MENTAL PRACTICE: CLINICAL AND EXPERIMENTAL RESEARCH IN IMAGERY AND ACTION OBSERVATION EDITED BY : Magdalena Ietswaart, Andrew J. Butler, Philip L. Jackson and Martin Gareth Edwards PUBLISHED IN : Frontiers in Human Neuroscience 1 May 2016 | Mental Practice, State of Play Frontiers in Human Neuroscience Frontiers Copyright Statement © Copyright 2007-2016 Frontiers Media SA. All rights reserved. All content included on this site, such as text, graphics, logos, button icons, images, video/audio clips, downloads, data compilations and software, is the property of or is licensed to Frontiers Media SA (“Frontiers”) or its licensees and/or subcontractors. The copyright in the text of individual articles is the property of their respective authors, subject to a license granted to Frontiers. The compilation of articles constituting this e-book, wherever published, as well as the compilation of all other content on this site, is the exclusive property of Frontiers. 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For the full conditions see the Conditions for Authors and the Conditions for Website Use. ISSN 1664-8714 ISBN 978-2-88919-839-9 DOI 10.3389/978-2-88919-839-9 About Frontiers Frontiers is more than just an open-access publisher of scholarly articles: it is a pioneering approach to the world of academia, radically improving the way scholarly research is managed. The grand vision of Frontiers is a world where all people have an equal opportunity to seek, share and generate knowledge. Frontiers provides immediate and permanent online open access to all its publications, but this alone is not enough to realize our grand goals. Frontiers Journal Series The Frontiers Journal Series is a multi-tier and interdisciplinary set of open-access, online journals, promising a paradigm shift from the current review, selection and dissemination processes in academic publishing. 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Find out more on how to host your own Frontiers Research Topic or contribute to one as an author by contacting the Frontiers Editorial Office: researchtopics@frontiersin.org 2 May 2016 | Mental Practice, State of Play Frontiers in Human Neuroscience MENTAL PRACTICE: CLINICAL AND EXPERIMENTAL RESEARCH IN IMAGERY AND ACTION OBSERVATION Photograph by John McPake Topic Editors: Magdalena Ietswaart, University of Stirling, UK Andrew J. Butler, Georgia State University, USA Philip L. Jackson, Universite Laval, Canada Martin Gareth Edwards, Université catholique de Louvain, Belgium There is now strong evidence demonstrating that the brain simulates action and other func- tions. Such action simulation can be evoked through conscious mental rehearsal of move- ment or imagery, but also through passive action observation watching movements in others. Furthermore, there is evidence to suggest that mental rehearsal of movement, or mental practice, can produce improvements normally attributed to practising actual movements. It is currently assumed that such improvements are due to neural activation associated with action simulation. However the neuroscience of mental practice efficacy is still poorly understood. The aim of this research topic is to clarify the underlying mechanisms of mental practice, bringing evidence from 3 May 2016 | Mental Practice, State of Play Frontiers in Human Neuroscience cognitive neuroscience, experimental neuropsychology, sport and movement science, and clinical neurology. It also attempts to address confusion regarding the concepts of imagery and obser- vation, which has hampered the progression of mental practice research both scientifically and applied. As well as reviews, theoretical, and position articles, this research topic includes original neuroimaging, experimental, and patient research addressing, among others, the following issues. Neuroimaging studies provide strong evidence for action simulation, but the link to behavioural change and functional outcome is weak. What is the evidence that mental practice efficacy is driven by neuroplasticity processes evoked by action simulation? This research topic includes contributions on neural correlates and behaviour with regards to imagery and action observation. Much of the mental practice efficacy evidence comes from longstanding research within sport science. However, what does mental practice entail in these contexts, and to what extent is it compatible with the cognitive neuroscience perspective of action simulation? This research topic will include contributions that consider both evidence and concepts with regards to imagery and action observation, in an attempt to build an interdisciplinary consensus on the nature and application of mental practice. Mental practice is perceived as a promising motor rehabilitation technique, but critically there is lack of clarity or consensus on what mental practice treatment should entail. It is also not clear what are the most appropriate outcomes to measure imagery ability and cognitive or behav- ioural change following mental practice. A further important issue that needs consideration as part of this research topic is dosage, as it is currently unclear how much mental practice is appropriate and whether this depends on patient variables such as age, cognitive functioning, motor function, or pathophysiology. Citation: Ietswaart, M., Butler, A. J., Jackson, P. L., Edwards, M. G., eds. (2016). Mental Practice: Clinical and Experimental Research in Imagery and Action Observation. Lausanne: Frontiers Media. doi: 10.3389/978-2-88919-839-9 4 May 2016 | Mental Practice, State of Play Frontiers in Human Neuroscience Table of Contents 06 Editorial: Mental practice: clinical and experimental research in imagery and action observation Magdalena Ietswaart, Andrew J. Butler, Philip L. Jackson and Martin G. Edwards Chapter 1: The Efficacy of Mental Practice 10 Kinesthetic imagery training of forceful muscle contractions increases brain signal and muscle strength Wan X. Yao, Vinoth K. Ranganathan, Didier Allexandre, Vlodek Siemionow and Guang H. Yue 16 Performance improvements from imagery: evidence that internal visual imagery is superior to external visual imagery for slalom performance Nichola Callow, Ross Roberts, Lew Hardy, Dan Jiang and Martin Gareth Edwards 26 The comparison between motor imagery and verbal rehearsal on the learning of sequential movements Arnaud Saimpont, Martin F. Lafleur, Francine Malouin, Carol L. Richards, Julien Doyon and Philip L. Jackson 35 The effects of mental practice in neurological rehabilitation; a systematic review and meta-analysis Susy Braun, Melanie Kleynen, Tessa van Heel, Nena Kruithof, Derick Wade and Anna Beurskens 58 Towards the integration of mental practice in rehabilitation programs. A critical review Francine Malouin, Philip L. Jackson and Carol L. Richards Chapter 2: Mechanisms of Mental Practice 78 Multiple roles of motor imagery during action observation Stefan Vogt, Franck Di Rienzo, Christian Collet, Alan Collins and Aymeric Guillot 91 Using action observation to study superior motor performance: a pilot fMRI study Carl-Johan Olsson and Peter Lundström 99 Does motor imagery share neural networks with executed movement: a multivariate fMRI analysis Nikhil Sharma and Jean-Claude Baron 107 A timely review of a key aspect of motor imagery: a commentary on Guillot et al. (2012) Dave Smith and Caroline Wakefield 111 The impact of sensorimotor experience on affective evaluation of dance Louise P. Kirsch, Kim A. Drommelschmidt and Emily S. Cross 5 May 2016 | Mental Practice, State of Play Frontiers in Human Neuroscience Chapter 3: Assessment and Mental Practice 121 Autonomic nervous system correlates in movement observation and motor imagery C. Collet, F. Di Rienzo, N. El Hoyek and A. Guillot 138 Congruency of gaze metrics in action, imagery and action observation Joe Causer, Sheree A. McCormick and Paul S. Holmes 146 Watch me if you can: imagery ability moderates observational learning effectiveness Gavin Lawrence, Nichola Callow and Ross Roberts 153 Motor imagery ability in stroke patients: the relationship between implicit and explicit motor imagery measures Sjoerd de Vries, Marga Tepper, Wya Feenstra, Hanneke Oosterveld, Anne M. Boonstra and Bert Otten Chapter 4 : Considerations for the Implementation of Mental Practice 163 A standardized motor imagery introduction program (MIIP) for neuro- rehabilitation: development and evaluation C. Wondrusch and C. Schuster-Amft 175 Mental representation and motor imagery training Thomas Schack, Kai Essig, Cornelia Frank and Dirk Koester 185 Motor imagery during action observation modulates automatic imitation effects in rhythmical actions Daniel L. Eaves, Lauren Haythornthwaite and Stefan Vogt 198 Mirror training to augment cross-education during resistance training: a hypothesis Glyn Howatson, Tjerk Zult, Jonathan P. Farthing, Inge Zijdewind and Tibor Hortobágyi EDITORIAL published: 15 October 2015 doi: 10.3389/fnhum.2015.00573 Frontiers in Human Neuroscience | www.frontiersin.org October 2015 | Volume 9 | Article 573 | Edited and reviewed by: Hauke R. Heekeren, Freie Universität Berlin, Germany *Correspondence: Magdalena Ietswaart magdalena.ietswaart@stir.ac.uk Received: 22 June 2015 Accepted: 01 October 2015 Published: 15 October 2015 Citation: Ietswaart M, Butler AJ, Jackson PL and Edwards MG (2015) Editorial: Mental practice: clinical and experimental research in imagery and action observation. Front. Hum. Neurosci. 9:573. doi: 10.3389/fnhum.2015.00573 Editorial: Mental practice: clinical and experimental research in imagery and action observation Magdalena Ietswaart 1 *, Andrew J. Butler 2 , Philip L. Jackson 3 and Martin G. Edwards 4 1 Psychology, University of Stirling, Stirling, UK, 2 Neuroscience Institute, Georgia State University, Atlanta, USA, 3 École de Psychologie and CIRRIS and CRIUSMQ, Université Laval, Québec, Canada, 4 Institut de Recherche en Sciences Psychologiques, Université Catholique de Louvain, Louvain-la-Neuve, Belgium Keywords: mental practice, action simulation, action observation, imagery, cognitive neuroscience This editorial accompanies 18 articles as part of a Frontiers research topic. The aim of this research topic was to clarify the underlying mechanisms involved in mental practice of action, bringing together evidence from a range of disciplines including cognitive neuroscience, experimental neuropsychology, sport and movement science, clinical neuropsychology and clinical neurology. The need to clarify the underlying mechanisms of mental practice is a pressing one. Mental practice of action has been explored in sport psychology for several decades, with the aim to use mental practice to improve sport performance. However, following the discovery of the mirror neuron system (see for example, Rizzolatti and Craighero, 2004), the perspective of mental practice has changed to a rationale based on neuroscience and to research focussed on understanding the neural processes of mental practice. Evidence that the brain simulates action has resulted in a common understanding of “functional equivalence” (Jeannerod, 1994): the idea that the mental representation of an action or percept in the person’s mind is the neural “equivalent” to the physical action or actual percept. This ability to mentally represent action using the motor system allows for action simulation, providing conscious mental rehearsal of movement (imagery), but also allows for a common percept when observing the movements of others. Finally, in recent years, the disciplines of clinical neuropsychology and neurology have begun to use mental rehearsal of action, or mental practice , to produce improvements normally attributed to practicing actual movements. At the heart of all of the research is the idea that mental practice of action uses equivalent neural processes to those used in action execution. Of course, there is debate on what one understands to be “equivalent,” but the common reasoning seems to be that because mental practice (motor imagery and action observation) is functionally or neurally equivalent to actual practice, the efficacy principle of mental practice is that the motor areas are “trained,” perhaps through Hebbian learning “firing-rewiring.” Although the scientific foundation of this idea of action simulation is very sound in neuroimaging research (e.g., Sharma and Baron, 2013, this issue), the link to behavioral evidence or efficacy is currently weak. The neural correlates of mental practice are just that: correlates and do not justify inference about function, efficacy, or critical causality. There nevertheless seems to be reluctance in the field to address the underlying mechanisms of mental practice efficacy. This comes maybe as no surprise. A functional equivalence rationale for mental practice is intuitive and appealing and will therefore attract interest and funding. It is hardly in the researchers’ interest to potentially undermine the idea by getting to the bottom of the matter. We are now 15 or maybe 20 years into mental practice efficacy research based on the neural equivalence premise (Jackson et al., 2001). What is apparent is that the above simple interpretation of equivalence is not reflected in emerging data. It seems that mental practice efficacy is much more complex than simple Hebbian learning. There may be an analogy with the development of our understanding of the supplementary motor area (SMA) over that same time period. Initially SMA was thought of as a simple planning neural strip, but we have since understood the operation 6 Ietswaart et al. Mental practice, state of play of SMA to be highly complex in the way it is involved in inhibition. For example, in studies using fMRI, motor imagery and action observation often do not activate the primary motor cortex (M1) because the SMA is thought to supresses the M1 activity (presumably preventing the individual from actually executing actions). The inherent role of inhibition in mental practice and the complexity of efficacy mechanisms still require further research. The popular notion that anything to do with the mirror neuron system is a simple matter of equivalence, or similarly that in applied contexts of mental practice equivalence is the end of the conversation, needs to change. We now need to go beyond what we are comfortable with and challenge what we know, even if we risk undermining the last decades of research. There are still a lot of things that we do not know about the mechanisms of mental practice of action. What does each part of the brain engaged in mental practice actually do; how do processes relate to one another; what happens when different areas in the network are damaged? There are indications that insufficient drive to address the fundamentals of mental practice is starting to become a real issue of concern. The systematic review in this issue by Braun et al. (2013) concludes that the clinical evidence for mental practice efficacy in neuro-rehabilitation is weakening. The reasons for this seems to be the lack of theory-driven interventions, conceptual confusion (what does mental practice actually entail in practice?) and general methodological malaise including feasibility, dose, responders/non-responders, and adherence issues in larger scale trials that are more representative of clinical practice. Alternatively, when neuroscience evidence is carefully implemented in theory-driven clinical evaluation of mental practice, this may not translate to earlier reported clinical benefit (Ietswaart et al., 2011). Indeed, Malouin et al. (2013) in this issue highlight significant issues with the translation of experimental findings into clinical practice. Malouin et al.’s critical review is constructive, however, by suggesting ways in which the value of mental practice can be redeemed by addressing underlying mechanisms of mental practice efficacy. They conclude that the field must now truly put the use of mental practice to the test. Mental practice may indeed benefit the large number of stroke patients in neuro-rehabilitation, but unless mental practice is truly put to the test, this application may be superseded by other clinical innovations, for example, robotic assisted therapy. The field needs to deliver the necessary clarity on what exactly are the “active ingredients” of mental practice; what are the things that do not work and are mere distractions; which complexities play a role. Only then can we formulate effective guidance on what mental practice should actually entail in clinical practice. In the meantime, mental practice therapy in neuro-rehabilitation is already currently recommended treatment in many clinical guidelines. This current position means that we need to act fast in order to understand the processes and benefits of mental practice for clinical use. However, the current questionable guidance, range of possible uses, lack of efficacy etc. will likely undermine clinicians’ willingness to adopt the treatment in the forthcoming years unless some clarity emerges. Currently, much of the research effort goes to further documenting the correlates of mental practice, i.e., the fact that imagery and observation resonate with other motoric processes. In that respect, a number of the studies reported in this issue are exceptions to this rule in the way these studies ambitiously delineate the mental practice process by for example comparing the quasi-visual and the verbal-cognitive element of mental practice efficacy (Saimpont et al., 2013, this issue), or by contrasting the efficacy of different visual perspectives in mental practice (Callow et al., 2013, this issue; Yao et al., 2013, this issue), or by separating the impact of active imagery and passive observation (Eaves et al., 2014, this issue). It is an issue of concern, however, that such experimental approaches are generally not pursued (nor funded) as part of clinical evaluations, when now is the time to establish the finer details of mental practice efficacy in clinical contexts. We therefore advocate more high risk, high gain evaluations of mental practice that can establish the real impact of mental practice on the lives of real people in the clinic. Further to bringing clarity with regards to the underlying mechanisms of mental practice, there is a real need to establish the modes of delivery and dosage. Clinicians furthermore need tools to make predictions of which patients will benefit and from what types of mental practice treatment. Lack of clarity on patient characteristics such as motor imagery ability can easily lead to miss-use of current findings exposing a risk of clinicians dismissing patients who they believe would not stand to benefit from mental practice–based rehabilitation. It would be great if we could say with some level of certainty whether a brain- damaged patient has an intact ability to use and benefit from mental practice therapies. Some authors would claim this can be done either through subjective methods such as vividness questionnaires, or through more objective methods such as mental chronometry (Milner, 1986), or monitoring automatic covert action simulation such as the cognitive hand mental rotation task established by Parsons (1987), or the response of the autonomic nervous system in mental practice as proposed by Collet et al. (2013, this issue). There is pressure on the research community to provide reliable measures of motor imagery ability on which clinicians can base a decision whether to provide a patient with mental practice rehabilitation. But quite possibly we do not (yet) have reliable tools on which such important decisions can be based. A study by de Vries et al. (2013, this issue) documenting motor imagery ability in stroke patients, showed that poor motor imagery ability as measured by subjective vividness questionnaires was not associated with poor performance also on objective imagery ability assessment. So although vividness scores suggested the patients had poor motor imagery, objective task performance in these stroke patients suggested that motor imagery was in fact intact. This situation could lead to the risk that clinicians when using only vividness scores could dismiss patients as poor imagers and therefore unable to benefit from mental practice-based rehabilitation, while the patients’ imagery ability would be deemed intact if measured in other ways. Although Lawrence et al. (2013, this issue) report that high motor imagery vividness is associated with an increased benefit of mental practice in novice gymnasts compared to the lower performance gains in those with low motor imagery Frontiers in Human Neuroscience | www.frontiersin.org October 2015 | Volume 9 | Article 573 | 7 Ietswaart et al. Mental practice, state of play vividness, this relationship may not be a simple one suitable for rehabilitation treatment decisions. This research topic aimed to address confusion regarding the concepts of imagery and observation which has hampered the progression of mental practice research both scientifically and in translation to clinical practice. Wondrusch and Schuster- Amft (2013, this issue) remarkably point to the need to address any confusion regarding mental practice even at a therapeutic level. They advocate a good understanding of theory and practice in recipients using mental practice rehabilitation techniques by describing ways to teach stroke patients mental practice. Other contributions in this issue broaden the concept of mental practice in a number of ways, such as Howatson et al.’s rationale for including the observation of one’s own movements within the mental practice concept (Howatson et al., 2013, this issue), Smith and Wakefield’s considerations with regards to the timing rate of mental practice (Smith and Wakefield, 2013, this issue), Kirsch et al.’s link between action simulation and aesthetic experience (Kirsch et al., 2013, this issue), Schack et al.’s novel theory of how mental practice develops cognitive mental representation structures (Schack et al., 2014, this issue), and importantly Vogt et al.’s meticulous review of the evidence of why mental practice should encompass both motor imagery and action observation (Vogt et al., 2013, this issue). Because neuroimaging studies provide strong evidence for action simulation, but the link to behavioral change is perhaps weak, we invited contributions to show that mental practice efficacy might be driven by neuroplasticity processes evoked by action simulation. The preliminary work by Olsson and Lundstrom (2013, this issue) shows that successful action anticipation, as a precursor of mental practice, appeared associated with motor and temporal regions of the brain. Future research needs to investigate evidence of the associations between mental practice performance benefits and brain plasticity in the motor network. It is possible that combination of techniques is needed, including functional magnetic resonance imaging (fMRI), diffusion tensor imaging (DTI), MEG, and EEG. In conclusion, in an attempt to build on interdisciplinary consensus on the nature and application of mental practice, this research topic integrated perspectives from the full range of the disciplines involved in mental practice research. It furthermore intentionally did not seek to limit mental practice to a narrow interpretation of conscious mental rehearsal of movement or motor imagery, but instead advocates to include imitation and action observation of self or others as an interpretation of mental practice as Action Simulation Therapy (AST). Such an interpretation of AST mental practice is justified in light of the evidence for neural equivalence. What the neuroscience of neural equivalence means for our understanding of behavior, mechanisms, and applied efficacy of mental practice, however, needs a much more sustained research effort devoid of complacency and supported by high-risk-high-gain research funding. With this shared and funded research drive it will be possible to accelerate our understanding and agreement on the core processes of mental practice, and therefore speed up the translation of evidence-based benefit of applied use of mental practice in sport and clinical practice. REFERENCES Braun, S., Kleynen, M., van Heel, T., Kruithof, N., Wade, D., and Beurskens, A. (2013). The effects of mental practice in neurological rehabilitation; a systematic review and meta-analysis. Front. Hum. Neurosci. 7:390. doi: 10.3389/fnhum.2013.00390 Callow, N., Roberts, R., Hardy, L., Jiang, D., and Edwards, M. G. (2013). Performance improvements from imagery: evidence that internal visual imagery is superior to external visual imagery for slalom performance. Front. Hum. Neurosci. 7:697. doi: 10.3389/fnhum.2013.00697 Collet, C., Di Rienzo, F., El Hoyek, N., and Guillot, A. (2013). Autonomic nervous system correlates in movement observation and motor imagery. Front. Hum. 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Neurosci. 8:328. doi: 10.3389/fnhum.2014.00328 Frontiers in Human Neuroscience | www.frontiersin.org October 2015 | Volume 9 | Article 573 | 8 Ietswaart et al. Mental practice, state of play Sharma, N., and Baron, J. (2013). Does motor imagery share neural networks with executed movement: a multivariate fMRI analysis. Front. Hum. Neurosci. 7:564. doi: 10.3389/fnhum.2013.00564 Smith, D., and Wakefield, C. (2013). A timely review of a key aspect of motor imagery: a commentary on Guillot et al.(2012). Front. Hum. Neurosci. 7:761. doi: 10.3389/fnhum.2013.00761 Vogt, S., Di Rienzo, F., Collet, C., Collins, A., and Guillot, A. (2013). Multiple roles of motor imagery during action observation. Front. Hum. Neurosci. 7:807. doi: 10.3389/fnhum.2013.00807 Wondrusch, C., and Schuster-Amft, C. (2013). A standardized motor imagery introduction program (MIIP) for neuro-rehabilitation: development and evaluation. Front. Hum. Neurosci. 7:477. doi: 10.3389/fnhum.2013.00477 Yao, W. X., Ranganathan, V. K., Allexandre, D., Siemionow, V., and Yue, G. H. (2013). Kinesthetic imagery training of forceful muscle contractions increases brain signal and muscle strength. Front. Hum. Neurosci. 7:561. doi: 10.3389/fnhum.2013. 00561 Conflict of Interest Statement: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Copyright © 2015 Ietswaart, Butler, Jackson and Edwards. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. Frontiers in Human Neuroscience | www.frontiersin.org October 2015 | Volume 9 | Article 573 | 9 ORIGINAL RESEARCH ARTICLE published: 26 September 2013 doi: 10.3389/fnhum.2013.00561 Kinesthetic imagery training of forceful muscle contractions increases brain signal and muscle strength Wan X. Yao 1† , Vinoth K. Ranganathan 2,3† , Didier Allexandre 4 , Vlodek Siemionow 2,3 and Guang H. Yue 2,3,4 * 1 Department of Health and Kinesiology, University of Texas at San Antonio, San Antonio, TX, USA 2 Department of Physical Medicine & Rehabilitation, Neurological Institute, The Cleveland Clinic, Cleveland, OH, USA 3 Department of Biomedical Engineering, Lerner Research Institute, The Cleveland Clinic, Cleveland, OH, USA 4 Human Performance and Engineering Laboratory, Kessler Foundation Research Center, West Orange, NJ, USA Edited by: Martin G. Edwards, Université catholique de Louvain, Belgium Reviewed by: Martin G. Edwards, Université catholique de Louvain, Belgium Magdalena Ietswaart, University of Stirling, UK Philip L. Jackson, Universite Laval, Canada *Correspondence: Guang H. Yue, Human Performance and Engineering Laboratory, Kessler Foundation Research Center, 1199 Pleasant Valley Way, West Orange, NJ 07052, USA e-mail: gyue@kesslerfoundation.org † These authors have contributed equally to this work. The purpose of this study was to compare the effect of training using internal imagery (IMI; also known as kinesthetic imagery or first person imagery) with that of external imagery (EMI; also known as third-person visual imagery) of strong muscle contractions on voluntary muscle strengthening. Eighteen young, healthy subjects were randomly assigned to one of three groups (6 in each group): internal motor imagery (IMI), external motor imagery (EMI), or a no-practice control (CTRL) group. Training lasted for 6 weeks ( ∼ 15 min/day, 5 days/week). The participants’ right arm elbow-flexion strength, muscle electrical activity, and movement-related cortical potential (MRCP) were evaluated before and after training. Only the IMI group showed significant strength gained (10.8%) while the EMI (4.8%) and CTRL ( − 3 3%) groups did not. Only the IMI group showed a significant elevation in MRCP on scalp locations over both the primary motor (M1) and supplementary motor cortices (EMI group over M1 only) and this increase was significantly greater than that of EMI and CTRL groups. These results suggest that training by IMI of forceful muscle contractions was effective in improving voluntary muscle strength without physical exercise. We suggest that the IMI training likely strengthened brain-to-muscle (BTM) command that may have improved motor unit recruitment and activation, and led to greater muscle output. Training by IMI of forceful muscle contractions may change the activity level of cortical motor control network, which may translate into greater descending command to the target muscle and increase its strength. Keywords: motor imagery training, muscle strength, electroencephalography (EEG), movement-related cortical potential (MRCP) INTRODUCTION Accumulating evidence suggests that mental training with- out physical or muscle exercise can improve voluntary muscle strength (Yue and Cole, 1992; Yue et al., 1996; Smith et al., 2003; Zijdewind et al., 2003; Ranganathan et al., 2004; Sidaway and Trzaska, 2005; Shackell and Standing, 2007). This finding could have significant application in rehabilitation medicine (Jackson et al., 2001) because numerous weak patients or frail older adults who find it difficult or unsafe to participate in conventional strength training (such as weightlifting) programs, may now be able to strengthen their muscles by using their mind. It has been shown that the main underlying mechanism for motor imagery (MI) training-induced strength gains is by adaptations occurring in the nervous system. For example, after 4 weeks of mental training, the strength of the little finger abduction force increased 22%; the augmentation accompanied an increase in the electromyographic (EMG) signal of the finger abductor that represented overall neural input to the muscle (Yue and Cole, 1992). In another study, two groups of volunteers had their lit- tle finger of the left hand immobilized for 4 weeks during which one group performed MI training of maximal voluntary con- tractions (MVC) and the other [control (CTRL) group] did not. After immobilization, both groups showed muscle atrophy but strength reduction only occurred in the CTRL group. The MI group maintained the strength with a significant increase in the EMG signal despite muscle atrophy caused by immobilization (Yue et al., 1996). In this case, the increase of neural (EMG) signal appeared to compensate for strength loss due to the atrophy. More recently, Ranganathan et al. (2004) demonstrated MI training- induced strength gains in a finger and upper-arm muscle that accompanied an elevation in the cortical signal directly related to the execution of strength-production muscle contractions. These observations support the hypothesis that the descend- ing command from the brain to target muscle for MVC can be strengthened by MI training alone, which in turn increases max- imal muscle force by recruiting additional motor units and/or increasing activation level of the participating motor units. Despite finding of significant strength gains by MI training in a majority of studies in this area, one investigation (Herbert et al., 1998) did not reported similar results. In this study, no significant strength gain specifically associated with MI training was observed following an 8-week training program. The dis- crepancy in the results between this (Herbert et al., 1998) and other MI strength training studies could have been caused by Frontiers in Human Neuroscience www.frontiersin.org September 2013 | Volume 7 | Article 561 | HUMAN NEUROSCIENCE 10 Yao et al. Motor imagery training and strength gain different imagery procedures adopted by the investigators. There are two common types of mental imagery—internal and exter- nal imagery. In internal imagery (IMI; also known as kinesthetic or first-person imagery), a person imagines or mentally creates the feeling of performing the exercise from within the body (i.e., from a first-person perspective). For example, mental strength training using internal imagery emphasizes that the subject gen- erates a similar feeling as he/she felt during a physical MVC (e.g., Ranganathan et al., 2004; Sidaway and Trzaska, 2005). In exter- nal imagery (EMI; also known as third-person visual imagery), the person sees or visualizes performing the task from outside the body—similar to watching oneself in a mirror performing an exercise (i.e., from a third-person perspective). Performing IMI generates significantly more physiological responses [such as heart rate (HR), blood pressure (BP), and respiration rate] compared to doing EMI (Ranganathan et al., 2004). Many stud- ies (Mumford and Hall, 1985; Murphy, 1994; White and Hardy, 1995; Reed, 2002) have indicated that IMI is superior to EMI for improving motor skills. Studies have reported that highly skilled athletes predominantly use IMI to enhance their perfor- mance (e.g., Roure et al., 1998). It is possible that participants in the study of Herbert et al. (1998) adopted EMI procedure for the mental training, which did not result in a significant strength gain. For those studies that demonstrated significant strength increases, the MI training procedure was clearly using IMI (Yue and Cole, 1992; Yue et al., 1996; Smith et al., 2003; Zijdewind et al., 2003; Ranganathan et al., 2004; Sidaway and Trzaska, 2005; Shackell and Standing, 2