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6. Electrical network mechanisms of responsiveness to continuous theta burst magnetic stimulation (cTBS)

Clinical Neurophysiology
DOI: 10.1016/j.clinph.2012.03.070
  • Medicine


Introduction Continuous (inhibitory) TBS can induce transitory neglect-like visual exploration behaviour in healthy participants when applied to the right posterior parietal cortex (PPC) (Nyffeler et al., 2008). However, the mechanisms of this effect remain poorly understood. In particular, it is unknown how cTBS affects the network organization of the brain and how network reorganization influences behavioural effects of cTBS. This study therefore aimed to investigate changes in electrical network interactions associated with behavioural responsiveness to cTBS. Methods Nine young healthy subjects underwent three sessions of cTBS (30Hz bursts applied at intervals of 100ms), separated by at least 1week. Stimulation of the right PPC was compared to sham stimulation and to stimulation of the frontal eye field (FEF), in counterbalanced order. Before and after stimulation, visual exploration of images was recorded with an eye tracking device, and high-resolution resting-state EEG recordings were obtained. The neural oscillations at each grey matter voxel were reconstructed using an adaptive spatial filter. The functional connectivity (FC) of each voxel with the rest of the brain was calculated and correlated with left-sided visual exploration behaviour. Results PPC but not sham or FEF stimulation induced neglect-like visual exploration behaviour in the majority, but not in all subjects (Fig. 1). Before stimulation, left-sided visual exploration was highly linearly correlated with the magnitude of alpha-band FC between the right temporo–parietal junction (TPJ) and the rest of the brain (Fig. 2, top). Inhibitory cTBS of the right PPC induced an increase of FC in the contralateral left temporo–parietal cortex at about 10, 21, and 30Hz, which correlated with the induced neglect-like effect (Fig. 2, middle). The behavioural effect of cTBS over the right PPC could be predicted by FC magnitude in the right TPJ before stimulation: only subjects with high alpha-band FC in the stimulated area showed the expected neglect-like effect. Conversely, inhibitory cTBS applied to subjects whose right TPJ was already poorly connected before stimulation induced the inverse effect of more left-sided exploration (Fig. 2, bottom). Conclusion Behavioural effects of cTBS are associated with changes in electrical network interactions at the stimulation frequencies. Effects are variable across subjects. An important proportion of this variability seems to depend on, and can be predicted by, network states before stimulation. Thus, FC analyses based on EEG might help optimize therapeutic cTBS in the future.

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