Guidelines for reporting action simulation studies (GRASS): Proposals to improve reporting of research in motor imagery and action observation.

Researchers from multiple disciplines have studied the simulation of actions through motor imagery, action observation, or their combination. Procedures used in these studies vary considerably between research groups, and no standardized approach to reporting experimental protocols has been proposed. This has led to under-reporting of critical details, impairing the assessment, replication, synthesis, and potential clinical translation of effects. We provide an overview of issues related to the reporting of information in action simulation studies, and discuss the benefits of standardized reporting. We propose a series of checklists that identify key details of research protocols to include when reporting action simulation studies. Each checklist comprises A) essential methodological details, B) essential details that are relevant to a specific mode of action simulation, and C) further points that may be useful on a case-by-case basis. We anticipate that the use of these guidelines will improve the understanding, reproduction, and synthesis of studies using action simulation, and enhance the translation of research using motor imagery and action observation to applied and clinical settings.

A theoretical perspective on action consequences in action imagery: internal prediction as an essential mechanism to detect errors.

Acting in the environment results in both intended and unintended consequences. Action consequences provide feedback about the adequacy of actions while they are in progress and when they are completed and therefore contribute to monitoring actions, facilitate error detection, and are crucial for motor learning. In action imagery, no actual action takes place, and consequently, no actual action consequences are produced. However, imagined action consequences may replace actual action consequences, serving a similar function and facilitating performance improvements akin to that occurring with actual actions. In this paper, we conceptualize action imagery as a simulation based on internal models. During that simulation, forward models predict action consequences. A comparison of predicted and intended action consequences sometimes indicates the occurrence of action errors (or deviations from optimal performance) in action imagery. We review research indicating that action errors are indeed sometimes imagined in action imagery. These results are compatible with the view that action imagery is based on motor simulation but incompatible with the view that action imagery is solely based on abstract knowledge. The outlined framework seems suitable to cover a wide range of action imagery phenomena and can explain action imagery practice effects.

Learning motor actions via imagery-perceptual or motor learning?

It is well accepted that repeatedly imagining oneself acting without any overt behavior can lead to learning. The prominent theory accounting for why imagery practice is effective, motor simulation theory, posits that imagined action and overt action are functionally equivalent, the exception being activation of the end effector. If, as motor simulation theory states, one can compile the goal, plan, motor program and outcome of an action during imagined action similar to overt action, then learning of novel skills via imagery should proceed in a manner equivalent to that of overt action. While the evidence on motor simulation theory is both plentiful and diverse, it does not explicitly account for differences in neural and behavioural findings between imagined and overt action. In this position paper, we briefly review theoretical accounts to date and present a perceptual-cognitive theory that accounts for often observed outcomes of imagery practice. We suggest that learning by way of imagery reflects perceptual-cognitive scaffolding, and that this 'perceptual' learning transfers into 'motor' learning (or not) depending on various factors. Based on this theory, we characterize consistently reported learning effects that occur with imagery practice, against the background of well-known physical practice effects and show that perceptual-cognitive scaffolding is well-suited to explain what is being learnt during imagery practice.

Imagined movement accuracy is strongly associated with drivers of overt movement error and weakly associated with imagery vividness.

Theories of motor imagery conflict in their account of what happens during an imagined movement, with some suggesting that movement is simulated while others suggest it involves creating and elaborating upon an internal representation of the movement. Here we report evidence that imagery involves the simulation of a movement and that it varies in accuracy. Two groups of participants performed a motor task focused on challenging movement execution either overtly or via motor imagery. Overt performance was used to model expected performance given required movement characteristics (i.e., speed, complexity, familiarity), which was then compared with self-reported accuracy during imagery. Movement characteristics had a large effect on self-reported accuracy compared with a small effect of imagery vividness. Self-reported accuracy improved across trials with familiar movements compared with novel movements in a similar manner for each group. The complexity of the imagined movement did not influence movement time during imagery or overt trials, further suggesting that imagined movements are simulated rather than abstractly represented. Our results therefore support models of motor imagery that involve the simulation of a movement and its viability, which may be the basis of imagery-based motor learning. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Portable wireless and fibreless fNIRS headband compares favorably to a stationary headcap-based system.

This study's purpose is to characterize the performance of a prototype functional near-infrared spectroscopy (fNIRS) headband meant to enable quick and easy measurements from the sensorimotor cortices. The fact that fNIRS is well-suited to ergonomic designs (i.e., their ability to be made wireless, their relative robustness to movement artifacts among other characteristics) has resulted in many recent examples of novel ergonomic fNIRS systems; however, the optical nature of fNIRS measurement presents an inherent challenge to measurement at areas of the brain underlying haired parts of the head. It is for this reason that the majority of ergonomic fNIRS systems that have been developed to date target the prefrontal cortex. In the present study we compared the performance of a novel, portable fNIRS headband compared with a stationary full headcap fNIRS system to measure sensorimotor activity during simple upper- and lower-extremity tasks, in healthy individuals >50 years of age. Both fNIRS systems demonstrated the expected pattern of hemodynamic activity in both upper- and lower-extremity tasks, and a comparison of the contrast-to-noise ratio between the two systems suggests the prototype fNIRS headband is non-inferior to a full head cap fNIRS system regarding the ability to detect a physiological response at the sensorimotor cortex during these tasks. These results suggest the use of a wireless and fibreless fNIRS design is feasible for measurement at the sensorimotor cortex.

Are observed effects of movement simulated during motor imagery performance?

Motor learning relies on adjusting the performance of movements via error detection and correction. How motor learning proceeds via motor imagery, the imagination of movement, is not understood. Motor imagery-based learning is thought to rely on comparing the predicted effect of movement, resulting from the forward model, against its intended effect. Whether motor imagery-based learning uses the observed effect of movement, simulated in motor imagery, to make comparisons to the intended effect to permit error detection and correction, is an open question. To address this, transcranial magnetic stimulation was used to inhibit the left inferior parietal lobe (L_IPL) after each trial of a task requiring participants to reproduce complex trajectories via motor imagery. From past work, we speculated the L_IPL was a candidate for integrating simulated feedback about task performance (simulated observed effects), hypothesizing inhibition of the L_IPL would impair learning, suggesting simulated observed effects of movement are used in motor imagery-based learning. Participants received stimulation to the L_IPL or over the vertex of the head after each trial. Learning was defined as reduced error on a repeated trajectory in comparison to randomly generated trajectories. Regardless of group participants learned, a finding countering our hypothesis, suggesting (a) observed effects of movement are not simulated in motor imagery; (b) the L_IPL is not involved in integrating simulated observed effects of movement; or (c) the timing of the stimulation did not align with the speculated role of the L_IPL. Results encourage further research probing simulated feedback in motor imagery and its neural correlates. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Modality of practice modulates resting state connectivity during motor learning.

When bookending skilled motor practice, changes in resting state functional magnetic resonance imaging (rs-fMRI; used to characterise synchronized patterns of activity when the brain is at rest) reflect functional reorganization that supports motor memory consolidation and learning. Despite its use in practice in numerous domains, the neural mechanisms underlying motor memory consolidation and learning that result from motor imagery practice (MIP) relative to physical practice are not well understood. The current study examined how rs-fMRI is modulated by skilled motor practice that results through either MIP or physical practice. Two groups of participants engaged in five days of MIP or physical practice of a dart throwing task. Performance and rs-fMRI were captured before and after training. Relative to physical practice, where focal changes in rs-fMRI within a cerebellar-cortical network were observed, MIP stimulated widespread changes in rs-fMRI within a frontoparietal network encompassing bilateral regions. Findings indicate functional reorganization that supports motor memory consolidation and learning is not equivalent across practice modalities. Ultimately, this work provides new information regarding the unique neural underpinnings MIP relies on to drive motor memory consolidation and learning.

Investigating the dose-response relationship between motor imagery and motor recovery of upper-limb impairment and function in chronic stroke: A scoping review.

The recovery of upper-limb impairment and dysfunction post-stroke is often incomplete owing to the limited time in therapy focused on upper-limb recovery and the severity of the impairment. In these cases, motor imagery (MI) may be used as a precursor to physical therapies to initiate rehabilitation early on when it would be otherwise impossible to engage in therapy, as well as to increase the dose of therapy when MI is used in adjunct to physical therapy. While previous reviews have shown MI to be effective as a therapeutic option, disparity in findings exists, with some studies suggesting MI is not an effective treatment for post-stroke impairment and dysfunction. One factor contributing to these findings is inconsistency in the dose of MI applied. To explore the relationship between MI dose and recovery, a scoping review of MI literature as a treatment for adult survivors of stroke with chronic upper-limb motor deficit was performed. Embase, Medline and CINHAL databases were searched for articles related to MI and stroke. Following a two-phase review process, 21 papers were included, and data related to treatment dose and measures of impairment and function were extracted. Effect sizes were calculated to investigate the effect of dosage on motor recovery. Findings showed a high degree of variability in dosage regimens across studies, with no clear pattern for the effect of dose on outcome. The present review highlights the gaps in MI literature, including variables that contribute to the dose-response relationship, that future studies should consider when implementing MI.

Imagining the way forward: A review of contemporary motor imagery theory.

Over the past few decades, researchers have become interested in the mechanisms behind motor imagery (i.e., the mental rehearsal of action). During this time several theories of motor imagery have been proposed, offering diverging accounts of the processes responsible for motor imagery and its neural overlap with movement. In this review, we summarize the core claims of five contemporary theories of motor imagery: motor simulation theory, motor emulation theory, the motor-cognitive model, the perceptual-cognitive model, and the effects imagery model. Afterwards, we identify the key testable differences between them as well as their various points of overlap. Finally, we discuss potential future directions for theories of motor imagery.

Examining the role of the supplementary motor area in motor imagery-based skill acquisition.

Motor imagery (MI) and physical practice (PP) have been seen as parallel processes that can drive acquisition of motor skills. Emerging evidence, however, suggests these two processes may be fundamentally different, whereby MI-based motor skill acquisition relies more on effector-independent encoding of movement relative to PP. This alternate view is supported by evidence where real and virtual lesions to brain areas involved in visuospatial processing impair MI-based skill acquisition, and via behavioural studies showing perceptual, but not motor, transfer impairs skill acquisition via MI whereas this effect is reversed in PP. This study further investigated the degree to which MI utilizes effector-independent encoding of movement by investigating the role of the supplementary motor area (SMA), an area involved in perceptual to motor transformations, in MI-based motor skill acquisition. Sixty-four participants completed a serial reaction time paradigm following assignment to one of four groups based on training modality (MI or PP) and stimulation type (sham stimulation or continuous theta burst stimulation to inhibit the SMA). Faster reaction times (RTs) to elements of a repeated sequence in comparison to randomly generated elements indicated that sequence-specific learning occurred. Learning occurred in both PP and MI, with the magnitude of learning significantly smaller in MI. Inhibitory stimulation impaired learning in both modalities. In the context of a framework that distinguishes effector-independent and -dependent components of learning, these findings indicate the SMA plays a role in developing motor chunks in both PP and MI facilitating effector-independent learning in both modalities.

Probing the Effect of Block Duration on Corticospinal Excitability during Motor Imagery Performance.

Considerable evidence exists related to the behavioral outcomes of motor imagery-based training (MI). Comparatively, there is a relative gap in the literature on how corticospinal excitability, a precursor for experience-dependent plasticity, changes over the course of an MI session, and more specifically if there is an effect of varying the duration of the blocks in which MI is performed. As such, we probed corticospinal excitability during MI, whereby the duration of MI blocks within the session were manipulated yet total exposure to MI was kept constant. Participants performed a total of 24 min of MI of common motor tasks in blocks of 2, 4 or 6 min. Transcranial magnetic stimulation was used to assess corticospinal excitability throughout MI performance. All groups demonstrated increased corticospinal excitability over the session. Owing to a decrease in corticospinal excitability when engaging in 6 min blocks and the variability noted when engaging in 2 min blocks, findings suggest that MI performed in 4 min blocks may be preferable for the generation and maintenance of corticospinal excitability, at least relative to 2 and 6 min blocks. Overall, our findings provide physiological evidence that informs the structure of MI training sessions to optimize their effectiveness.

Leveraging the effector independent nature of motor imagery when it is paired with physical practice.

While considered analogous to physical practice, the nature of imagery-based skill acquisition-specifically whether or not both effector independent and dependent encoding occurs through motor imagery-is not well understood. Here, motor imagery-based training was applied prior to or after physical practice-based training to probe the nature of imagery-based skill acquisition. Three groups of participants (N = 38) engaged in 10 days of training of a dart throwing task: 5 days of motor imagery prior to physical practice (MIP-PP), motor imagery following physical practice (PP-MIP), or physical practice only (PP-PP). Performance-related outcomes were assessed throughout. Brain activity was measured at three time points using fMRI (pre/mid/post-training; MIP-PP and PP-MIP groups). In contrast with physical practice, motor imagery led to changes in global versus specific aspects of the movement. Following 10 days of training, performance was greater when motor imagery preceded physical practice, although remained inferior to performance resulting from physical practice alone. Greater activation of regions that support effector dependent encoding was observed mid-, but not post-training for the PP-MIP group. Findings indicate that changes driven by motor imagery reflect effector independent encoding, providing new information regarding how motor imagery may be leveraged for skill acquisition.

Neural and Behavioral Outcomes Differ Following Equivalent Bouts of Motor Imagery or Physical Practice.

Despite its reported effectiveness for the acquisition of motor skills, we know little about how motor imagery (MI)-based brain activation and performance evolves when MI (the imagined performance of a motor task) is used to learn a complex motor skill compared to physical practice (PP). The current study examined changes in MI-related brain activity and performance driven by an equivalent bout of MI- or PP-based training. Participants engaged in 5 days of either MI or PP of a dart-throwing task. Brain activity (via fMRI) and performance-related outcomes were obtained using a pre/post/retention design. Relative to PP, MI-based training did not drive robust changes in brain activation and was inferior for realizing improvements in performance: Greater activation in regions critical to refining the motor program was observed in the PP versus MI group posttraining, and relative to those driven via PP, MI led only to marginal improvements in performance. Findings indicate that the modality of practice (i.e., MI vs. PP) used to learn a complex motor skill manifests as differences in both resultant patterns of brain activity and performance. Ultimately, by directly comparing brain activity and behavioral outcomes after equivalent training through MI versus PP, this work provides unique knowledge regarding the neural mechanisms underlying learning through MI.

Efficacy of low-cost wireless neurofeedback to modulate brain activity during motor imagery.

OBJECTIVES: Motor imagery can be used as an adjunct to traditional stroke rehabilitation therapies for individuals who have hand and arm impairment resulting from their stroke. The provision of neurofeedback during motor imagery allows individuals to receive real time information regarding their motor imagery-related brain activity. However, the equipment required to administer this feedback is expensive and largely inaccessible to many of the individuals who could benefit from it. Available EEG-based technology provides an accessible, low-cost, wireless alternative to traditional neurofeedback methods, with the tradeoff of lower gain and channel count resulting in reduced signal quality. This study investigated the efficacy of this wireless technology for the provision of motor imagery-related neurofeedback. APPROACH: Twenty-eight healthy individuals participated in a 2-group, double-blinded study which involved imagining performing a unimanual button pressing task while receiving neurofeedback that is either a direct transform of their motor imagery-related brain activity (i.e., real) or is related to someone else's brain activity (i.e., sham). The change in amplitude of 15-30 Hz (beta) rhythmic brain activity elicited during the task blocks was calculated and analyzed across sessions and groups. MAIN RESULTS: We found that individuals who received real neurofeedback showed a statistically significant positive trajectory in modulating the amplitude of the beta rhythm across sessions, while those who received sham feedback showed a negative trajectory. Our results did not indicate a trend of increased lateralization across sessions, as has been shown in previous studies. SIGNIFICANCE: Our main findings replicated previous results with research-grade equipment indicating that there is potential for introducing this wireless technology for the provision of neurofeedback. Given the marginal longitudinal effect of neurofeedback in our study, further study is required to address the limitations associated with this technology before our protocol can be implemented in a clinical setting.

Probing the temporal dynamics of movement inhibition in motor imagery.

Beyond the lack of overt movement in motor imagery (MI), MI is thought to be functionally equivalent to motor execution (ME). Two theories appear viable to explain the neural mechanism underlying the inhibition of movement in MI, with one suggesting the inhibition of movement in MI occurs early in the planning process, and the other suggesting it occurs after the planning for movement is compete. Here we sought to generate evidence related to the timing of movement inhibition in MI. Participants performed a motor task via MI and ME that had distinct preparation and performance phases, with brain activity obtained throughout. Analysis of sensor-level data was performed to isolate event related desynchrony (ERD) in the mu and beta frequency bands in both a sensorimotor and left parietal region of interest (ROI). The magnitude of ERD in the sensorimotor ROI was significantly greater in ME than MI during both the preparatory and performance phases. The reduced ERD in the mu and beta frequency bands in the sensorimotor ROI during the preparatory phase for MI, compared to ME, suggests that movement planning is inhibited (or at least reduced) in MI, contributing to the lack of movement. While past work has shown that the networks of functional brain activity underlying MI and ME are heavily overlapping, differences in the temporal dynamics of this activity suggest that MI and ME are not equivalent processes.

Correction: Mild Traumatic Brain Injury (mTBI) and chronic cognitive impairment: A scoping review.

[This corrects the article DOI: 10.1371/journal.pone.0174847.].

Disruption of motor imagery performance following inhibition of the left inferior parietal lobe.

The left inferior parietal lobe (IPL), a brain region localized to the ventro-dorsal stream, is known to be critical to motor imagery (MI) performance. Yet its specific role in processes underlying MI, namely the generation, maintenance, manipulation, and controllability of motor images, is conflicting in the literature. To determine the specific role of the left IPL in MI, the current study sought to examine the effect inhibition of the left IPL has on performance on two disparate measures thought to probe different MI processes within the same participants. Participants (N = 31) completed the hand laterality judgment task (HLJT), employed to probe processes related to manipulation and controllability, and mental chronometry, employed to probe processes related to generation and maintenance, after receiving either inhibitory transcranial magnetic stimulation to the left IPL (Active-TMS group), or with the coil angled away from the scalp (Sham group). Impaired performance on the HLJT was observed following active TMS relative to sham. Similar mental chronometry performance resulted regardless of left IPL inhibition. In showing that inhibition of the left IPL selectively disrupted performance on the HLJT but not mental chronometry, our findings indicate that the left IPL is specifically involved in image manipulation and controllability during MI. Ultimately, the current study extends our understanding of the role of the left IPL in MI.

Intensity of acute aerobic exercise but not aerobic fitness impacts on corticospinal excitability.

Aerobic exercise (AE) modulates cortical excitability. It can alter both corticospinal excitability and intra-cortical networks, which has implications for its use as a tool to facilitate processes such as motor learning, where increased levels of excitability are conducive to the induction of neural plasticity. Little is known about how different intensities of AE modulate cortical excitability or how individual-level characteristics impact on it. Therefore, we investigated whether AE intensities, lower than those previously employed, would be effective in increasing cortical excitability. We also examined whether the aerobic fitness of individual participants was related to the magnitude of change in AE-induced cortical excitability. In both experiments we employed transcranial magnetic stimulation to probe corticospinal excitability before and after AE. We show that 20 min of continuous moderate- (40% and 50% of heart rate reserve, HRR), but not low- (30% HRR) intensity AE was effective at increasing corticospinal excitability. We also found that while we observed increased corticospinal excitability following 20 min of continuous moderate-intensity (50% HRR) AE, aerobic fitness was not related to the magnitude of change. Our results suggest that there is a lower bound intensity of AE that is effective at driving changes in cortical excitability, and that while individual-level characteristics are important predictors of response to AE, aerobic fitness is not. Overall these findings have implication for the way that AE is used to facilitate processes such as motor learning, where increased levels of cortical excitability and plasticity are favourable.

Movement related sensory feedback is not necessary for learning to execute a motor skill.

Sensory feedback has traditionally been considered critical for motor learning. While it has been shown that motor learning can occur in the absence of visual or somatosensory feedback, it is thought that at least one must be present. This assumption contrasts with literature demonstrating that motor imagery (MI) - the mental rehearsal of a movement - is capable of driving motor learning even though the lack of actual execution precludes sensory feedback related to movement. However, studies of MI typically employ simple tasks that do not require improvements in motor execution per se, suggesting that MI might improve task performance primarily through perceptual mechanisms. To avoid this limitation, we designed a novel motor task requiring the repeated execution of unfamiliar kinematic trajectories where learning was assessed through changes in the speed-accuracy function (SAF) across five sessions. General task performance was controlled for by assessing performance on randomly generated trajectories. Groups included physical practice (PP; with and without added visual feedback), MI, and perceptual control (PC), the latter of which only observed the trajectories. All groups performed physically on the final session. Upon the final session, the MI group performed better than the PC group, and better than initial session PP performance. These results suggest that motor learning occurred in the MI group despite the lack of sensory feedback related to the movement, and that this learning was not simply the result of perceptual learning. Our results question long-standing assumptions about MI based learning and the necessity of feedback in motor learning generally.

Experience modulates motor imagery-based brain activity.

Whether or not brain activation during motor imagery (MI), the mental rehearsal of movement, is modulated by experience (i.e. skilled performance, achieved through long-term practice) remains unclear. Specifically, MI is generally associated with diffuse activation patterns that closely resemble novice physical performance, which may be attributable to a lack of experience with the task being imagined vs. being a distinguishing feature of MI. We sought to examine how experience modulates brain activity driven via MI, implementing a within- and between-group design to manipulate experience across tasks as well as expertise of the participants. Two groups of 'experts' (basketball/volleyball athletes) and 'novices' (recreational controls) underwent magnetoencephalography (MEG) while performing MI of four multi-articular tasks, selected to ensure that the degree of experience that participants had with each task varied. Source-level analysis was applied to MEG data and linear mixed effects modelling was conducted to examine task-related changes in activity. Within- and between-group comparisons were completed post hoc and difference maps were plotted. Brain activation patterns observed during MI of tasks for which participants had a low degree of experience were more widespread and bilateral (i.e. within-groups), with limited differences observed during MI of tasks for which participants had similar experience (i.e. between-groups). Thus, we show that brain activity during MI is modulated by experience; specifically, that novice performance is associated with the additional recruitment of regions across both hemispheres. Future investigations of the neural correlates of MI should consider prior experience when selecting the task to be performed.