The aim of the present study was to evaluate the differences in the effects on the sensorimotor control capacity of a hand across conditions including baseline, standard MST, and PHL-based MST conditions. A passive haptic learning-based music-supported therapy system suitable for sensorimotor control training was developed in this study. The findings partially supported the premise that the PHL-based MST would enhance the sensorimotor performance of a hand in healthy older adults. Participants in the PHL-based MST group exhibited better performance in a pinch-holding task that required online sensory feedback, as compared to those in the standard MST and the baseline groups. In addition, the results also showed that there were better effects of the PHL-based MST on improving sensory functions related to barognosis and roughness differentiation, as compared to those under the baseline and standard MST conditions. This result clearly demonstrated the effectiveness of the PHL-based MST on the sensorimotor performance of healthy older adults.
The FRpeak under the baseline condition in the current study was higher when compared with data obtained from young subjects . A higher pinch-to-load force ratio, which indicated inaccurate pinch force modulation to the momentum-induced load changes , might have been due to the decline in the sensorimotor functioning of the hands. In the case of healthy participants, pinch force predictions and adjustments during execution of a functional task is an automatic motor response corresponding to the dynamics of arm movement ; however, recent evidence suggested that sensory deficits of a hand appear to remarkably decrease the capacity of momentary motor adjustment  because task-based sensorimotor processing depends on not only a feed-forward control mechanism but also on peripheral sensory events . Since aging leads to slowing of sensorimotor functions , an intervention program involving the use of passive sensory stimulation drives plastic reorganizational changes in the sensorimotor cortex based on the Hebbian forms of plasticity , which promotes precision pinch performance in the hands of healthy seniors. The better performance in precision pinch performance when receiving the PHL-based MST intervention was supported by recent studies based on sensory augmentation systems used to explain the potential mechanisms of sensory restoration, sensorimotor integration, and substitution in motor control [37, 38]. That is, integration of afferent vibration signals in the form of haptic feedback in hand therapy contributes to enhancing hand performance .
In addition, the results of the SWM test and MTT test obtained under the baseline condition in the older adults examined in the present study revealed lower sensitivity in both the touch threshold  and discriminative sensory function . The PHL-based MST had superior training effects on the results of the SWM test and the MTT barognosis subtest. Compared to the standard MST, the sensitivity related to both tactile and proprioceptive sensation increased through activation of sensory receptors during the transmission of vibrotactile stimulation to the finger tips, which enhanced sensory restoration in the participants receiving the PHL-based MST intervention. A recent report indicated that the vibrotactile component of the haptic feedback that occurs when playing a musical instrument leads to increased quality of hand perceptions . Due to central mechanisms, vibrotactile stimulation on the fingertips results in not only activating cutaneous mechanoreceptors, but it also enhances the mechanical coupling between the contacted skin, tendons, and bones, which significantly improves the of touch-pressure threshold, as tested by Semmes-Weinstein monofilament  and the capacity of active force perception as measured when participants manipulate objects .
The standard MST in this study also presented some training effects on improving hand functions, which might concur with the findings of a previous review report regarding MST training used to enhance the motor functions of stroke survivors . MST provides visual and auditory information intended to guide the temporal and spatial organization of sequential motor responses based on rhythmic keyboard pressing and enhances the coordinated actions of the hand . Furthermore, the predictability of motor sequences serve as a facilitative factor for motor control based on a cognitive processing mechanism that occurs during the preparation and execution of movements such as keyboard pressing during MST . Therefore, music-based paradigms have been suggested to be an effective strategy for motor learning and rehabilitation. Also, the results of the roughness differentiation and stereognosis after the standard MST revealed statistically significant changes compared to the baseline condition in the current study, which might have been correlated with the enrichment of the sensorimotor network through functional motor training in a multisensory environment .
This may be the first study examining the effects of an intervention using a music-supported therapy with multiple sensory feedback on sensorimotor performance of the hands of healthy senior participants. The present study established a passive haptic learning-based music-supported therapy (PHL-based MST) system acting as a sensory augmentation approach to dealing with impaired motor control capabilities. It is noteworthy that better training results in precision pinch performance, hand function, and sensory function of healthy older adults have been obtained for participants even when only receiving the PHL-based MST intervention once comparing with receiving the standard MST. Also, both the PHL-based MST and standard MST interventions had beneficial effects on hand performance and sensory function compared to the baseline condition. However, several limitations exist in the present study. First of all, the protocol only provided immediate outcome measures, which make it impossible to clearly understand the delayed effects on the sensorimotor performance that occurred in both the PHL-based MST and standard MST interventions. The other limitation was that the design of current study was only a single session intervention, which did not permit us to observe effects of a longer training period of music-supported therapy on the sensorimotor performance of hands. In spite of these limitations, the results indeed have merit related to supporting future studies investigating the impacts of PHL-based MST and standard MST on hand sensorimotor performance or sensory functioning in participants with marked sensorimotor deficits.