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DOI: 10.1055/s-0034-1371201
Are individual responses to theta-burst rTMS in cortical excitability related to changes in motor network connectivity?
Intermittent theta-burst stimulation (iTBS) effectively increases cortical excitability within the human brain (Huang et al., 2005). However, individual after-effects of iTBS vary between subjects, with a large proportion not responding at all in terms of changes in excitability (Ridding and Ziemann, 2010; Hamada et al., 2013). We here investigated whether subjects responding to iTBS show differential changes in resting-state functional connectivity (rsFC) within the cortical motor system compared to subjects with no response.
14 healthy, right-handed subjects (m = 7, 27 ± 3 years) received iTBS over the left primary motor cortex (M1) on two days, separated by at least one week. Before and after iTBS-application (i) motor-evoked potentials (MEPs) or (ii) resting-state functional magnetic resonance imaging (fMRI) was assessed. Seed-based whole-brain rsFC was computed for the stimulated M1.
Subjects were divided into groups of responders and non-responders according to iTBS-induced changes in MEPs (criterion: increase of at least 10% compared to baseline). Following iTBS, rsFC between M1 and premotor areas was significantly higher in responders compared to both baseline (p ≤0.001, FWE-corrected) and non-responders (p ≤0.001, FWE-corrected). Furthermore, non-responders featured higher levels of pre-interventional rsFC compared to responders (p = 0.061, FWE-corrected). Individual changes in MEPs and rsFC did not correlate.
Significant iTBS-induced modulations of rsFC were exclusively found for the group of iTBS-responders, suggesting that changes in cortical excitability and motor network rsFC are related. However, no linear correlation between changes in MEP-amplitudes and rsFC was evident. Furthermore, higher levels of pre-interventional rsFC might prevent iTBS-induced strengthening of premotor-M1 connections, possibly underlying the non-responsiveness to iTBS.
References:
Hamada M, Murase N, Hasan A, Balaratnam M, Rothwell JC (2013) The role of interneuron networks in driving human motor cortical plasticity. Cereb Cortex 23:1593 – 1605.
Huang YZ, Edwards MJ, Rounis E, Bhatia KP, Rothwell JC (2005) Theta burst stimulation of the human motor cortex. Neuron 45:201 – 206.
Ridding MC, Ziemann U (2010) Determinants of the induction of cortical plasticity by non-invasive brain stimulation in healthy subjects. J Physiol 588:2291 – 2304.