[Abstract] EXOPINCH – A Robotic Mirror Therapy System for Hand Rehabilitation

Abstract

Introduction ExoPinch is a robotic mirror therapy system for hand rehabilitation, focusing to increase the corticospinal excitability for the patients with hemiparesis. We propose that specific type of visual stimuli may be implemented in the action observation treatment to have a positive additional impact by activating the mirror neuron system (MNS) in premotor cortex. Recently, mirror therapy (MT) has been used as an alternative treatment for stroke of upper and lower limbs. In MT, the patient places the intact limb on the reflective side of a mirror and the non-intact limb on the non-reflective side of the mirror. Observation of the healthy limb’s reflection gives the illusion that the affected limb is functioning as instructed [1]. The underlying mechanism of the MT of stroke patients has mainly been related to the activation of the neurons with mirror-like properties. They were first discovered in the macaque monkey ventral premotor area F5 [2]. These mirror neurons discharge both when a particular action is done by an individual and when that same action done by another individual is observed. MT together with robotic assistive devices in the field of rehabilitation has led researchers to the robotics neuro-rehabilitation [3] and robots are particularly suitable for the application of motor learning principles to neurorehabilitation [4]. In the robotic mirror therapy systems, the motion of the functional hand is tracked by the intact hand using the robotic system. Based on the properties of the MNS and its role in motor learning, this system has been activated as a novel approach for training in the rehabilitation of patients with motor impairment of the upper limb following stroke. In this study, unlike the conventional mirror therapy where the functional hand motion is observed through a mirror, selected motions which provide higher activation for the mirror neuron system are observed through the prepared video streams aiming to improve the efficacy of the therapy. ExoPinch assists the patient’s index and thumb fingers to track the observed and imagined pinching actions. The selected motions are determined by the experiments on healthy subjects. In general, MNS is supposed to decode the kinematics of the observed motion. During the experiments, it is seen that the observed actions that include kinetic features (imposing force or torque) also increase the MNS activity. Therefore, the selected motions for the robotic mirror therapy system include features enforcing the kinetics, as well. This approach is supported by the motor learning principles where the kinematic and kinetic aspects are both concerned [6]. Methods ExoPinch is an exoskeletal type of rehabilitation robot. The index and thumb fingers are the parts of fully-actuated mechanisms with 2 degrees of actuation and 1 degree of actuation respectively. The exoskeleton mechanism of ExoPinch is synthesized using genetic algorithm over a multiobjective objective function. The mechanism design is based on the kinematic synthesis and the optimization of the transmission angles during the pinching motion. Dynamical models are built in MATLAB and Simechanics. Passive joint torques of the index and thumb fingers with spasticity are modeled as well to introduce the resistances to the motion. 10 healthy volunteers participated in this study. In the experiments, the suppression (desynchronization) in mu band (8-12 Hz) power as an index of the human mirror neuron system (MNS) [7] was studied while subjects observed object-directed hand actions with varying kinetics and kinematics contexts: squeezing a hard and a soft spring; grasping a long and a short stick, Fig.1. Our main purpose was to explore whether observation of any of these actions may have a relatively strong effect on MNS activity. The activation of mirror neurons in premotor cortex during action observation plays a crucial role in observational learning [5],[8] and rehabilitation is a motor relearning process [6]. Therefore, the recruitment of MNS in this respect with action observation might provide an effective neurorehabilitative program for patients with strok that may lead to a personal optimal therapy in the future. Figure 1. Video library elements imposing kinetic and kinematic features Electroencephalography (EEG) method was used to investigate the activity of the MNS. EEG data were recorded continuously (bandpass, 0.1-100 Hz; sampling rate, 250 Hz) with the 16 channel 32-bit A/D converter using OpenBCI. UltraCortex Mark 2 dry electrode headset was used conforming international 10-20 electrode placement. EEG data were processed offline using EEGLAB. The mean mu (8-12 Hz) band power values (in dB) were extracted at a number of frontal (F7, F8), central (C3, C4) and parietal (P3, P4) channels since these regions almost exclusively included regions that have been associated with the MNS in the literature. Event Related Spectral Perturbation (ERSP) method was used for analyzing the mirror neuron activity in time-frequency domain. A two-way repeated measures of ANOVA revealed the main effect of video stimuli of squeezing soft/hard springs, at the frontal channels close to ventral premotor cortex area of the brain. These results showed that the observed actions imposing kinetic features can increase the MNS activity. Therefore, the selected motions to be observed by the patients will include the features that impose the kinetics, as well, aiming to improve the efficacy of the therapy.

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