Changes in perceived peripersonal space following the rubber hand illusion

Abstract

Peripersonal space (PPS), the region immediately surrounding the body is essential for bodily protection and goal directed action. Previous studies have suggested that the PPS is anchored to one's own body and in the current study we investigated whether the PPS could be modulated by changes in perceived body ownership. While theoretically important, this anchoring can also have implications for patients with altered body perception. The rubber hand illusion (RHI) is a way to manipulate body ownership. We hypothesized that after induction of a left hand RHI, the perceived space around the body shifts to the right. Sixty-five participants performed a landmark task before and after a left hand RHI. In the landmark task, participants had to determine whether a vertical landmark line was left or right from the center of a horizontal screen. One group of the participants was exposed to synchronous stroking, the other group experienced asynchronous stroking. Results showed a shift in space to the right (e.g. away from the own arm), but only for the 'synchronous stroking' group. These results suggest that the relevant action space becomes linked to the fake hand. Critically, subjective ownership experience did not correlate with this shift, but proprioceptive drift did. This suggests that multisensory integration of bodily information drives this shift in space around the body and not feelings of ownership.

Figure 1

Timeline of the design: From left to right: SAP = straight ahead pointing, PD = proprioceptive drift, LM = Landmark, RHI = Rubber Hand Illusion, SG = synchronous stroking group, AG = asynchronous stroking group, 1 indicates the pre-measure, 2 indicated the post-measure. See Task/Stimuli for details of the tasks.

Figure 2

(A) Experimental set-up (not drawn to scale) and dimensions for the landmark task, top = experimenter, bottom = participant, one trial of the landmark is shown. (B) Set-up of proprioceptive drift with an occluder covering the lower arms (C) All possible landmarks (not drawn to scale). Only one of these landmarks was shown each trial. Each trial started with a static dot (either left or right from the center of the screen) that disappeared when the landmark appeared. (D). Hand positioning during the rubber hand illusion. Note that the dotted (real) arm was occluded by a black occluder. Only the added left rubber hand and the real right hand were visible to the participant.

Figure 3

Boxplot of the data on the ownership questionnaire (see text for details). The panel shows the average score on the ownership scale (question 1–3) and control scale (question 4–10) for the Synchronous stroking Group and Asynchronous stroking Group (including individual scores). Error bars represent the 95% confidence intervals. (see also Supplementary Table 1).

Figure 4

Average estimates in cm of proprioceptive localisation (i.e. difference between pre- and post-illusion) for the synchronous stroking and asynchronous stroking groups for the left index finger. Error bars represent standard error of the mean (see also Supplementary Table 1).

Figure 5

Average shifts in point of subjective equality (PSE) on the landmark task (i.e. difference between pre- and post-illusion) for the synchronous stroking and asynchronous stroking groups. The PSE is depicted in mm for convenience but has been analyzed in pixels. The error bars depict the standard error of the mean (see also Supplementary Table 1).

Figure 6

Average shifts in pointing straight ahead (i.e. difference between pre- and post-illusion) for the synchronous stroking and asynchronous stroking groups. The error bars depict the standard error of the mean (see also Supplementary Table 1).