Effect of painful and non-painful sensorimotor manipulations on subjective body midline

Abstract

Patients with chronic pain often show disturbances in their body perception. Understanding the exact role played by pain is however complex, as confounding factors can contribute to the observed deficits in these clinical populations. To address this question, acute experimental pain was used to test the effect of lateralized pain on body perception in healthy subjects. Subjects were asked to indicate the position of their body midline (subjective body midline, SBM) by stopping a moving luminescent dot projected on a screen placed in front of them, in a completely dark environment. The effect of other non-painful sensorimotor manipulations was also tested to assess the potential unspecific attentional effects of stimulating one side of the body. SBM judgment was made in 17 volunteers under control and three experimental conditions: (1) painful (heat) stimulation; (2) non-painful vibrotactile stimulation; and (3) muscle contraction. The effects of the stimulated side and the type of trial (control vs. experimental condition), were tested separately for each condition with a 2 × 2 repeated measures ANOVA. The analyses revealed a significant interaction in both pain (p = 0.05) and vibration conditions (p = 0.04). Post hoc tests showed opposite effects of pain and vibration. Pain applied on the right arm deviated the SBM toward the right (stimulated) side (p = 0.03) while vibration applied on the left arm deviated the SBM toward the right (not stimulated) side (p = 0.01). These opposite patterns suggest that the shift in SBM is likely to be specifically linked to the stimulation modality. It is concluded that acute experimental pain can induce an SBM shift toward the stimulated side, which might be functionally beneficial to protect the painful area of the body. Interestingly, it appears to be easier to bias SBM toward the right side, regardless of the modality and of the stimulated side.

Keywords: body perception; egocentric frame of reference; neglect; neuropathic pain; spatial attention.

Figure 1

Experimental set-up used to test the subjective body midline. Note that the screen on which the laser was projected was curved to ensure that the speed of the projected laser dot remained constant.

Figure 2

Group data for each modality are presented. Data are normalized to their respective controls (see Materials and Methods). Negative values indicate a shift of the SBM position toward the left and conversely, a positive value indicates a shift toward the right. Error bars indicate the standard deviation on the mean normalized data. *Indicates a statistically significant difference between the test SBM (e.g., with pain/vibration/contraction) and the control SBM measured in the same block.

Figure 3

Individual data for all subjects are shown for the right pain condition (A) and the left vibration condition (B). In addition, data from one subject in the right pain condition (C) and the left vibration (D) are shown. Data in C and D were each obtained in a single block, and each dot represents a single trial. “X” indicates trials in the control condition (i.e., no stimulation, trials 1–6 of a given block), gray circles in right pain condition, and black squares in left vibration condition (respectively trials 7–16 of a given block). All data are normalized against the average SBM obtained in the control condition.