## 1 Introduction  
Parkinson's disease (PD) is one of the most common neurodegenerative disorders. Clinical assessment and diagnosis of PD are based on patient symptoms, and the accuracy of the diagnosis is considered to be approximately 75-95% [1]. However, PD symptoms may vary depending on the patient, and the importance of early diagnosis and treatment of PD is increasing because of the possibility of developing other diseases such as dementia. Signs of PD can be separated into the motor and non-motor symptoms. Among them, motor symptoms mainly include bradykinesia, muscle stiffness, resting tremor, and gait disturbances, and in particular, the prevalence of fractures due to gait disorders is noted to be high [2]. The problems associated with the gait worsen as the disease progresses, which is a significant disease burden that significantly affects the independence and quality of life [3,4]. Many complementary and alternative medicines, including oriental medicines, have recently been used to treat PD [5]. Among them, acupuncture is one of the most active fields of study relevant to PD and has been proven useful in basic and clinical studies [6]. Concerning the effectiveness of acupuncture for treating gait disturbances in PD patients, several clinical studies have also demonstrated its ability to improve gait speed [7; 8]. However, systematic research on the mechanism of acupuncture in clinical trials is relatively scarce, and there is also debate about the scientific validity of the effects of acupuncture. To confirm the mechanism of acupuncture treatment, it is necessary to monitor changes in the cortical region because of neuroplasticity caused by the treatment. Damaged brain function promotes neuroplasticity through repetitive treatment and rehabilitation, such that brain tissue in the cortical region can be reorganized to restore damaged function [9]. Neuroimaging has recently been used as one of the tools to identify the mechanism of acupuncture treatment. Functional magnetic resonance imaging (fMRI) is the most commonly used tool for functional neuroimaging in acupuncture studies because it can indirectly measure brain activity and brain function changes without harmful radiation and invasive procedures. In studies where fMRI was used to measure the effects of acupuncture, each acupoint was found to have a specific effect, and acupuncture was shown to improve symptoms in diseased patients [10; 11]. However, fMRI has a limited ability to measure gait-related movements. Moreover, it cannot be used for real-time gait measurements under gravitational acceleration, an essential element of walking. These examples expose one confusing aspect of neuroimaging: it cannot be concluded that fMRI can identify all brain regions involved in behavioral rehabilitation or treatment unless the fMRI procedure is precisely the same as that used in therapy. Recently, a new technique for brain imaging, functional near-infrared spectroscopy (fNIRS), has gradually gained acceptance and become widely used in clinical practice as a method for diagnosis in psychiatry and neurosurgery [12]. Similar to fMRI, fNIRS is a non-invasive brain imaging technique that can measure blood flow changes in the cerebral cortex. fNIRS can be designed to be relatively insensitive to head motion and uses compact instruments that make it possible to apply to patients with PD at the bedside or in the rehabilitation facility. Several studies have shown, using fNIRS, that oxy-hemoglobin (HbO) levels within the prefrontal cortex are increased in patients with PD during the gait [13; 14]. Our previous study also applied fNIRS to PD patients and demonstrated the feasibility of using fNIRS to assess the effects of gait disturbance therapies in PD patients [15]. However, previous studies only showed hemodynamic responses in the cortices following tasks. In the case of PD, it is important to monitor the connectivity of patients with Parkinson's brain regions and brain activity because the nerve cells that transmit signals are damaged.