1. Current status of music therapy

Music is a sound wave composed of elements such as melody, rhythm, and harmony and is defined as a tone or sound arranged in time through a continuous combination of notes ^[1]. Music activates the "auditory-motor pathway," activating motor neurons through the rhythms contained in the music itself, causing muscles to contract and synchronize body movements with the beat ^[2]. For example, music with a strong rhythm makes people want to be in sync with it, and the listener's fingers or feet will automatically tap the beat. The main reason for this is that the auditory processing area of the cerebral cortex is adjacent to the motor area. Due to the interaction between the auditory and motor systems, the human body will show corresponding motor performance when stimulated by music ^[3]. This motor response of the brain to auditory stimuli is called thalamus response. Based on this principle, more and more scholars have applied music to clinical therapy, resulting in Music Therapy. MT plays a role through the cross-activity of the hypothalamic-pituitary-adrenal axis, nervous system, cardiovascular system and digestive system to regulate the psychological and physiological functions of the human body, so it is widely used in the field of medicine and psychotherapy ^[4]. MT is also used as a special rehabilitation tool for gait abnormalities caused by neurological diseases.

2. Forms of music therapy conducted

MT can be divided into active and passive treatments, depending on the treatment method. Active therapy involves patients playing musical instruments and participating in singing and opera performances under the guidance of a music therapist in a self-fulfilling form. It is a process involving all the sensory organs. The melody and rhythm of music are used as specific stimuli to obtain certain motor and emotional responses, thus combining the movement and stimulation of different sensory pathways, i.e., auditory and tactile (multisensory stimulation). Passive therapy does not require the patient to actively participate in music performance or singing activities, just to listen to music ^[5]. Passive music therapy is most vital in the treatment effect of PD patients with gait disorders, while active therapy is mainly in the form of teamwork, focusing more on improving PD patients' cognition, mood, quality of life, and other aspects.

3. Modalities of MT to improve gait in PD patients

Current clinical treatment of gait disorders in PD patients consists mainly of Rhythmic Auditory Stimulation (RAS), Therapeutic Singing (TS), and Therapeutic Instrumental Music Playing (TIMP). Treating patients by synchronizing or asynchronizing their walking rhythms with music to adjust walking parameters in this way ^[6].

3.1 RAS

RAS is a Neurological Music Therapy (NMT) technology that refers to auditory rhythm cues to promote the synchronization of gait movement ^[7] and is the main method for the intervention of gait disorders in PD patients. A large number of experiments have found that RAS can reshape sensorimotor rhythm and frontal-central parietal lobe/time connection and restore the brain mechanism that generates regular walking rhythm through the intervention of RAS in PD patients with gait disorders. In addition, other studies have shown that RAS has a good therapeutic effect on abnormal gait in PD patients. After treatment, bilateral ankle dorsiflexion and all gait parameters are signifi-cantly improved, and fall index decreases. Spatio-temporal gait parameters, such as stride speed and stride length, are significantly improved ^[8,9].

3.2 TS

TS is spontaneous behavior, is a kind of internal clue, different from the external clues, such as listening to music rhythm, patients can sing loudly (loud, soft) and in the heart silent in two ways, to synchronize their walk with the pace of the singing, has large variability, can at any time according to the adjustment of the state of patients, and the synchronized rhythm. Compared with external cues, singing does not require any equipment to play music, nor does it require the difficult task of consistently synchronizing with the music rhythm. It can be controlled according to the patient's personal rhythm, reducing spatial and temporal variability ^[10] and making it easier for patients to complete. Therefore, TS has been regarded as an important method to improve gait disorders in PD patients in recent years.

3.3 TIMP

TIMP has been used in several neurological disorders, including PD, in which patients actively play rhythmic melodies or beats to improve overall and fine motor skills through rhythmic entrainment. Playing Musical Instruments can provide immediate auditory feedback [11]. The instruments used here are mainly percussion instruments that can emphasize obvious beats, such as drums. Such instant auditory feedback can coordinate rhythm and movement, and percussive rhythm can stimulate the movement of patients to synchronize with it, and the movement of playing Musical Instruments can reduce the amplitude of involuntary movement of PD patients. TIMP is mostly used in group cooperation. Some people play Musical Instruments, while others walk and do other body movements according to the rhythm of the music. This form can treat not only movement disorders but also improve the cognitive and emotional quality of life.

4. Study on gait mechanism of MT intervention in PD patients

4.1 Compensation mechanisms for recruiting cerebellum

Since BG is damaged in the brainstem of PD patients and the cerebellum is capable of processing temporal perception and motor execution, the temporal rhythmic perception of musical rhythms and performing synchronized movements are mainly accomplished by recruiting the cerebellum to compensate for the missing function.

4.2 The rhythm of entrainment

The musical rhythm serves as an activation signal to stimulate the PD patient's motor system to synchronize with the musical rhythm. In practice, this rhythmic entrainment synchronizes the walking movement with the musical rhythm and thus intervenes in the patient's gait.

4.3 Accelerated motor learning

When people listen to the rhythm of music, the motor area in the brain will become active, and the auditory environment and incentive effect of music may lead to the acceleration of motor learning ^[11].

4.4 Stimulate neural coherence

When listening to music, music stimulates the nerve to produce excitement. These neural networks are coherent and orderly, and work together to play a role in the coordination of the human body's motor system in the synchronous walking movement with rhythm music, and intervene in gait.

4.5 Increased cortical activity

Perceiving musical rhythms is a process of perceiving time, and walking in sync with it is a process of integrating the timing and sequence of movements that work through the relevant areas of the cerebral cortex. Rhythm music combines the sensory cortex and motor cortex regions.

This entry is adapted from the peer-reviewed paper 10.3390/ijerph19159568