Effect of Non-Invasive Neuromodulation on Primary Chronic Pain, Including Post-COVID-19

The aim of this study is to analyse whether there is an advantage in adding low-intensity electrical stimulations to the vagus nerve with electrodes in the outer ear to control pain and fatigue in people with primary chronic pain, and in people who have had COVID-19. Several studies have shown that many people with primary chronic pain and also people who have had COVID patients continue to have pain and fatigue that may be due to changes that occur in the brain and the vagus nerve that passes through the neck might cause inflammation. Therefore the aim of this study is to investigate if these electrical stimulations can help improve the condition of the brain and nerve and consequently, the inflammation so that these complaints improve. This study will be organized as the following: It will begin with an assessment, divided into a part with questionnaires and another with specific tests. The questionnaires are aimed at understanding volunteer's characteristics, pain, fatigue, inflammation, sleep, quality of life, and the presence of symptoms of anxiety and depression. Next, the volunteer will undergo two tests to assess the behavior of his/her brain. They are performed using EEG electroencephalography and transcranial magnetic stimulation (TMS). * Quantitative EEG at rest - In this technique, a cap with electrodes is placed on volunteer's head, in contact with hair, which are connected through a gel. The volunteer needs to remain still for 6 minutes with eyes closed, sitting in a comfortable chair. * TMS with EEG - First, the volunteer will be asked to honestly answer the questions in the safety questionnaire for using this technique. Honest answers will eliminate the risks associated with this evaluation. Next, the volunteer's skin will be cleaned with alcohol and a special paste and place three self-adhesive electrodes on the muscle of the volunteer's hand, in a muscle called the interosseous, on the side where he/she writes. These electrodes will only capture