Test-Retest Reliability of Homeostatic Plasticity Induced and Assessed in the Human Primary Motor Cortex Using Thetaburst Stimulation

Homeostatic plasticity (HMP) regulates neural plasticity to prevent excessive neuronal excitability or silencing. Transcranial magnetic thetaburst stimulation (TBS) has been shown to induce HMP in the human cortex, however, the reliability is unknown. Reliability of TBS induced HMP was examined in the primary motor cortex (M1) of 20 healthy individuals using an excitatory and an inhibitory priming-test paradigm and assessed using motor evoked potentials (MEPs) from the first dorsal interosseous (FDI) muscle. MEPs were reliable for both protocols (ICCs between 0.61-0.99). HMP was not induced as expected (F(8) = 1.04, p = 0.408), although when accounting for order effects, HMP was evident (F(9)=3.85, p < 0.001), but contingent on first implemented protocol (t = -5.098, p < 0.001). Our findings provide important insights for designing HMP induction protocols, and this discovery of order effects suggests HMP may operate on a longer timescale than previously thought, warranting further investigation into the reliability of these methods.