Increasing motor cortex activation during grasping via novel robotic mirror hand therapy: a pilot fNIRS study

I bet your hospital hasn't even bought mirrors and created protocols for their use. They certainly won't buy expensive robots.

Increasing motor cortex activation during grasping via novel robotic mirror hand therapy: a pilot fNIRS study

• Dong Hyun Kim,
• Kun-Do Lee,
• Thomas C. Bulea &
• Hyung-Soon Park 

Journal of NeuroEngineering and Rehabilitation volume 19, Article number: 8 (2022) Cite this article

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Abstract

Background

Mirror therapy (MT) has been used for functional recovery of the affected hand by providing the mirrored image of the unaffected hand movement, which induces neural activation of the cortical hemisphere contralateral to the affected hand. Recently, many wearable robots assisting the movement of the hand have been developed, and several studies have proposed robotic mirror therapy (RMT) that uses a robot to provide mirrored movements of the unaffected hand to the affected hand with the robot controlled by measuring electromyography or posture of the unaffected hand. In some cases of RMT a mirror is placed to allow the person to observe only the unaffected hand but in others users simply observe the robotically assisted hand performing the mirrored movements, as was the case in this study. There have been limited evaluations of the cortical activity during RMT compared to MT and robotic therapy (RT) providing passive movements despite the difference in the modality of sensory feedback and the involvement of motor intention, respectively.

Methods

This paper analyzes bilateral motor cortex activation in nine healthy subjects and five chronic stroke survivors during a pinching task performed in MT, RT, and RMT conditions using functional near infrared spectroscopy (fNIRS). In the MT condition, the person moved the unaffected hand and observed it in a mirror while the affected hand remained still. In RT condition passive movements were provided to the affected hand with a cable-driven soft robotic glove, while, in RMT condition, the posture of the unaffected hand was measured by a sensing glove and the soft robotic glove mirrored its movement on the affected hand.

Results

For both groups, the RMT condition showed the greatest mean cortical activation on the motor cortex contralateral to the affected (non-dominant for the healthy group) hand compared to other conditions. Individual results indicate that RMT induces similar or greater neural activation on the motor cortex compared to MT and RT conditions. The interhemispheric activations of both groups were balanced in RMT condition. In MT condition, significantly greater activation was shown on the hemisphere ipsilateral to the affected (dominant for the healthy group) hand for both subject groups, while the contralateral side showed significantly greater activation for the healthy group in RT condition.

Conclusion

The experimental results indicate that combining visual feedback, somatosensory feedback, and motor intention are important for greater stimulation on the contralateral motor cortex of the affected hand. RMT that includes these factors is hypothesized to achieve a more effective functional rehabilitation due to greater and more balanced cortical activation.

Introduction

Individuals who experience stroke tend to lose motor function, and more than 70% of them have the upper limb affected. Particularly, hand function is most severely affected and also shows the worst response to standard of care therapy [1, 2]. Hand motor function can be improved by intense and repeated practice of functional movements through rehabilitation therapy. Repeated motor training is believed to improve motor functions because it induces neuroplastic changes that construct a new neural network in the intact cortical area, which replaces the function of the damaged area [3,4,5]. For effective rehabilitation, repeatedly providing neural stimulation around the motor and somatosensory cortex is important [6, 7]. A greater activation level of the cortical area near the motor cortex is observed after functional recovery likely indicating neuroplastic changes [8, 9].

Mirror therapy (MT) is a rehabilitation method of placing a mirror between the arms or legs so that the reflected movement of the non-affected limb gives an illusion of normal movement in the affected limb [10, 11]. MT is particularly used for the rehabilitation of individuals post-stroke who do not have the ability to conduct voluntary movements. The contralateral motor cortex of the affected limb is known to be stimulated by mirror therapy, although no voluntary movements are conducted [12,13,14]. However, the magnitude of the cortical activation is small compared to that of the unaffected limb as it does not convey actual movements and the corresponding somatosensory feedback.

Wearable robotic technologies enable assistance of limb movements for individuals with paralysis and other movement pathologies, and the use of wearable robots has expanded to rehabilitation therapy for various functional tasks. Recently, many researches have attempted to apply mirror therapy using wearable robots (robotic-mirror therapy, RMT) [15,16,17]. In RMT, the movement or muscle activation of the unaffected hand is measured and the wearable robot donned on the affected hand induces identical movement corresponding to the measurements. While some versions of RMT in the previous studies include a mirror to provide visual feedback of the mirrored less affected hand, other RMT systems do not. In this latter case, the term “mirror” indicates that the robotic assistance provided to the more affected hand is “mirroring” the movement of less affected hand. Unlike MT that only provides visual feedback, RMT provides both visual feedback and somatosensory feedback by providing passive movements. However, the neural effect of the RMT compared to MT is still a question and needs to be studied to understand the effect on actual functional recovery.

There are various methods available to observe neural activation of the brain during functional movements. Functional near-infrared spectroscopy (fNIRS) is one of these methods, which non-invasively measures brain activation by analyzing the hemodynamics of the cerebral vessels through near-infrared light. Movement-related cortical activity, including both the area and magnitude of activations, can be quantified with fNIRS by placing multiple light-emitters and detectors on the scalp around the motor cortex. Previous studies have evaluated the effectiveness of robotic and/or sensorized gloves using fNIRS and observed increased cortical activation [18, 19]. By spatial analysis of fNIRS previous studies have also identified the correlation between the variation of functional recovery of MT among subjects and the shift of cortical activation on the precuneus region [20], and analyzed the functional laterality according to the time after a stroke [21].

In this paper, we analyzed the neural effect of MT, robotic therapy (RT), and RMT in repetitive pinching movements. RT was tested, in addition, to observe the neural effect when no movement intention was involved. The RMT was conducted by measuring the movement of the unaffected hand with a custom-designed sensor glove and inducing movement of the affected hand with a soft robotic glove that could assist 4-DOF movements [22] previously developed from our research group. RT was conducted by moving the affected hand with the soft robotic glove without involving movement of the unaffected hand. fNIRS was used to measure neural activity in the brain during MT, RT, and RMT. Neural effects of each condition on healthy subjects and stroke survivors were analyzed and compared.

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