Sensory Disturbances, but Not Motor Disturbances, Induced by Sensorimotor Conflicts Are Increased in the Presence of Acute Pain

Abstract

Incongruence between our motor intention and the sensory feedback of the action (sensorimotor conflict) induces abnormalities in sensory perception in various chronic pain populations, and to a lesser extent in pain-free individuals. The aim of this study was to simultaneously investigate sensory and motor disturbances evoked by sensorimotor conflicts, as well as to assess how they are influenced by the presence of acute pain. It was hypothesized that both sensory and motor disturbances would be increased in presence of pain, which would suggest that pain makes body representations less robust. Thirty healthy participants realized cyclic asymmetric movements of flexion-extension with both upper limbs in a robotized system combined to a 2D virtual environment. The virtual environment provided a visual feedback (VF) about movements that was either congruent or incongruent, while the robotized system precisely measured motor performance (characterized by bilateral amplitude asymmetry and medio-lateral drift). Changes in sensory perception were assessed with a questionnaire after each trial. The effect of pain (induced with capsaicin) was compared to three control conditions (no somatosensory stimulation, tactile distraction and proprioceptive masking). Results showed that while both sensory and motor disturbances were induced by sensorimotor conflicts, only sensory disturbances were enhanced during pain condition comparatively to the three control conditions. This increase did not statistically differ across VF conditions (congruent or incongruent). Interestingly however, the types of sensations evoked by the conflict in the presence of pain (changes in intensity of pain or discomfort, changes in temperature or impression of a missing limb) were different than those evoked by the conflict alone (loss of control, peculiarity and the perception of having an extra limb). Finally, results showed no relationship between the amount of motor and sensory disturbances evoked in a given individual. Contrary to what was hypothesized, acute pain does not appear to make people more sensitive to the conflict itself, but rather impacts on the type and amount of sensory disturbances that they experienced in response to that conflict. Moreover, the results suggest that some sensorimotor integration processes remain intact in presence of acute pain, allowing us to maintain adaptive motor behavior.

Keywords: acute pain; body image; body schema; sensorimotor integration; virtual reality.

Figure 1

Experimental design. (A) The experiment was carried out on two experimental sessions separated by ~7 days for each participant. Each session comprised two blocks of trials, one block corresponded to one of the four somatosensory conditions. (B) Somatosensory conditions. Black and red rectangles indicate the site of application of vibrators or capsaicin cream, respectively.

Figure 2

Trial timeline and visual conditions. The participant saw exclusively the virtual upper limbs (Step 1 and 2) as well as the red targets (Step 1). Blue lines depict the real position of the upper limbs. The size and the center of rotation of virtual upper limbs were adjusted to correspond to the real upper limbs of the participant. In Step 1, two red targets were alternating in anti-phase at 1.25 Hz and participants were instructed to reach successively toward the targets, in order to create a bilateral anti-phase movement. In Step 2, the red targets were disappearing and one of the four visual conditions depicted was presented, providing either congruent or incongruent visual feedback (VF) about the limb movement.

Figure 3

Experimental set up. Exoskeleton robot (A) and 2D virtual environment (B) are the 2 elements of the KINARM. The exoskeleton is fitted to the anthropometric characteristics of the participant. The virtual environment consists in the projection of virtual upper limbs on a semi-transparent mirror (47″) thanks to a television. Upper limbs rest on the exoskeleton under the semi-transparent mirror and are obstructed from the participant' view.

Figure 4

Average amount of sensory disturbances reported in each experimental condition. Error bars represent the standard error of the mean.

Figure 5

Number of individuals who reported at least one disturbance for a given item, reported as a function of the experimental condition.

Figure 6

Motor disturbances. (A) Individual data for two representative participants, in absence of somatosensory stimulation. Black circles and black dashed lines represent, respectively, targets and trajectory of ULs during the baseline phase (Step 1). Blue and red lines represent, respectively, the trajectory of the left and right ULs during the experimental phase (Congruent VF, No VF, Flipped VF or Mirror VF—Step 2). (B,C) Amplitude asymmetry between left and right ULs (B) and medio-lateral drift (C). A positive value indicates a degradation of motor performance relative to baseline. Error bars represent the standard error of the mean.

Figure 7

Motor and sensory disturbances induced by sensorimotor conflict in the No Stimulation condition. (A) represents the amount of sensory disturbances across participants for the three sensorimotor conflict conditions during No Stimulation condition. (B,C) compares the average medio-lateral drift (black bars) and amount of sensory disturbances (red circles) between groups with Minimal vs. High sensory disturbances, in the No VF and Flipped VF conditions, respectively. (D) compares the average amplitude asymmetry (black bars) and amount of sensory disturbances (red circles) between groups with Minimal vs. High sensory disturbances in the Mirror VF condition. Error bars represent the standard error of the mean. P-values are reported only for motor disturbances (as groups were formed based on amount of sensory disturbances).