tDCS has been shown to be an effective treatment for depression. However, tDCS is a relatively new clinical tool and more needs to be understood about its use. This study hopes to further the field of knowledge by examining how tDCS should be optimally used. Application of tDCS in clinical trials of depression is typically to the prefrontal cortex, but in this project, tDCS application will be to the motor cortex as it provides a more ready measure of excitability. Excitability will be measured using Transcranial Magnetic Stimulation (TMS) to the motor cortex and electromyography (EMG) recordings from peripheral muscles stimulated. Using a cross-over three-arm design this study aims to investigate whether daily tDCS administered in increasing intensity across sessions leads to greater and lasting effects on brain excitability than keeping the intensity at a same dose across the days and whether the excitatory effect could be enhanced with D-cycloserine, a medication known to prolong the excitatory effects of a single session of tDCS. This in turn will inform on how to optimize tDCS for therapeutic applications, e.g treatment of depression. The study hypothesis is that 5 sessions of tDCS with a dose of D-cycloserine given on the Monday and Thursday sessions will result in more sustained effect on motor cortex excitability than 5 sessions of tDCS alone. The second hypothesis is that the gradational increases in tDCS intensity over 5 sessions will result in greater motor cortex excitability than 5 sessions of tDCS where intensity is kept constant across sessions.