Time course of changes in corticospinal excitability induced by motor imagery during action observation combined with peripheral nerve electrical stimulation

A commentary on Eaves et al. with a special focus on clinical neurorehabilitation

We very much appreciate the theoretical foundations and considerations of AO, MI, and their combination AO + MI by Eaves et al. In their exploratory review, the authors highlight the beneficial effects of the combined use of AO and MI, with a particular focus on synchronous AO and MI. From a neurorehabilitation perspective, different processes may apply to patients, particularly after a stroke. As suggested by Eaves et al., the cognitive load might prevent the use of synchronous AO + MI and the asynchronous application of AO and MI might be indicated. Furthermore, some aspects should be considered when applying AO + MI in rehabilitation: screening for the patients' cognitive capabilities and MI ability, and a familiarisation programme for AO and MI, before starting with an AO + MI training. With their review, Eaves et al. propose a number of research questions in the field of neurorehabilitation that urgently need to be addressed: the use of asynchronous vs. synchronous AOMI, observation and imagination with or without errors, or use of different MI perspectives and modes in different learning stages. This commentary provides some additional suggestions on patients' MI ability and cognitive level, MI familiarisation and detailed reporting recommendations to transfer Eaves et al. findings into clinical practice.

Virtual Reality Action Observation and Motor Imagery to Enhance Neuroplastic Capacity in the Human Motor Cortex: A Pilot Double-blind, Randomized Cross-over Trial

Neuroplasticity is important for learning, development and recovery from injury. Therapies that can upregulate neuroplasticity are therefore of interest across a range of fields. We developed a novel virtual reality action observation and motor imagery (VR-AOMI) intervention and evaluated whether it could enhance the efficacy of mechanisms of neuroplasticity in the human motor cortex of healthy adults. A secondary question was to explore predictors of the change in neuroplasticity following VR-AOMI. A pre-registered, pilot randomized controlled cross-over trial was performed. Twenty right-handed adults (13 females; mean age: 23.0 ± 4.53 years) completed two experimental conditions in separate sessions; VR-AOMI and control. We used intermittent theta burst stimulation (iTBS) to induce long term potentiation-like plasticity in the motor cortex and recorded motor evoked potentials at multiple timepoints as a measure of corticospinal excitability. The VR-AOMI task did not significantly increase the change in MEP amplitude following iTBS when compared to the control task (Group × Timepoint interaction p = 0.17). However, regression analysis identified the change in iTBS response following VR-AOMI was significantly predicted by the baseline iTBS response in the control task. Specifically, participants that did not exhibit the expected increase in MEP amplitude following iTBS in the control condition appear to have greater excitability following iTBS in the VR-AOMI condition (r = -0.72, p < 0.001). Engaging in VR-AOMI might enhance capacity for neuroplasticity in some people who typically do not respond to iTBS. VR-AOMI may prime the brain for enhanced neuroplasticity in this sub-group.

Attentional State-Dependent Peripheral Electrical Stimulation During Action Observation Enhances Cortical Activations in Stroke Patients

Brain-computer interface (BCI) is a promising technique that enables patients' interaction with computers or machines by analyzing specific brain signal patterns and provides patients with brain state-dependent feedback to assist in their rehabilitation. Action observation (AO) and peripheral electrical stimulation (PES) are conventional methods used to enhance rehabilitation outcomes by promoting neural plasticity. In this study, we assessed the effects of attentional state-dependent feedback in the combined application of BCI-AO with PES on sensorimotor cortical activation in patients after stroke. Our approach involved showing the participants a video with repetitive grasping actions under four different tasks. A mu band suppression (8-13 Hz) corresponding to each task was computed. A topographical representation showed that mu suppression of the dominant (healthy) and affected hemispheres (stroke) gradually became prominent during the tasks. There were significant differences in mu suppression in the affected motor and frontal cortices of the stroke patients. The involvement of both frontal and motor cortices became prominent in the BCI-AO+triggered PES task, in which feedback was given to the patients according to their attentive watching. Our findings suggest that synchronous stimulation according to patient attention is important for neurorehabilitation of stroke patients, which can be achieved with the combination of BCI-AO feedback with PES. BCI-AO feedback combined with PES could be effective in facilitating sensorimotor cortical activation in the affected hemispheres of stroke patients.

Non-invasive brain-spine interface: Continuous control of trans-spinal magnetic stimulation using EEG

Brain-controlled neuromodulation has emerged as a promising tool to promote functional recovery in patients with motor disorders. Brain-machine interfaces exploit this neuromodulatory strategy and could be used for restoring voluntary control of lower limbs. In this work, we propose a non-invasive brain-spine interface (BSI) that processes electroencephalographic (EEG) activity to volitionally control trans-spinal magnetic stimulation (ts-MS), as an approach for lower-limb neurorehabilitation. This novel platform allows to contingently connect motor cortical activation during leg motor imagery with the activation of leg muscles via ts-MS. We tested this closed-loop system in 10 healthy participants using different stimulation conditions. This BSI efficiently removed stimulation artifacts from EEG regardless of ts-MS intensity used, allowing continuous monitoring of cortical activity and real-time closed-loop control of ts-MS. Our BSI induced afferent and efferent evoked responses, being this activation ts-MS intensity-dependent. We demonstrated the feasibility, safety and usability of this non-invasive BSI. The presented system represents a novel non-invasive means of brain-controlled neuromodulation and opens the door towards its integration as a therapeutic tool for lower-limb rehabilitation.

Changes in corticospinal and spinal reflex excitability through functional electrical stimulation with and without observation and imagination of walking

Functional electrical stimulation (FES), a method for inducing muscle contraction, has been successfully used in gait rehabilitation for patients with deficits after neurological disorders and several clinical studies have found that it can improve gait function after stroke and spinal cord injury. However, FES gait training is not suitable for patients with walking difficulty, such as those with severe motor paralysis of the lower limbs. We have previously shown that action observation combined with motor imagery (AO + MI) of walking induces walking-related cortical activity. Therefore, we combined FES, which alternately generates dorsiflexion and plantar flexion, with AO + MI as an alternative to gait training. The present study investigates the transient effects of 20-min of FES simultaneously with and without AO + MI of walking on corticospinal and spinal reflex excitability in able-bodied participants. We measured motor evoked potentials and Hoffmann-reflexes to assess corticospinal and spinal reflex excitability at rest before and after the 20-min FES with and without the AO + MI. Our results show that FES without AO + MI did not change excitability (p &gt; 0.05), while FES with AO + MI facilitated corticospinal excitability (p &lt; 0.05). This facilitation likely occurred due to the synchronization of sensory inputs from FES and cortical activity during AO + MI. Facilitation was observed only in the dorsiflexor but not the plantar flexor muscle (p &lt; 0.05), suggesting muscle specificity of the facilitation. These results demonstrate the effectiveness of combining FES with AO + MI and pave the way for novel neurorehabilitation strategies for patients with neurological gait deficits.

Brain-computer interface-based action observation combined with peripheral electrical stimulation enhances corticospinal excitability in healthy subjects and stroke patients

Objective.Action observation (AO) combined with brain-computer interface (BCI) technology enhances cortical activation. Peripheral electrical stimulation (PES) increases corticospinal excitability, thereby activating brain plasticity. To maximize motor recovery, we assessed the effects of BCI-AO combined with PES on corticospinal plasticity.Approach.Seventeen patients with chronic hemiplegic stroke and 17 healthy subjects were recruited. The participants watched a video of repetitive grasping actions with four different tasks for 15 min: (A) AO alone; (B) AO + PES; (C) BCI-AO + continuous PES; and (D) BCI-AO + triggered PES. PES was applied at the ulnar nerve of the wrist. The tasks were performed in a random order at least three days apart. We assessed the latency and amplitude of motor evoked potentials (MEPs). We examined changes in MEP parameters pre-and post-exercise across the four tasks in the first dorsal interosseous muscle of the dominant hand (healthy subjects) and affected hand (stroke patients).Main results.The decrease in MEP latency and increase in MEP amplitude after the four tasks were significant in both groups. The increase in MEP amplitude was sustained for 20 min after tasks B, C, and D in both groups. The increase in MEP amplitude was significant between tasks A vs. B, B vs. C, and C vs. D. The estimated mean difference in MEP amplitude post-exercise was the highest for A and D in both groups.Significance.The results indicate that BCI-AO combined with PES is superior to AO alone or AO + PES for facilitating corticospinal plasticity in both healthy subjects and patients with stroke. Furthermore, this study supports the idea that synchronized activation of cortical and peripheral networks can enhance neuroplasticity after stroke. We suggest that the BCI-AO paradigm and PES could provide a novel neurorehabilitation strategy for patients with stroke.

The acute effects of action observation on muscle strength/weakness and corticospinal excitability in older adults

Muscle weakness is a critical problem facing many older adults. Interventions targeting nervous system plasticity may show promise in enhancing strength. The purpose of this study was to examine the acute effects of action observation on muscular strength characteristics and corticospinal excitability in older adults. Isometric wrist flexion strength characteristics and corticospinal excitability of the first dorsal interosseous (FDI) were measured in 14 older adults (mean age = 73 years) in response to observation of (1) STRONG contractions of the hand/wrist, (2) WEAK contractions of the hand/wrist, and (3) a CONTROL condition. Results from repeated measures analyses of variance (ANOVAs) indicated that rate of torque development at 200 ms (RTD200) significantly decreased from PRE to POST observation for CONTROL and WEAK, but not STRONG. No other ANOVAs were significant. However, effect sizes indicated that maximal voluntary contraction (MVC) peak torque showed moderate declines following WEAK (d = − 0.571) and CONTROL (d = − 0.636), but not STRONG (d = 0.024). Similarly, rate of torque development at 30 (RTD30), 50 (RTD50), and 200 (RTD200) ms showed large declines from PRE to POST after WEAK and CONTROL, but small changes following STRONG. FDI motor-evoked potential (MEP) amplitude tended to increase over time, but these results were variable. There was a pronounced effect from PRE to 8MIN (d = 0.954) during all conditions. Action observation of strong contractions may exert a preservatory effect on muscular strength. More work is needed to determine whether this is modulated by increased corticospinal excitability. The study was prospectively registered (ClinicalTrials.gov Identifier: NCT03946709).

Effects of action observation and motor imagery of walking on the corticospinal and spinal motoneuron excitability and motor imagery ability in healthy participants

Action observation (AO) and motor imagery (MI) are used for the rehabilitation of patients who face difficulty walking. Rehabilitation involving AO, MI, and AO combined with MI (AO+MI) facilitates gait recovery after neurological disorders. However, the mechanism by which it positively affects gait function is unclear. We previously examined the neural mechanisms underlying AO and MI of walking, focusing on AO+MI and corticospinal and spinal motor neuron excitability, which play important roles in gait function. Herein, we investigated the effects of a short intervention using AO+MI of walking on the corticospinal and spinal motor neuron excitability and MI ability of participants. Twelve healthy individuals participated in this study, which consisted of a 20 min intervention. Before the experiment, we measured MI ability using the Vividness of Movement Imagery Questionnaire-2 (VMIQ-2). We used motor evoked potential and F-wave measurements to evaluate the corticospinal and spinal motor neuron excitability at rest, pre-intervention, 0 min, and 15 min post-intervention. We also measured corticospinal excitability during MI of walking and the participant’s ability to perform MI using a visual analog scale (VAS). There were no significant changes in corticospinal and spinal motor neuron excitability during and after the intervention using AO+MI (p>0.05). The intervention temporarily increased VAS scores, thus indicating clearer MI (p<0.05); however, it did not influence corticospinal excitability during MI of walking (p>0.05). Furthermore, there was no significant correlation between the VMIQ-2 and VAS scores and changes in corticospinal and spinal motor neuron excitability. Therefore, one short intervention using AO+MI increased MI ability in healthy individuals; however, it was insufficient to induce plastic changes at the cortical and spinal levels. Moreover, the effects of intervention using AO+MI were not associated with MI ability. Our findings provide information about intervention using AO+MI in healthy individuals and might be helpful in planning neurorehabilitation strategies.

Observation of others' actions during limb immobilization prevents the subsequent decay of motor performance

There is rich clinical evidence that observing normally executed actions promotes the recovery of the corresponding action execution in patients with motor deficits. In this study, we assessed the ability of action observation to prevent the decay of healthy individuals' motor abilities following upper-limb immobilization. To this end, upper-limb kinematics was recorded in healthy participants while they performed three reach-to-grasp movements before immobilization and the same movements after 16 h of immobilization. The participants were subdivided into two groups; the experimental group observed, during the immobilization, the same reach-to-grasp movements they had performed before immobilization, whereas the control group observed natural scenarios. After bandage removal, motor impairment in performing reach-to-grasp movements was milder in the experimental group. These findings support the hypothesis that action observation, via the mirror mechanism, plays a protective role against the decline of motor performance induced by limb nonuse. From this perspective, action observation therapy is a promising tool for anticipating rehabilitation onset in clinical conditions involving limb nonuse, thus reducing the burden of further rehabilitation.

Why brain-controlled neuroprosthetics matter: mechanisms underlying electrical stimulation of muscles and nerves in rehabilitation

Delivering short trains of electric pulses to the muscles and nerves can elicit action potentials resulting in muscle contractions. When the stimulations are sequenced to generate functional movements, such as grasping or walking, the application is referred to as functional electrical stimulation (FES). Implications of the motor and sensory recruitment of muscles using FES go beyond simple contraction of muscles. Evidence suggests that FES can induce short- and long-term neurophysiological changes in the central nervous system by varying the stimulation parameters and delivery methods. By taking advantage of this, FES has been used to restore voluntary movement in individuals with neurological injuries with a technique called FES therapy (FEST). However, long-lasting cortical re-organization (neuroplasticity) depends on the ability to synchronize the descending (voluntary) commands and the successful execution of the intended task using a FES. Brain-computer interface (BCI) technologies offer a way to synchronize cortical commands and movements generated by FES, which can be advantageous for inducing neuroplasticity. Therefore, the aim of this review paper is to discuss the neurophysiological mechanisms of electrical stimulation of muscles and nerves and how BCI-controlled FES can be used in rehabilitation to improve motor function.

Neuromuscular electrical stimulation-promoted plasticity of the human brain

The application of neuromuscular electrical stimulation (NMES) to paretic limbs has demonstrated utility for motor rehabilitation following brain injury. When NMES is delivered to a mixed peripheral nerve, typically both efferent and afferent fibres are recruited. Muscle contractions brought about by the excitation of motor neurons are often used to compensate for disability by assisting actions such as the formation of hand aperture, or by preventing others including foot drop. In this context, exogenous stimulation provides a direct substitute for endogenous neural drive. The goal of the present narrative review is to describe the means through which NMES may also promote sustained adaptations within central motor pathways, leading ultimately to increases in (intrinsic) functional capacity. There is an obvious practical motivation, in that detailed knowledge concerning the mechanisms of adaptation has the potential to inform neurorehabilitation practice. In addition, responses to NMES provide a means of studying CNS plasticity at a systems level in humans. We summarize the fundamental aspects of NMES, focusing on the forms that are employed most commonly in clinical and experimental practice. Specific attention is devoted to adjuvant techniques that further promote adaptive responses to NMES thereby offering the prospect of increased therapeutic potential. The emergent theme is that an association with centrally initiated neural activity, whether this is generated in the context of NMES triggered by efferent drive or via indirect methods such as mental imagery, may in some circumstances promote the physiological changes that can be induced through peripheral electrical stimulation.

Kinaesthetic illusion shapes the cortical plasticity evoked by action observation

KEY POINTS

The combination of action observation (AO) and a peripheral nerve stimulation has been shown to induce plasticity in the primary motor cortex (M1). However, using peripheral nerve stimulation little is known about the specificity of the sensory inputs. The current study, using muscle tendon vibration to stimulate muscle spindles and transcranial magnetic stimulation to assess M1 excitability, investigated whether a proprioceptive stimulation leading to a kinaesthetic illusion of movement (KI) was able to evoke M1 plasticity when combined with AO. M1 excitability increased immediately and up to 60 min after AO-KI stimulation as a function of the vividness of the perceived illusion, and only when the movement directions of AO and KI were congruent. Tactile stimulation coupled with AO and KI alone were not sufficient to induce M1 plasticity. This methodology might be proposed to subjects during a period of immobilization to promote M1 activity without requiring any voluntary movement.

ABSTRACT

Physical practice is crucial to evoke cortical plasticity, but motor cognition techniques, such as action observation (AO), have shown their potentiality in promoting it when associated with peripheral afferent inputs, without the need of performing a movement. Here we investigated whether the combination of AO and a proprioceptive stimulation, able to evoke a kinaesthetic illusion of movement (KI), induced plasticity in the primary motor cortex (M1). In the main experiment, the role of congruency between the observed action and the illusory movement was explored together with the importance of the specificity of the sensory input modality (proprioceptive vs. tactile stimulation) to induce plasticity in M1. Further, a control experiment was carried out to assess the role of the mere kinaesthetic illusion on M1 excitability. Results showed that the combination of AO and KI evoked plasticity in M1, with an increase of the excitability immediately and up to 60 min after the conditioning protocol (P always <0.05). Notably, a significant increase in M1 excitability occurred only when the directions of the observed and illusory movements were congruent. Further, a significant positive linear relationship was found between the amount of M1 excitability increase and the vividness of the perceived illusion (P = 0.03). Finally, the tactile stimulation coupled with AO was not sufficient to induce changes in M1 excitability as well as the KI alone. All these findings indicate the importance of combining different sensory input signals to induce plasticity in M1, and that proprioception is the most suitable sensory modality to allow it.

Effects of Leg Motor Imagery Combined With Electrical Stimulation on Plasticity of Corticospinal Excitability and Spinal Reciprocal Inhibition

Motor imagery (MI) combined with electrical stimulation (ES) enhances upper-limb corticospinal excitability. However, its after-effects on both lower limb corticospinal excitability and spinal reciprocal inhibition remain unknown. We aimed to investigate the effects of MI combined with peripheral nerve ES (MI + ES) on the plasticity of lower limb corticospinal excitability and spinal reciprocal inhibition. Seventeen healthy individuals performed the following three tasks on different days, in a random order: (1) MI alone; (2) ES alone; and (3) MI + ES. The MI task consisted of repetitive right ankle dorsiflexion for 20 min. ES was percutaneously applied to the common peroneal nerve at a frequency of 100 Hz and intensity of 120% of the sensory threshold of the tibialis anterior (TA) muscle. We examined changes in motor-evoked potential (MEP) of the TA (task-related muscle) and soleus muscle (SOL; task-unrelated muscle). We also examined disynaptic reciprocal inhibition before, immediately after, and 10, 20, and 30 min after the task. MI + ES significantly increased TA MEPs immediately and 10 min after the task compared with baseline, but did not change the task-unrelated muscle (SOL) MEPs. MI + ES resulted in a significant increase in the magnitude of reciprocal inhibition immediately and 10 min after the task compared with baseline. MI and ES alone did not affect TA MEPs or reciprocal inhibition. MI combined with ES is effective in inducing plastic changes in lower limb corticospinal excitability and reciprocal Ia inhibition.