I recommend you read this paper: 'Cortical Plasticity Induction by Pairing Subthalamic Nucleus Deep-Brain Stimulation and Primary Motor Cortical Transcranial Magnetic Stimulation in Parkinson's Disease'.
link: Article Cortical Plasticity Induction by Pairing Subthalamic Nucleus...
The authors studied plasticity changes in cortex in relation to TMS and DBS in Parkinson's disease. Even though you ask about tDCS you might find it of interest.
They found that 'repeated pairing of STN-DBS with TMS at short (∼ 3 ms) and medium (∼ 23 ms) intervals increased cortical excitability that lasted for up to 45 min, whereas the control condition (fixed latency of 167 ms) had no effects on cortical excitability. '.