Attenuation of Pain Perception Induced by the Rubber Hand Illusion

Abstract

Adaptive behavior usually requires accurate representations of body positions and ownership, which rely on integration of multiple sources of sensory information. The rubber hand illusion (RHI) presents a compelling example demonstrating that the combination of visual and tactile signals strongly influences the subjective experience of body ownership. However, it still remains unclear how the perception of body ownership in turn alters other aspects of sensory processing, such as pain perception. In the present study, we examined whether the RHI could modulate the subjective experience of pain. We set three conditions corresponding to different levels of ownership of the rubber hand: the synchronous condition in which the rubber and the real hand were simultaneously stroked; the asynchronous condition in which the two hands were asynchronously stroked; the own-hand-only condition in which only the real hand was stroked. Results from the screening experiment indicated that subjects experienced the stronger RHI in the synchronous condition, compared with the strength of RHI in the other two conditions. In the main experiment, subjects were requested to report the intensity and unpleasantness of pain evoked by laser stimuli under the three stroking conditions. Results showed that pain ratings were significantly lower under the synchronous condition than those under the other two conditions, suggesting the RHI could induce a significant analgesic effect. Furthermore, the correlation analysis showed that the degree of the analgesic effect was positively correlated with the RHI strength across individuals. Taken together, these results suggest an analgesic effect of the RHI and support the potential usage of visual illusions in future translational research on pain.

Keywords: analgesic; body ownership; laser evoked pain; pain perception; rubber hand illusion.

Figures

FIGURE 1

Experimental setup (A) and task schematic diagram (B). (A) The laser point and five red points indicate six sites where pain stimulation is delivered to the subject’s real hidden hand. (B) A trial starts with an LED flash (∼0.5 s), immediately after which, continuous stroking begins. One minute after the onset of stroking, there is a second LED flash (∼0.5 s) indicating an upcoming pain stimulus. About 2 s after the second flash, stroking stops, and about 1 s later, a laser stimulus is delivered to the subject’s real hand. The trial ends after the subject rates pain intensity induced by the laser stimulus.

FIGURE 2

The RHI strength in subject screening. (A) Rating scores on all nine questions in three stroking conditions. Error bars represent SEMs across subjects. All nine questions can be classified into two categories: illusion questions (Q1–Q3) and control questions (Q4–Q9). (B) Averaged rating scores on illusion questions and control questions in each stroking condition. RHI strength scores are higher in the synchronous condition than the other two conditions. These results suggest that our stroking manipulation indeed produces significant RHI effects in the synchronous condition but not in the other two conditions.

FIGURE 3

Attenuation by the RHI on perceived pain intensity and unpleasantness. (A) Rating scores of the RHI strength in the main experiment. Results indicate that three stroking conditions indeed produce different levels of the RHI. (B) Rating scores of pain intensity and unpleasantness in the three stroking conditions. Both scores reflect the level of perceived pain, which is lower in the synchronous condition compared to the other two conditions. For both (A,B), significant symbol conventions are ∗p < 0.05; ∗∗p < 0.01. (C) Pain intensity reduction index reflects the strength of the analgesic effect in the synchronous condition compared to the asynchronous condition. A larger value represents a stronger analgesic effect (see section “Materials and Methods”). Pain intensity reduction indices of individual subjects are significantly correlated with their illusion strength indices (r = 0.42, p = 0.040), which reflect the difference of the RHI strength in two stroking conditions (see section “Materials and Methods”). (D) Pain unpleasant reduction indices and illusion strength indices show a trend in correlation analysis at the individual level (r = 0.33, p = 0.11) similar to the analysis of pain intensity. For both (C,D), the shaded area represents the 95% confidence interval for correlations.