Parkinson's disease (PD) has been classically regarded as a movement disorder, so earlier work has focused on treating motor symptoms only. As PD patients now have longer life expectancy, the relatively slowly progressing cognitive deficits (compared to their motor deficits) have become one of the major challenges. Approximately 80% of PD patients eventually become demented. Therefore cognitive dysfunction is one of the most significant factors affecting the quality of life of patients with PD. While dementia in Parkinson's disease is routinely treated by cholinesterase inhibitors (e.g., donepezil and rivastigmine), their efficacy on mild cognitive impairment found in non-demented PD is questionable. Alternative approaches have been proposed including transcranial direct current stimulation (tDCS) but no consensus has been reached. This can be attributed mainly to: (1) imprecise knowledge of the underlying functional circuitry mediating this disease manifestation and (2) inter-individual variability. Here, the investigators will utilize a novel personalized network analysis approach to elucidate on the underlying mechanisms of the effect of tDCS on cognitive dysfunction in non-demented PD patients. It has been well documented that the caudate nucleus plays an important role in cognitive dysfunction found in PD. In the investigators' preliminary resting-state functional magnetic resonance imaging (fMRI) study, they have shown that the connectivity of the right caudate nucleus is correlated to cognitive status of PD patients measured by the Montreal Cognitive Assessment (MoCA). The investigators hypothesize that tDCS on the left and/or right dorsolateral prefrontal cortex may restore the functional connectivity of the right caudate nucleus which may in turn improve patients' cognitive performance.