TMS-Induced Modulation of EEG Functional Connectivity Is Affected by the E-Field Orientation

Abstract

Coregistration of transcranial magnetic stimulation (TMS) and electroencephalography (EEG) allows non-invasive probing of brain circuits: TMS induces brain activation due to the generation of a properly oriented focused electric field (E-field) using a coil placed on a selected position over the scalp, while EEG captures the effects of the stimulation on brain electrical activity. Moreover, the combination of these techniques allows the investigation of several brain properties, including brain functional connectivity. The choice of E-field parameters, such as intensity, orientation, and position, is crucial for eliciting cortex-specific effects. Here, we evaluated whether and how the spatial pattern, i.e., topography and strength of functional connectivity, is modulated by the stimulus orientation. We systematically altered the E-field orientation when stimulating the left pre-supplementary motor area and showed an increase of functional connectivity in areas associated with the primary motor cortex and an E-field orientation-specific modulation of functional connectivity intensity.

Exploring Motor Network Connectivity in State-Dependent Transcranial Magnetic Stimulation: A Proof-of-Concept Study

“…It should be noted that functional connectivity before the stimulation might be influenced by the previous pulse (or pulses) in a way that is still debated in the literature. Post-stimulation increases have been observed in Pieramico et al [35], while a reduction in global EEG connectivity was observed in the healthy subject's cohort of Vlachos et al [76]. Nevertheless, even if the observed Motor Network connectivity pattern was influenced by the previous stimulation, this still represents the actual connectivity state of the network, which we found to be related to the MEP amplitude.…”

“…In this framework, a brain state can be described as the evolving dynamics of one or more large-scale networks [31], including the so-called resting-state networks [32], that constrain ongoing activity in the absence of any externally imposed task. Several studies have investigated the effects of invasive and non-invasive stimulation on resting-state networks and, more in general, on remote regions connected to the stimulation site [33][34][35]. However, while the techniques for connectomic neuromodulation studies appear mature [36][37][38][39][40][41], scarce evidence has been provided, so far, for the impact of network dynamics on stimulation effects, explicitly using functional connectivity approaches [42].…”

Network perturbation-based biomarkers of depression and treatment response

“…Likewise, both local and remote EEG responses depend on the specific parameter settings, e.g., the exact coil position, orientation, and stimulation intensity, which define the induced electric field in the relevant brain tissue. 8 Currently, it is not clear which stimulation parameters are optimal for targeting and engaging a specific brain network. In this context, the current study provided evidence of a role of the DLPFC-OFC-HPC network in the pathophysiology of depression and its alleviation by rTMS.…”

Number of Trials and E-Field Orientation during Continuous Theta Burst Stimulation May Impact Modulation of Motor-Evoked Potentials

“…Varying the local EFO by e.g. rotating the TMS coil has been demonstrated to influence both the distribution of the cortical electric field as well as outcome parameters such as EEG-derived changes or motor-evoked potentials (MEPs) elicited in singlepulse protocols [17][18][19][20][21][22] . Aside from the shape and strength of the electric field itself, the heterogeneous shape and occurrence of different neuron populations across cortical layers contribute to the differential sensitivities regarding TMS applied at varying EFO 23 .…”