Abstract
Theta burst stimulation (TBS) has been used as a tool to induce synaptic plasticity and improve neurological disorders. However, there is high interindividual variability in the magnitude of the plastic changes observed after TBS, which hinders its clinical applications. The electric field induced by transcranial magnetic stimulation (TMS) is strongly affected by the depth of the stimulated brain region. Therefore, it is possible that the variability in the response to TBS over the lower-limb motor cortex is different for the hand area. This study investigated the variability of TBS-induced synaptic plasticity in the lower-limb motor cortex, for intermittent TBS (iTBS), continuous TBS (cTBS), and sham iTBS, in 48 healthy young participants. The motor cortical and intracortical excitability of the tibialis anterior was tested before and after TBS using TMS. The results showed that iTBS had facilitatory effects on motor cortex excitability and intracortical inhibition, whereas cTBS exerted opposite effects. Twenty-seven percent of individuals exhibited enhanced motor cortical plasticity after iTBS, whereas 63% of participants showed enhanced plasticity after cTBS. In addition, the amount of TBS-induced plasticity was correlated with the intracortical excitability and the variability of the motor evoked potential prior to TBS. Our study demonstrated the high variability of the iTBS-induced lower-limb motor cortical plasticity, which was affected by the sensitivity of intracortical interneuronal circuits. These findings provide further insights into the variation of the response to TBS according to the anatomy of the stimulated brain region and the excitability of the intracortical circuit.
Highlights
Theta burst stimulation (TBS) is a non-invasive brain stimulation (NIBS) that can modulate the cortical excitability of the human brain depending on the pulse frequency (Huang et al, 2005)
No significant differences were recorded at Post-45. These results indicate that Intermittent TBS (iTBS) and continuous TBS (cTBS) have the potential to modulate the lower-limb corticospinal excitability
The current study provides evidence of the induction of an increase in corticospinal excitability after the application of iTBS to the lower-limb motor cortex
Summary
Theta burst stimulation (TBS) is a non-invasive brain stimulation (NIBS) that can modulate the cortical excitability of the human brain depending on the pulse frequency (Huang et al, 2005). Previous reviews showed that these TBS-induced effects of motor cortex excitability are known as long-term potentiation (LTP)-like and long-term depression (LTD)-like synaptic plasticity, respectively (Suppa et al, 2016; Huang et al, 2017). The inter- and intra-individual variability of cortical plasticity after TBS is a key issue of this tool (Hamada et al, 2013; Hinder et al, 2014; López-Alonso et al, 2014; Hordacre et al, 2017; Sasaki et al, 2018). The variability of the TBS-induced plasticity hinders its clinical applications as a potential therapy for neurological disorders (Terranova et al, 2019). Several factors contribute to the variation that occurs in response to TBS, such as biological factors including age, genetics, sex, and anatomy of the neural circuits (Suppa et al, 2016; Huang et al, 2017)
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