Effects of High-frequency RTMS on Brain Connectivity

Abnormal connectivity patterns interfere with the normal function of a given neuronal network, thus leading to circuit dysfunction and, subsequently, chronic pain. In the last few years, neuroscience has been heavily influenced by network science. This synergistic association provided a new framework for understanding brain function in health and how dysfunction in specific neuronal brain circuits can lead to symptoms. A network comprises nodes (e.g., areas of the brain) and edges (functional connections between nodes). An effective network can process and share large amounts of information while maintaining specificity and not allowing noise to contaminate the flow of information across the circuits. The network approach to brain functioning has been able to integrate what has been known for several decades as spatial structural anatomy with the time-varying streaming of information (connectivity) in a dynamic perspective. In this context, symptoms of diseases are seen as being correlated with specific network abnormalities, and therapeutic interventions as being associated with the normalisation of these abnormal patterns of connection. Non-invasive neuromodulatory approaches, such as repetitive transcranial magnetic stimulation (rTMS), have entered the guidelines for the management of major depression refractory to pharmacological treatment (FDA clearance in 2008 - K061053) and several chronic pain conditions, providing low-adverse event, rapid-to-perform and safe non-pharmacological treatment possibilities for neuropsychiatric disorders. rTMS is based on the induction of a high-intensity magnetic field on the scalp and on the subsequent creation of an induced electric current that penetrates the skull and influences neuronal firing over a very restricted volume of the brain. Depending on stimulation parameters, rTMS also acts by facilitating or depressing the activity of specific brain networks non-invasively. Despite important advances in the use of rTMS, and similar to what is also obtained from pharmacological treatments, up to 40% of patients remain symptomatic after treatment. This study aims to investigate the responses of specific neuronal brain circuits to repetitive transcranial magnetic stimulation (rTMS) applied to the primary motor cortex in healthy volunteers. It has been hypothesised that the local-to-global connectivity pattern obtained by the stimulation of different cortical hubs (primary motor cortex, dorsolateral prefrontal cortex, anterior cingulate cortex and posterior insula) will be described by TMS-EEG responses in healthy individuals. The modification in cortical connectivity in response to a repetitive transcranial magnetic stimulation (rTMS) applied to the primary motor cortex will be described and compared with a sham rTMS stimulation.