The Role of Interneuron Networks in Driving Human Motor Cortical Plasticity

TLDR

Variability in rTMS after‑effects is largely due to individual differences in which cortical neurons are recruited rather than differences in synaptic plasticity. The study examined rTMS effects in 56 healthy volunteers and introduced a novel protocol using closely timed pairs of oppositely directed TMS pulses across the central sulcus. The authors applied excitatory and inhibitory theta‑burst stimulation and the new paired‑pulse protocol to probe motor cortical plasticity. Responses to TBS were highly variable, correlated with the latency of motor‑evoked potentials evoked by anterior–posterior current, and support the view that interneuron network recruitment drives rTMS plasticity variability.

Abstract

The after-effects of repetitive transcranial magnetic stimulation (rTMS) are highly variable between individuals. Because different populations of cortical neurons are stimulated more easily or are more excitable in different people at different times, the variability may not be due to differences between individuals in the plasticity of cortical synapses, but may instead be due to individual differences in the recruitment of cortical neurons. In this study, we examined the effects of rTMS in 56 healthy volunteers. The responses to excitatory and inhibitory theta burst stimulation (TBS) protocols were highly variable between individuals. Surprisingly, the TBS effect was highly correlated with the latency of motor-evoked potentials (MEPs) evoked by TMS pulses that induced an anterior–posterior (AP) directed current across the central sulcus. Finally, we devised a new plasticity protocol using closely timed pairs of oppositely directed TMS current pulses across the central sulcus. Again, the after-effects were related to the latency of MEPs evoked by AP current. Our results are consistent with the idea that variation in response to rTMS plasticity probing protocols is strongly influenced by which interneuron networks are recruited by the TMS pulse.