1. Introduction
Stroke is one of the leading causes of physical disability seriously affecting 5 million people’s quality of life out of the 15 million who suffer from stroke around the world [
1]. About 80% of stroke survivors have residual mobility limitations usually associated with a foot-drop. That means a lower limb impairment that combines a weak dorsiflexor and an increased plantar flexor stiffness reducing the capacity to maintain balance and posture while walking [
2,
3,
4]. Post-stroke rehabilitation therapy aims to restore the patient’s physical, neurological, and psychological capacities to achieve the highest level of functional independence [
5]. In fact, robotic devices like lower-limb exoskeletons in motor rehabilitation programs have been shown to improve automatic repetitive training and promote new motor skill acquisition after stroke [
6,
7]. User’s intention in this field is usually detected and predicted through control approaches based on the sensing of human biomechanics (i.e., through inertial sensors, direct contact operation, or external transducers) [
7,
8]. Therefore, conventional robotic control systems generally do not include efficient and natural interaction methods between users and exoskeletons [
9]. In this way, the possibility of enhancing and involve the patient increasingly is a clear objective to improve the user skills in a short-term period with better results.
Brain–computer interfaces (BCI), mostly based on the acquisition of electroencephalography (EEG) biological signals, provides a promising communication and control channel to improve the patient’s involvement with the system. It has been shown to generate neuroplasticity progressively throughout the development of neuromotor abilities and the mental practice of movements [
10,
11]. Besides, this technology has emerged as a potential tool to command robotic exoskeletons (e.g., lower-body powered exoskeletons) in the assistance and rehabilitation fields [
9]. One of the few clinical studies exploring the BCI-based rehabilitation systems showed the viability of this tool based on motor-related events when a user is commanding a lower-limb exoskeleton [
12]. Other research, also focused exclusively on the ankle-foot orthosis, showed a fast and effective approach for inducing cortical plasticity through BCI having a huge prospective in motor function rehabilitation after stroke [
13].
In the case of the control of robotic exoskeletons by means of a BCI, several paradigms exists based on rhythms related to the brain activity [
12]. One of the most used strategies to decode brain activity is the motor imagery (MI) study [
14]. Motor imagery is a technique that requires a dynamic mental image of the desired motor output [
15]. Its use in the BCI field has been relevant to detect neurological patients’ movement intention. Specifically throughout the Event-Related Desynchronization/Synchronization (ERD/ERS) modality, it is possible to recognize the beta band’s variations in power after performing a real or an imagined movement [
16,
17,
18]. Generally, the alpha and beta power decrease in the resting state and keeps a reduced power during the motor imagination or planning (ERD). However, about 300 or 500 ms after the end of the motor imagery, the beta rebound emerges through one second approximately (ERS). This last event occurs particularly in motor areas representing a simple idle activity and/or an active inhibition of the motor network [
19,
20]. ERD/ERS pattern has been widely studied in MI-BCI modalities as a potentially effective strategy for detecting and measuring commands to control a system [
21,
22]. For instance, the beta cortical oscillations control signals were effective to actuate an upper-limb exoskeleton with motor execution and motor imagination [
23], and to trigger a robot-assisted action during lower limb motor imagery tasks [
24,
25]. In particular, one of the inspirational developments in the BCI-based beta rebound system was used to control a virtual spaceship takeoff using real or imaginary foot movements. The design of this strategy resulted in effective commands that easily interpret neural signals as motor intentions to activate the animation in the virtual reality (VR) environment without MI training [
26,
27,
28].
In general, motor imagery-based BCIs are commonly related to low performance and reliability due to imperfect signal processing algorithms and most users’ complexity to display a vivid picture of the movement [
8,
29]. According to Lotte et al. [
30], the user, beyond the processing techniques, is one of the most critical components of the BCI loop. The subject’s inability to correctly perform the desired mental commands hinders the capacity of any algorithm to properly detect them [
30]. In this sense, the user must be properly guided to be able to effectively use and control the BCI system [
30,
31,
32]. Several sources agree the proper induction of MI is a suitable and beneficial alternative for patients in their rehabilitation process [
33,
34]. Precisely, MI-BCI strategies induce neural activity and increase motor and cognitive performance by generating a change in brain cortical activity [
33,
35]. In addition, the specific modulation of the brain for planning and control voluntary exoskeleton movements triggers neuroplasticity in post-stroke patients [
36,
37]. These neurofeedback mechanisms generate brain reorganization to restore the lost function and consequently prompt a motor recovery [
36,
38]. Some studies reinforce this idea by including other signals, cues, feedback systems, and even other modalities within the therapy protocol [
39].
Motor imagery-associated stimuli has been considered as an effective strategy to proficiently regulating motor imagery [
15]. Motivation and compatibility with the therapy have been increasingly involved in the BCI systems protocols making users possible to learn to regulate electrocortical activity in the sensorimotor cortex. Usually, visual stimulus is most applied in this field to support users’ motor imagery task [
30]. Neuper et al. [
40] have shown the control of an MI-based BCI system can develop a better precision in its performance through visual developments. Nevertheless, haptic stimulus has been reported to be more engaging and functional than the visual in MI-BCI systems [
41,
42]. According to Kauhanen et al. [
43], haptic stimulus has emerged as complement to regulate motor imagery generation. In this way, MI-based BCI with a haptic stimulus can be an effective alternative when the visual channel is overloaded or when it is needed for the performance of additional tasks beyond the BCI system [
41,
43].
Following this line of research, this preliminary study seeks to develop a BCI-controlled ankle exoskeleton system based on motor imagery to activate neuronal and motor patterns in post-stroke recovery. Furthermore, this report looks to evaluate the best strategy to induce MI through a comparison of visual and visual with haptic stimuli modalities. From this, it is expected to introduce a complete and portable system to actively involve stroke survivors in robotic therapies. To do that, BCI motor imagination accuracy, offline EEG signal analysis, and user level of satisfaction are presented.
4. Discussion
The results of the proposed system demonstrated the viability of acquiring the beta rebound signal to command the exoskeleton. Subjects successfully performed all tasks where mental ability was required with a good performance accuracy in visual or visual with haptic stimulation. However, neither generated differential electrical activity throughout the session stages.
As an initial approach, the algorithm for real-time detection of MI was ideal as a proof of concept in the BCI-based device control. The calibration strategy made it possible to individually adapt and define the user’s basal level of brain activity at which changes in beta rebound were detected. Moreover, the thresholding technique further limited MI detection for random events resulting from even haptic stimulation. Nevertheless, analysis of the offline signals demonstrated imagery activity during passive or non-imagery events. This behavior is related as a consequence of the user’s lack of concentration or as a leftover effect from the active state. In any case, the results suggest the need to establish longer intermediate times to pass from one state to another, and a much more robust machine-learning algorithm to guarantee the beta rebound detection in more complex contexts.
On the other hand, the most relevant MI accuracy results of this study showed an average difference of 17.3% of the MIVH test over MIV active periods. All patients had a greater number of successful attempts at motor imagery when haptic stimulation was present. These results were related to the proprioceptive delivery, to the CNS, of a neural representation or mimic of the natural body mechanics to facilitate the creation of the movement mental image [
39,
58,
59]. In contrast, both topographically and statistically in PSD results revealed no significant differences between the stationary therapy without MI, and the MI with the visual and visual-haptic stimulus strategies. The above data suggest visual and haptic stimuli improve the subject’s accuracy in performing motor imagery but do not generate differential brain activity. Similar results were reported by Kauhanen et al. [
43], who did not report differences between haptic or visual stimuli with upper limb exoskeleton. In the case of the present study, only the Cpz channel in the ST and MIVH tests presented significant differences, indicating the neural variations during cognitive tasks and the activation of the somatic sensory association area as a result of the haptic stimulation [
60,
61].
Furthermore, within these results, it was also possible to identify data variability in terms of detection time, accuracy, and PSD power. As could be observed, the comparison performance between subjects 4 and 5 was the opposite, and in general, each participant revelated variable results. Initially, it could be associated with environmental or clinical variables that vary between patients (e.g., age, stroke year, and even the type of treatment). However, it is mostly related to intersubject variability in the motor imagery performing, where not all participants have the same facility to create a visual image of a movement. Emami et al. [
31], have made findings of the significant role of the distractor factors affecting the MI-BCI performance results.
In the same way, topographical maps showed potential changes over the contralateral and the ipsilateral hemisphere. Although the proposed BCI-controlled exoskeleton system did not contemplate the beta rebound laterality for the device control, the offline analysis over alfa and beta signals (8–32 Hz) exhibited a significant impact on the C1 and C2 channels during the active periods of MI (
Figure 8). In this case, the movement imagery-related lateralization had a higher discriminative power over the imagery of left foot movement (i.e., C2 channel). However, only two of the five patients had left hemisphere involvement. This way, the remaining participants presented, in some cases, a compensatory effect of the healthy side on the loss of functions of the paretic side. This behavior could have the opposite consequences to those expected with the BCI system, whose emphasis should be on the recovery of the paretic hemisphere at the neuronal and motor level. Therefore, training approaches should be considered involving the cerebral-affected side in BCI strategies.
Now, according to the results presented in
Table 5, a favorable result was concluded regarding user perception. Besides, no patient exhibited affectations in the locomotor system, pressure points, fatigue, stress, or anxiety during the experimental procedure. Within the assessment, the rehabilitation technology weight and the instructions at the time of use were remarkable for patients. Moreover, the reliability and the easiness of the learning process were optimal according to the patients’ perception in the extended test due to the simple versions of the task. This last criterion was beneficial since one of the most encountered problems in current BCI systems with neurological patients was the task learning system [
8]. According to Zickler et al. [
62], this result was helpful for the study in general, since this designed technology is aimed at rehabilitation. Therefore, it is conclusive that the technology complied with a sufficient design for the use of patients. This survey did not eliminate some of the shortcomings previously presented, but it did contribute to subjective patient satisfaction, which may benefit possible long-term studies with T-FLEX.
One of the strengths of this preliminary study is the system integrated strategy to command the portable and low-cost T-FLEX exoskeleton with inter-device connection strategies, relaxation tactics before the experimental session, and straightforward stimulus strategies. However, it was limited in terms of the number of patients, the number of sessions, and the number of mapped channels for offline EEG analysis, from which it was not possible to find statistically significant differences between the stimuli approach.
5. Conclusions
This study presents the BCI integration system to the T-FLEX lower-limb exoskeleton combining two different stimulus modes for post-stroke patients. The experimental results demonstrated the proposed system’s ability to detect MI with an increase on average from 50.7% to 68% when the stimulus was not only visual. Nevertheless, no significant differences were found in the PSD mean of active periods between the ST, MIV, and MIVH tests. Only the Cpz channel appeared to represent differences in ST and MIVH tests related to the sensorial cue and the higher neural activity required during the MI process. In addition, PSD topographic maps showed the contralateral MI activity, which was indispensable to demonstrate the intrasubject variability and the healthy hemisphere response.
In terms of the user’s subjective perception, the BCI system implementation is viable since has a good acceptance. However, deeper and long-term assessments monitoring correlations between muscle and brain activity are required to allow evidence about neuroplasticity induction. Future works should focus on additional data processing and classification procedures to better quantify beta rebound power activity in more complex contexts and considering the MI laterality over the affected side. Likewise, the assessment of the BCI-controlled ankle exoskeleton system in long-term sessions with a more extensive sample of post-stroke patients is indispensable to evaluate the efficiency and effect of the system in a broad spectrum. Moreover, in a larger scope, stimuli with informational and additional feedback strategies should be implemented to improve the MI performance sought by BCI systems.