People suffering from chronic pain exhibit changes in the way the central nervous system processes pain. Some of the changes in the central nervous system are associated with how the brain adapts to the process of different stimuli. There are several physiological mechanisms that regulates how the brain adapts to changes and one of these mechanisms is called homeostatic plasticity (or equilibrium plasticity ). In healthy participants homeostatic plasticity mechanisms have been tested and considered normal, whereas in patients with chronic conditions, such as low back pain, this mechanism was shown to be dysfunctional. However, it is unknown when this difference in the pain system develops. It is possible that homeostatic mechanism becomes impaired over a period of time. Current studies have investigate a cohort of patients and there is a lack of longitudinal designs. In order to investigate the long-term effects of pain on homeostatic plasticity mechanisms it is important to first investigate the reliability of the methods. This study will investigate the reliability of two protocols of homeostatic plasticity induction.

The aim of this study is to investigate the corticomotor excitability changes provoked by two homeostatic plasticity induction protocols, specifically the duration and the test-retest reliability of such corticomotor excitability changes.

A within-subject repeated-measures design will be used to evaluate the aftereffects and the reliability of two homeostatic plasticity induction protocols using cathodal transcranial direct current stimulation (tDCS) (experiment 1) and anodal tDCS (experiment 2). Each participant will take part in two experimental sessions during which homeostatic plasticity and corticomotor excitability will be induced and measured in the left primary motor cortex.

A sample size calculation was conducted using α of 0.05, β of 0.80 and effect size of 0.48 based on motor evoked potential (MEP) analysis of previous studies (Thapa, Schabrun, 2018, Thapa et al., 2018) resulting in a target of 13 participants. To account for differences in study designs and for the possibility of participant withdrawal/dropout, the investigators set target recruitment at 15 participants for each experiment.

Each participant will attend two identical experimental sessions on the same time in two consecutive days. During the experiment, participants will be seated comfortably with hands and arms at rest. First, the electromyography electrodes will be placed at the right hand muscle to be used for assessing the corticomotor excitability by recordings of motor evoked potentials by transcranial magnetic stimulation (TMS) on the left primary motor cortex. Then, the neoprene cap for tDCS on the left primary motor cortex will be mounted. The optimal scalp position (hot spot) for TMS stimulation will be identified and marked with a pen on the cap for standardisation. The corticomotor excitability in response to the homeostatic plasticity protocol (cathodal tDCS in experiment 1 or anodal tDCS in experiment 2) will be measured before and immediately post paradigm (time point 0-min), and then every 10 minutes for 70 minutes.

Homeostatic plasticity will be induced in the left primary motor cortex using tDCS applied for 7 minutes followed by an interval of 3 minutes and another block of 5 minutes of stimulation (Thapa, Schabrun, 2018, Thapa et al., 2018). A constant current of 1mA will be transmitted through the tDCS system (Starstim, Neuroelectrics, Barcelona, Spain), using two 3.14 cm2 Ag/AgCl gelled electrodes placed into holes of a neoprene cap corresponding to the international 10/10 EEG system, placed on participants head with the central Cz position aligned to the vertex of the head. In experiment 1 the cathode will be placed at C3 and return electrode placed at Fp2. In experiment 2 the anode will be placed at C3 and return electrode placed at Fp2.

Data distribution will be assessed using the Shapiro-Wilk test. A repeated measures analysis of variance (ANOVA) will be conducted on mean MEPs with factors Session (Day1 and Day2) and Time (baseline, 0 min, 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, 70 min). A Greenhouse-Geisser correction will be used if Mauchly's test shows that sphericity cannot be assumed. Adjustments will be made for multiple post-hoc comparisons using the Bonferroni correction. Results will be interpreted according to the level of statistical significance p≤0.05 and effect size reported as partial eta squared.

Inclusion Criteria:

• Healthy, aged between 18-60 years, right-handed and can speak and understand English.

Exclusion Criteria:

• Lack of ability to cooperate
• History or present chronic pain or current acute pain
• Pregnancy
• Drug addiction defined as the use of cannabis, opioids or other drugs
• Present and previous neurological disorders such as epilepsy, Alzheimer's disease, dementia, stroke, migraine and other headache disorders, multiple sclerosis, Parkinson's disease, neuroinfections, brain tumors and head trauma.
• Present or previous musculoskeletal disorders such as tendonitis, degenerative disc disease, mechanical back syndrome, and ruptured/herniated disc.
• Present or previous mental illnesses such as depression, bipolar disorder, and schizophrenia.
• Current use of medications that may affect the trial (e.g. analgesics, anti-inflammatories, anti-depressives)
• Contraindications to TMS application (history of epilepsy, metal implants in head or jaw, etc.)
• Failure to pass the tDCS screening questionnaire
• Failure to pass the "TASS questionnaire"