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The physiological basis of the effects of intermittent theta burst stimulation of the human motor cortex
Theta burst stimulation (TBS) is a form of repetitive transcranial magnetic stimulation (TMS). When applied to motor cortex it leads to after-effects on corticospinal and corticocortical excitability that may reflect LTP/LTD-like synaptic effects. An inhibitory form of TBS (continuous, cTBS) suppres...
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Published in: | The Journal of physiology 2008-08, Vol.586 (16), p.3871-3879 |
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Main Authors: | , , , , , , , , , , |
Format: | Article |
Language: | English |
Subjects: | |
Citations: | Items that this one cites Items that cite this one |
Online Access: | Get full text |
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Summary: | Theta burst stimulation (TBS) is a form of repetitive transcranial magnetic stimulation (TMS). When applied to motor cortex
it leads to after-effects on corticospinal and corticocortical excitability that may reflect LTP/LTD-like synaptic effects.
An inhibitory form of TBS (continuous, cTBS) suppresses MEPs, and spinal epidural recordings show this is due to suppression
of the I1 volley evoked by TMS. Here we investigate whether the excitatory form of TBS (intermittent, iTBS) affects the same
I-wave circuitry. We recorded corticospinal volleys evoked by single pulse TMS of the motor cortex before and after iTBS in
three conscious patients who had an electrode implanted in the cervical epidural space for the control of pain. As in healthy
subjects, iTBS increased MEPs, and this was accompanied by a significant increase in the amplitude of later I-waves, but not
the I1 wave. In two of the patients we tested the excitability of the contralateral cortex and found a significant suppression
of the late I-waves. The extent of the changes varied between the three patients, as did their age. To investigate whether
age might be a significant contributor to the variability we examined the effect of iTBS on MEPs in 18 healthy subjects. iTBS
facilitated MEPs evoked by TMS of the conditioned hemisphere and suppressed MEPs evoked by stimulation of the contralateral
hemisphere. There was a slight but non-significant decline in MEP facilitation with age, suggesting that interindividual variability
was more important than age in explaining our data. In a subgroup of 10 subjects we found that iTBS had no effect on the duration
of the ipsilateral silent period suggesting that the reduction in contralateral MEPs was not due to an increase in ongoing
transcallosal inhibition. In conclusion, iTBS affects the excitability of excitatory synaptic inputs to pyramidal tract neurones
that are recruited by a TMS pulse, both in the stimulated hemisphere and in the contralateral hemisphere. However the circuits
affected differ from those influenced by the inhibitory, cTBS, protocol. The implication is that cTBS and iTBS may have different
therapeutic targets. |
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ISSN: | 0022-3751 1469-7793 |
DOI: | 10.1113/jphysiol.2008.152736 |