[Can we see pain?]

Pain is a highly subjective experience that is difficult to measure objectively due to its varied expression. It is defined as a complex sensory-emotional experience, it is modulated by cognitive factors and involves a broad neural system. Functional neuroimaging has helped to define that neural circuit involved in the perception, modulation and response to painful experience, both in healthy controls and in patients with acute and chronic pain disorders. However, functional activation of the so-called "pain matrix" may also be differentially modulated by sensory and emotional processing components. The latter, for example, can influence the intensity to which a stimulus is perceived as painful. Such a threshold seems to be lower in patients with clinical diagnosis of fibromyalgia (FM) and has been linked to an abnormal pattern of activation of the "pain matrix" when assessed with functional magnetic resonance imaging (fMRI), considering a "syndrome of central susceptibility". Supporting an aetiological explanation for FM, studies have noted that a significant proportion of patients with FM demonstrate this abnormal pattern of activation to stimuli of low intensity. Additionally, there is an important and significant temporal dimension to this activation pattern observed in FM patients, where areas commonly associated with the emotional experience of pain show a prolonged response to painful stimuli compared to healthy subjects. Accordingly, fMRI may assist in objectifying the experience of pain in patients with FM in response to nociceptive stimulation.