Date Published: June 29, 2012
Publisher: Public Library of Science
Author(s): Sam C. C. Chan, Chetwyn C. H. Chan, Anne S. K. Kwan, Kin-hung Ting, Tak-yi Chui, Andreas Keil. http://doi.org/10.1371/journal.pone.0040215
Research has shown that people with chronic pain have difficulty directing their attention away from pain. A mental strategy that incorporates focused attention and distraction has been found to modulate the perception of pain intensity. That strategy involves placing attention on the nociceptive stimulus felt and shifting attention to a self-generated sub-nociceptive image and rehearsing it. Event-related potential was used to study the possible processes associated with the focus-then-orient strategy.
Eighteen pain-free participants received different levels of 50-ms nociceptive stimulations elicited by electric shocks at the right lateral malleolus (ankle). In perception trials, participants maintained the perceived nociceptive stimulus in working memory for 3,000 ms. In imagery trials, participants mentally generated and maintained the corresponding sub-nociceptive image they had learned previously. After both types of trials, participants evaluated the pain intensity of the incoming stimulus by recalling the feeling of the nociceptive stimulation at the beginning of the trial.
Shifting attention from the incoming nociceptive to a self-generated sub-nociceptive image elicited central P2 and centro-parietal P3 waves, which were found to correlate with proportional scores on the Stroop Test. They were followed by a frontal N400 and a parietal P600, denoting generation of sub-nociceptive images in working memory. The voltages elicited in these potentials correlated moderately with attenuation of the pain ratings of the recalled nociceptive stimulations.
Focus-and-orient attention across nociceptive and sub-nociceptive images appears to be related to response inhibition. Mental rehearsal of the sub-nociceptive images was found to modulate the perception of the nociceptive sensation felt prior to the imagery. Such modulation seems to be mediated by generating and maintaining sub-nociceptive images in working memory. Future studies should explore the mental processes associated with orienting attention for pain modulation among people with pathological pain and frontal lobe dysfunction.
The somatosensory cortices process afferent nociceptive sensation. Researchers have found that the lateral pathway to the somatosensory cortices mediates feelings of pain intensity, while the medial pathway to the limbic system mediates the affect resulting from that pain , . When people down-regulate the sensation of pain, the central nervous system exerts efferent inhibitory control. This control is mediated by the periaqueductal grey, situated in the midbrain. Wiech and colleagues  found that conscious down-regulation was associated with activity in the so-called “pain control center” located in the dorsolateral prefrontal cortex.