Transcranial Biphasic Quadro-Pulse Stimulation with One but not Two Full-Sine Cycles Induces Long-Term Depression-Like Changes in Corticomotor Excitability
Aims: Neuronal plasticity in form of long-term potentiation (LTP) and long-term depression (LTD) is considered to be the neurophysiological mechanism of learning and memory. There is growing evidence that the induction of LTD-like effects plays a crucial role in mediating therapeutic effects of transcranial magnetic stimulation. We previously demonstrated that biphasic transcranial quadripulse stimulation (QPS) is slightly more effective in inducing LTP-like effects in human primary motor cortex (M1), when the biphasic pulse consisted of two full-sine cycles as opposed to one full-sine cycle. Here, we studied the effectiveness of QPS with one and two full-sine cycles to induce LTD-like effects of synaptic plasticity.
Methods: Healthy volunteers (n = 7 per protocol) received quadripulse stimulation of M1 using one (QPS-1) or two (QPS-2) full-sine cycles (duration: 160 µs, respectively). The QPS protocol consisted of 1,440 pulses. Pulses were separated by an interstimulus interval (ISI) of 50 milliseconds and an interburst interval of 200 milliseconds. Resting motor threshold (rMT), and motor-evoked potential (MEP) amplitudes were measured before (Pre) as well as 0 minutes (Post1), 15 minutes (Post2), 30 minutes (Post3), and 60 minutes (Post4) after the QPS session.
Results: We found a significant decrease of MEP amplitudes after QPS-1 at all the time points lasting for 1 hour (Post 1–4) but no increase or decrease in MEP amplitude after QPS-2. No significant changes in rMT and no adverse events were observed.
Conclusion: QPS with one full-sine cycle induces LTD-like changes in human M1 at interstimulus intervals of 50 milliseconds. No LTD-like effect was elicited with QPS when biphasic stimuli consisted of two full-sine cycles. This divergence in the capability to induce corticomotor plasticity suggests different underlying neurophysiological mechanisms. This may be relevant in basic research, for example, investigations of pathophysiological mechanisms in developmental disorders, as well as if QPS or other patterned stimulation protocols are used therapeutically.
Keywords: neuronal plasticity, transcranial magnetic stimulation, developmental disorders, long-term depression.