Research Article Test-Retest Reliability of Homeostatic Plasticity in the Human Primary Motor Cortex Tribikram Thapa and Siobhan M. Schabrun Brain Rehabilitation and Neuroplasticity Unit, School of Science and Health, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia Correspondence should be addressed to Siobhan M. Schabrun; s.schabrun@westernsydney.edu.au Received 27 December 2017; Revised 16 April 2018; Accepted 26 April 2018; Published 10 June 2018 Academic Editor: Sergio Bagnato Copyright © 2018 Tribikram Thapa and Siobhan M. Schabrun. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Homeostatic plasticity regulates synaptic activity by preventing uncontrolled increases (long-term potentiation) or decreases (long- term depression) in synaptic efficacy. Homeostatic plasticity can be induced and assessed in the human primary motor cortex (M1) using noninvasive brain stimulation. However, the reliability of this methodology has not been investigated. Here, we examined the test-retest reliability of homeostatic plasticity induced and assessed in M1 using noninvasive brain stimulation in ten, right-handed, healthy volunteers on days 0, 2, 7, and 14. Homeostatic plasticity was induced in the left M1 using two blocks of anodal transcranial direct current stimulation (tDCS) applied for 7 min and 5 min, separated by a 3 min interval. To assess homeostatic plasticity, 15 motor-evoked potentials to single-pulse transcranial magnetic stimulation were recorded at baseline, between the two blocks of anodal tDCS, and at 0 min, 10 min, and 20 min follow-up. Test-retest reliability was evaluated using intraclass correlation coefficients (ICCs). Moderate-to-good test-retest reliability was observed for the M1 homeostatic plasticity response at all follow-up time points (0 min, 10 min, and 20 min, ICC range: 0.43–0.67) at intervals up to 2 weeks. The greatest reliability was observed when the homeostatic response was assessed at 10 min follow-up (ICC > 0 61). These data suggest that M1 homeostatic plasticity can be reliably induced and assessed in healthy individuals using two blocks of anodal tDCS at intervals of 48 hours, 7 days, and 2 weeks. 1. Introduction Synaptic plasticity is fundamental to learning and memory in the human brain. However, synaptic plasticity operates via a positive feedback loop and, as a result, has the potential to destabilise neural networks through excessive synaptic strengthening (long-term potentiation-like effects, LTP) or excessive synaptic weakening (long-term depression-like effects, LTD) [1]. To avoid destabilization, LTP-like and LTD-like changes are subject to homeostatic plasticity mech- anisms that maintain the neural activity within an optimal physiological range. Homeostatic plasticity is theorised to rely on the “sliding threshold” rule, such that the threshold for the induction of LTP or LTD is dependent on the activity in the postsynaptic neuron; high postsynaptic activity favors LTD, whereas low postsynaptic activity favors LTP [2–4]. Although early stud- ies investigating homeostatic plasticity occurred in slice prep- arations in vitro, a growing body of research has used noninvasive brain stimulation to investigate this mechanism in the human cortex [2–8]. Typically, one noninvasive brain stimulation protocol is used to “prime” (or condition) the synaptic effects of a subsequent noninvasive brain stimula- tion protocol, and LTP-like and LTD-like effects are indexed using transcranial magnetic stimulation (TMS). For example, when a 5 min block of anodal transcranial direct current stimulation (tDCS) is preceded at a short interval (3 min) by an additional 7 min block of anodal tDCS, the LTP-like (facilitatory) effect of anodal tDCS on the primary motor cor- tex (M1) is reversed toward LTD (observed as a reduction in Hindawi Neural Plasticity Volume 2018, Article ID 6207508, 9 pages https://doi.org/10.1155/2018/6207508