Date Published: June 5, 2019
Publisher: Public Library of Science
Author(s): Irene Peláez, David Ferrera, Paloma Barjola, Roberto Fernandes, Francisco Mercado, Thilo Kellermann.
Pain experience involves a complex relationship between sensory and both emotional and cognitive factors, which appear to be mediated by different neural pathways. Previous evidence has shown that whereas conscious processing of unpleasant stimuli enhances pain perception, the influence of emotions on pain under unaware conditions is much less known. The need to better characterise the relationship between pain processing and emotional factors is crucial for dealing with chronic pain conditions. Therefore, the present study aimed to explore the neural correlates relating to the influence of visual masking emotional stimulation on the processing of painful stimuli in chronic pain patients suffering from fibromyalgia (FM). Twenty FM and 22 healthy control (HC) women participated in the study. The experimental masking paradigm consisted of a rapid succession of two types of stimuli, where a masked picture (neutral, negative or pain-related) was followed by a laser stimulus (painful or not painful). LEP activity was recorded at sixty scalp electrodes. An LEP-amplitude approach was used to quantify the main cerebral waves linked to pain response. ANOVAs indicated that the posterior regions of the P1 component were sensitive to experimental manipulation (p<0.05). Specifically, FM patients showed higher amplitudes to painful stimuli preceded by pain-related pictures compared with painful trials preceded by other emotional pictures. The FM group also showed greater amplitudes than those in the HC group in P2a and P2b waves. In addition to the scalp data, at the neural level the posterior cingulate cortex, lingual gyrus and insular cortex showed higher activation in the FM group than in the HC group. Our findings show an early cerebral modulation of pain (as reflected by the P1) in FM patients, suggesting that only pain-related information, even when it is unconsciously perceived, is capable to enhance exogenous (automatic) attention, increasing the neural activity involved in processing painful stimulation. Further research is needed to fully understand unconscious emotional influences on pain in fibromyalgia.
Pain is generally understood as a subjective and unique experience, usually accompanied by an unpleasant emotional experience . In healthy states, the main function of the pain processing system is to either prevent or deal with physical harm. However, during chronic pain conditions, as in the case of fibromyalgia, pain loses its protective meaning . Although, the origin of this syndrome is still unknown and under debate, growing scientific evidence has suggested that its symptomatology is mediated by central pain-processing mechanisms [3–5], leading to the appearance of widespread and diffuse musculoskeletal pain as the main symptom [3,6]. Such dysfunctional regulation of pain could be modulated by affective and cognitive factors, in particular those relating to negative emotions or pain-related information [7,8].
The aim of the present study was to explore neural correlates relating to the influence of visual masking emotional stimulation on pain processing in fibromyalgia patients. Brain activity results showed that the emotional charge conveyed by subliminal pictures modulated pain responses in these patients. In particular, masking pain-related pictures enhanced pain processing, as reflected in higher P1 amplitudes. Additionally, activity within occipital and limbic regions (the lingual gyrus and PCC) showed higher activation in the fibromyalgia group compared with the healthy control group. In accordance with the previous literature [44,62], our LEP analysis showed the presence of a classical N2-P2 complex in response to CO2 laser stimulation. P2a and P2b presented greater amplitudes for fibromyalgia patients than for healthy controls. Furthermore, P2b amplitude was sensitive to the intensity of laser stimulation, being higher for painful than for painless events. In addition to the LEP data, the activity detected within the insular cortex, postcentral gyrus and posterior cingulum was also strongest for pain conditions and fibromyalgia patients, compared with the other stimuli and the control group, respectively. However, we did not observe any behavioural effect relating to the emotional modulation of pain. A careful interpretation of our results is given as follows.