Focused attention modulating pain perception in people with chronic pain : an event-related potential study

Abstract

People with chronic pain were shown to have difficulty to direct attention away from pain as in distraction due to hypervigilance to pain. Focused attention, a mental strategy which directs individuals' focus on the objective aspects of pain, was found to be effective for down-regulating pain intensity among people with chronic pain. This study can extend our understanding of the mental processes underlying focused attention that modulates nociceptive perception. This study aimed (1) to investigate the neural processes associated with focused attention through imagery of sub-nociceptive sensation; (2) to examine modulation of pain perception due to sub-nociceptive imagery in pain-free subjects and chronic pain patients; (3) to investigate the neural processes associated with modulation of pain perception using sub-nociceptive imagery and compare these processes between pain-free subjects and chronic pain patients. Seventeen patients with chronic low back pain (mean age=41.53 years; pain history years=4.05 years) and eighteen pain-free subjects (mean age=35.78 years). After familiarization training on 5 levels of sub-nociceptive and nociceptive stimuli, the subjects were asked to participate in a perception/imagery experiment with concurrent 128-channel electroencephalogram recording. In the perception trials, the participants mentally maintained and rehearsed the nociceptive images. They then rated the recalled nociceptive images. In the imagery trials, the participants received the nociceptive stimulations, and mentally generated and rehearsed nociceptive images that had previously learnt. They were then to rate the recalled nociceptive images. Though no significant between-group differences were revealed by three-way repeated measures ANOVA, post-hoc t-test showed significant difference in pain normalized pain rating between two conditions in Level 2 pain in chronic pain group (t(16)=-2.208, p<0.050). Significant differences in normalized pain rating were found between two conditions in Levels 1-3 pain (t(17)=-2.630 to -3.223, p<0.050) in pain-free group. Two-way (midline sites) and three-way (lateral sites) repeated measures ANOVA showed more positive amplitudes in P2 (p<0.01), P3 (p<0.01] and P600 (p<0.01) and less negative N400 (p<0.01) in Imagery task among pain-free group. Three-way and four-way repeated measures ANOVA revealed significant between-group differences in midline and lateral electrode sites of P2, P3 and P600. Further analysis revealed that some chronic pain patients (n=6) ("respondents") showed an ability to attenuate pain when compared to the others (n=11) ("non-respondents"). ERP analyses confirmed that the respondent group has significantly larger amplitude of P2 component in the respondents. Behavioral data suggested that the magnitude of down-regulation was shown to be lessened in chronic pain group, reflecting hypervigilance to pain possibly due to plastic cortical changes. Neurophysiologically, it was shown that some chronic pain people were able to modulate pain perception using somatosensory imagery technique. Fronto-central P2 component was shown to be the key marker for successful focused attention to nociceptive stimulation that would lead to pain attenuation. The sub-nociceptive somatosensory image was generated, as reflected by frontal N400 component. The results support that the somatosensory imagery technique has the potential to be a therapeutic technique for some patients with chronic pain to down-regulate nociceptive perception.