Consistency of MEPs elicited at two TMS intensities

Abstract Background: The mechanisms of cortico-motor plasticity can be evaluated in human motor cortex by comparing the amplitude of motor evoked potentials (MEPs) elicited by single-pulse transcranial magnetic stimulation (spTMS) after a repetitive transcranial magnetic stimulation (rTMS) regimen. Post-rTMS increases/decreases in MEPs are interpreted as long-term potentiation-/depression-like changes, respectively. Objective: To compare the two most common spTMS intensities—120% resting motor threshold (120%RMT) and the intensity to elicit an average ∼1mV MEP (1mV-MEP)—on within-session and between-sessions consistency of MEPs. Methods: Seven healthy adults (21-70 years; 5 males) consented to participate in this ongoing IRB-approved study. Participants completed two identical sessions (test-retest; ≥1 day apart), each consisting of four blocks of 120 neuronavigated spTMS trials. Blocks 1-2 were administered using either 120%RMT or 1mV-MEP (counterbalanced) and separated by 15 minutes where participants remained seated (simulating rTMS). After a 20-minute break where participants could move around, Blocks 3-4 were administered at the other intensity with the same 15-minute rest break. Results: PRE refers to blocks 1+3 and POST refers to blocks 2+4. Data are mean±SD. In the Test-visit, MEPs (mV) were 1.4±0.5 (PRE) and 1.5±0.5 (POST) for 1mV-MEP and 2.6±2.1 (PRE) and 2.0±2.0 (POST) for 120%RMT. In the Retest-visit, MEPs were 1.4±0.7 (PRE) and 1.3±0.6 (POST) for 1mV-MEP and 1.8±1.5 (PRE) and 1.7±1.2 (POST) for 120%RMT. The within-session percent change (%Δ) from PRE was 15.2±38.5 (Test-visit) and 4.9±45.5 (Retest-visit) for 1mV-MEP and -29.2±21.3 (Test) and 5.2±42.4 (Retest-visit) for 120%RMT. The between-sessions %Δ from the Test-visit was 4.1±41.1 for 1mV-MEP and 32.6±149.1 for 120%RMT. Conclusions: Compared to 120%RMT, MEPs elicited at a 1mV-MEP intensity show lower inter-individual variance and smaller changes within-session and between sessions. These results suggest 1mV-MEP is better suited to index changes induced by rTMS. Research Category and Technology and Methods Basic Research: 10. Transcranial Magnetic Stimulation (TMS) Keywords: transcranial magnetic stimulation, motor evoked potentials, variability, resting motor threshold