Abstract
Neuronal plasticity is considered to be the neurophysiological correlate of learning and memory and changes in corticospinal excitability play a key role in the normal development of the central nervous system as well as in developmental disorders. In a previous study, it was shown that quadri-pulse theta burst stimulation (qTBS) can induce bidirectional changes in corticospinal excitability (1). There, a quadruple burst consisted of four single-sine-wave (SSW) pulses with a duration of 160 μs and inter-pulse intervals of 1.5 ms to match I-wave periodicity (666 Hz). In the present study, the pulse shape was modified applying double-sine-waves (DSW) rather than SSW pulses, while keeping the pulse duration at 160 μs. In two separate sessions, we reversed the current direction of the DSW pulse, so that its second component elicited either a mainly posterior-to-anterior (DSW PA-qTBS) or anterior-to-posterior (DSW AP-qTBS) directed current in the precentral gyrus. The after-effects of DSW qTBS on corticospinal excitability were examined in healthy individuals (n = 10) with single SSW TMS pulses. For single-pulse SSW TMS, the second component produced the same preferential current direction as DSW qTBS but had a suprathreshold intensity, thus eliciting motor evoked potentials (PA-MEP or AP-MEP). Single-pulse SSW TMS revealed bidirectional changes in corticospinal excitability after DSW qTBS, which depended on the preferentially induced current direction. DSW PA-qTBS at 666 Hz caused a stable increase in PA-MEP, whereas AP-qTBS at 666 Hz induced a transient decrease in AP-MEP. The sign of excitability following DSW qTBS at I-wave periodicity was opposite to the bidirectional changes after SSW qTBS. The results show that the pulse configuration and induced current direction determine the plasticity-effects of ultra-high frequency SSW and DSW qTBS at I-wave periodicity. These findings may offer new opportunities for short non-invasive brain stimulation protocols that are especially suited for stimulation in children and patients with neurological or neurodevelopmental disorders.
Highlights
Synaptic plasticity is considered to be the neurophysiological correlate of learning and memory and changes in corticospinal excitability play a key role in the normal development of the central nervous system as well as in developmental disorders [2, 3]
We observed no changes in hotspots between single pulse SSW TMS and single pulse Double-Sine Wave (DSW) TMS which was administered prior to the DSW quadri-pulse theta-burst stimulation (qTBS) in AP and PA directed currents in the precentral motor hand representation
Ten volunteers participated in Experiment 1 (DSW PA-qTBS) and Experiment 2 (DSW AP-qTBS), assessing the effect of DSW qTBS at I-wave periodicity with ISI of 1.5 ms
Summary
Synaptic plasticity is considered to be the neurophysiological correlate of learning and memory and changes in corticospinal excitability play a key role in the normal development of the central nervous system as well as in developmental disorders [2, 3]. Regular or patterned repetitive transcranial magnetic stimulation (rTMS) of the precentral motor representations can induce lasting bidirectional changes in corticomotor excitability revealed by a lasting change in the mean amplitude of the motor evoked potential (MEP). This change in corticomotor excitability is attributed to changes in synaptic efficacy in the stimulated corticospinal system and referred to as long-term potentiation (LTP)-like or long-term depression (LTD)-like plasticity [4]. Depending on the preferential current direction, our novel SSW qTBS protocol consistently induced lasting bidirectional changes in corticospinal excitability in the human precentral motor hand representation [1]
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