Horizontal body position reduces cortical pain-related processing: evidence from late ERPs

Abstract

The present study investigated the influence of short-term horizontal body position on pain-related somatosensory processing, by measuring subjective and cortical responses to electrical pain stimulation. Twenty-eight healthy women were randomly assigned to either the experimental horizontal group (Bed Rest, BR) or to the sitting control group (Sitting Control, SC). After 90 minutes in either horizontal or sitting position, the individual pain thresholds were assessed and EEG/self-evaluations recorded during the administration of 180 stimuli delivered to the left forearm. Electrical pain stimuli, calibrated to subjects' individual pain thresholds, consisted of two different intensity levels: no pain (40% below pain threshold) and pain (40% above pain threshold). Compared with control, BR condition significantly inhibited subjective sensitivity to painless stimuli, whereas electrophysiological results pointed to a reduced slow cortical wave (interval: 300-600 ms) at all stimulus intensities, and smaller amplitude in BR's right vs. left prefrontal sites. sLORETA analysis revealed that cortical responses were associated with a decreased activation of superior frontal gyrus and anterior cingulate cortex (BA 6/24). Interestingly, BR group only showed significant negative correlations between self-evaluation of painful intensities and frontal cortical negativity, revealing increasingly differentiated responses in bed rest: indeed those BR participants who reported lower pain ratings, displayed reduced negativity within anterior regions. Taken together, results indicate that short-term horizontal position is able to inhibit a fronto-parietal pain network, particularly at the level of central prefrontal regions typically involved in cognitive, affective and motor aspects of pain processing.

Figure 1. Analysis of subjective pain evaluation: significant two-way Group (BR vs. SC) by Intensity (no-pain vs. pain) interaction.

Mean ratings and Standard Error (SE) are depicted for Bed Rest group (grey bars) and Controls (black bars).

Figure 2. ERP waveforms from the four clusters of electrodes, including the experimental BR (grey line) and the SC control group (black line), in the no-pain and pain conditions.

Time-scale is from -100 to 700 ms. Negativity is displayed upward.

Figure 3. Analysis of the late potentials measured in the 300- to 600-ms epoch after electrical stimuli: significant three-way Group (BR vs. SC) by Caudality (Anterior vs. Posterior regions) by Laterality interaction (Left vs. Right Hemisphere; LH vs. RH).

Mean activity and Standard Error (SE) are depicted for Bed Rest group (grey lines and bars) and Sitting Controls (black lines and bars).

Figure 4. Source localization computed with sLORETA in the 300- to 600-ms epoch: BR group, compared with SC, showed decreased activations of superior frontal gyrus/ACC only in pain condition.

Figure 5. Pearson’s correlations, for the painful condition, between subjective evaluations and the amplitude of the late potentials (300-600-ms epoch) at anterior clusters in the BR and SC groups.