Introduction:
Chronic Pain (CP) presents a complex and prevalent issue that significantly affects individuals and society. Exploring the complexities of CP involves analyzing Functional Connectivity (FC), a process that identifies how different brain regions communicate across distances. Magnetoencephalography (MEG) is particularly effective for FC analysis, offering advantages over Electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) due to its superior temporal resolution. Most studies on FC in CP have focused on resting-state analyses, leaving a gap in research on connectivity responses to noxious stimuli in CP.

Study aim:
The overarching goal of my exploring study is to investigate FC differences in response to noxious stimuli between individuals with CP and Healthy Controls (HCs) across different frequency bands, using MEG. This encompasses the comparison of FC patterns within pain-related brain regions between these two groups, the analysis of their response to a noxious stimulus, and the synthesis of these findings to identify potential differences in how the two groups respond to noxious stimuli.

Methods:
The study involved 17 individuals with CP and 17 HCs, each undergoing MEG sessions within a conditioned pain modulation (CPM) paradigm. During each CPM block, 22 noxious stimuli were applied to the right tibial nerve. FC was computed between pain-processing regions using phase and amplitude-based metrics in different frequency bands. Connectivity patterns were compared between the groups using a non-parametric permutation test. Connectivity was also evaluated on a time-scale to observe potential changes in the FC in response to the stimulus. These results were taken together to observe potential differences in the groups in response to the stimulus.

Results:
In comparing FC patterns across the entire epoch between the HC and CP groups, there is a predominant observation of increased FC in the CP group relative to the HC group. The insula and Dorsolateral Prefrontal Cortex (DLPFC) emerged as central hubs, and these alterations were most prominent in the beta (13-29 Hz) and gamma-low bands (30-45 Hz). An increase in FC in the mean response over all scout pairs and both groups was observed immediately following the stimulus, particularly in the theta band (5-7 Hz). Additionally, in investigating the specific hypothesis that there may be distinct FC responses to noxious stimuli between the HC and CP group, the findings indicate subtle differences rather than clear, pronounced patterns, with findings in the theta, alpha and gamma-low bands.

Conclusion:
My study explored FC differences in response to noxious stimuli between individuals with CP and HCs across different frequency bands, using MEG. Higher FC was predominantly observed in the CP group, suggesting more interconnected pain-processing networks. Key regions demonstrating this increased FC included the insula and the DLPFC, suggesting an altered insula-DLPFC network potentially influenced by underlying physiological factors of the CP group. Specifically examining differences in FC response to the noxious stimulus between the HC and the CP group yielded in subtle differences rather than clear, distinct patterns. This study stands out as the first using MEG to identify FC in CP in response to noxious stimuli. Future research should focus on refining connectivity as a biomarker for treatment follow-up and potential outcome predictor.

Introduction
Chronic pain is an increasing problem in terms of prevalence and disease-related costs. Due to its complexity, it is difficult to treat. Spinal cord stimulation (SCS) is a neurostimulation therapy with a relatively good success rate for patients with severe, intractable chronic pain. The mechanisms of action (MOAs) of SCS are considered to rely on spinal and supraspinal mechanisms. It has been suggested that newer SCS paradigms, such as burst SCS, may act through different MOAs than the traditional tonic SCS paradigm. Tonic and burst SCS are both postulated to act on the lateral pain pathway, which is associated with the location and character of a stimulus, whereas burst SCS is postulated to additionally act on the medial pain pathway, which is associated with the emotional/attentional processing of a stimulus. Somatosensory evoked responses (SERs) can be used to evaluate the processing of somatosensory stimuli and may aid in the unraveling of the MOAs of SCS.

Aim
The aim of this thesis is to assess how burst and tonic SCS affect the supraspinal SERs elicited by non-painful transcutaneous electrical stimulation of the tibial nerve as well as of the median nerve. The two distinct SERs are evaluated using magnetoencephalography (MEG).

Methods
26 chronic pain patients treated with SCS underwent MEG sessions after receiving tonic and burst SCS for one week. Four of these patients additionally underwent a MEG session before SCS treatment. During each session, approximately 200 non-painful electrical stimuli were applied to the median nerve as well as to the tibial nerve to elicit SERs. The SERs were compared in various cortical and subcortical regions of interest (ROIs). The following comparisons were made: 1) SERs in chronic pain patients before SCS implantation versus SERs in the same individuals during SCS, 2) SERs elicited by tibial nerve stimulation versus SERs elicited by median nerve stimulation, 3) SERs during tonic SCS versus SERs during burst SCS, and 4) the SERs in four case studies of two good and two poor responders to the tonic and/or burst SCS paradigms.

Results
22 patients were included for analysis. The number of patients varied among comparisons to facilitate within-patient comparisons. The results suggested an inhibitory effect of SCS on the SER elicited by tibial nerve stimulation, whereas the amplitude of the SER elicited by median nerve stimulation tended to increase during SCS. For both the SERs elicited by tibial nerve and by median nerve stimulation, the SER amplitudes were predominantly higher during burst SCS compared to tonic SCS. Differences in SER amplitude that were observed in the case studies did not correlate with pain relief.

Conclusion
The results suggested a spinal MOA of SCS on the SER, however, supraspinal MOAs likely play a role as well. The results did not suggest that burst SCS additionally acts on the emotional/attentional processing compared to tonic SCS. No evidence was found to support a correlation between the effect of SCS on the SER and the effect of SCS on the pain, underscoring the complexity of the relationship between somatosensory processing and pain perception in the context of SCS.

Introduction: Spinal Cord Stimulation (SCS) is a successful last-resort treatment for chronic pain patients, although its exact mechanisms of action (MOAs) still need to be unraveled. The MOAs of SCS partly rely on spinal mechanisms (gate control theory) and supraspinal mechanisms likely play a role as well. Pain processing involves a complex network of cortical structures and can be modulated. Conditioned pain modulation (CPM) is a measure to describe the modulation of pain perception. CPM relies on the 'pain inhibits pain' theory, where a nociceptive test stimulus (TS) is modulated by applying a nociceptive conditioning stimulus (CS). CPM is often less efficient in chronic pain patients, but this might be improved by effective SCS.
Objective: The aim of this thesis is to assess how effective SCS affects the supraspinal mechanisms of pain modulation in chronic pain patients. The supraspinal mechanisms are evaluated using magnetoencephalography (MEG) to assess the cortical response to TS before, during and after CPM in chronic pain patients treated with SCS.

Methods: Chronic pain patients treated with SCS underwent MEG sessions after receiving tonic and burst SCS for one week. Each session consisted of three CPM blocks: before, during and after CPM. During each CPM block 22 TS were administered accompanied by CS during CPM, after each block the subject reported a subjective pain rating of TS (and CS). The cortical response to TS, measured using MEG, was analyzed in the time and time-frequency (TF) domain using Brainstorm and Matlab software. TF decomposition was computed in several pain related regions of interest using complex Morlet wavelets. We examined how CPM affected event related spectral perturbations, induced by TS, in the alpha (8-12 Hz) and beta (13-30 Hz) frequency ranges. The average cortical response during tonic and burst SCS was evaluated in all subjects, and separately in the five clearest SCS responders (effective) and the five clearest SCS non-responders (non-effective).
Results: 17 subjects were included. On average a decrease in subjective pain ratings of TS was observed during CPM. In the time domain, TS evoked activity in areas related to the sensory-discriminative aspect of pain. In the TF domain, TS induced event related desynchronization (ERD) followed by event related synchronization (ERS) in the alpha and beta frequency ranges in the bilateral sensorimotor cortices. During CPM the power of beta ERS was significantly decreased during burst SCS. A trend was observed towards a decrease in power of beta ERS for the effective burst SCS group, whereas no decrease was observed for the non-effective burst, effective tonic and non-effective tonic SCS groups.
Conclusion: The results suggest that on average, effective burst SCS decreases the power of beta ERS during CPM, this decrease indicates successful modulation of pain. Therefore, I suggest that effective burst SCS is capable of improving the supraspinal mechanisms of pain modulation, whereas effective tonic SCS is not capable of doing so. This suggestion indicates a partially different MOA for tonic and burst SCS. Future studies containing larger group sizes should validate these findings.