Boosting Action Observation and Motor Imagery to Promote Plasticity and Learning

Department of Experimental Medicine, Section of Human Physiology, Centro Polifunzionale di Scienze Motorie, University of Genoa, Genoa, Italy Center for Neuroprosthetics, School of Life Science, Swiss Federal Institute of Technology (Ecole Polytechnique Fédérale de Lausanne), Campus Biotech, Geneva and Campus SUVA, Sion, Switzerland Laboratory of Cognitive Neuroscience, Brain Mind Institute, School of Life Science, Swiss Federal Institute of Technology (Ecole Polytechnique Fédérale de Lausanne), Campus Biotech, Geneva, Switzerland INSERM, U1093, Cognition Action Plasticité Sensorimotrice, Université de Bourgogne, Dijon, France Fondazione Istituto Italiano di Tecnologia, Centro di Neurofisiologia Traslazionale c/o Sezione Fisiologia Umana, Ferrara, Italy Neural Control of Movement Laboratory, ETH Zurich, Zurich, Switzerland

In a continuum from fundamental to applied research, many significant scientific contributions in interdisciplinary research fields such as cognitive neuroscience, sport science, and neurorehabilitation provided convincing evidence that action observation (AO, the process of observing actions performed by other people) and motor imagery (MI, the mental execution of action without an overt motor output) might enhance the efficacy of motor training and/or motor recovery by stimulating the activity of the sensorimotor system [1,2]. The scientific rationale behind this idea is that AO and MI activate neural substrates partially overlapped with those activated by movement execution [3][4][5]. The existence of a shared neural representation would support the hypothesis that AO and MI may promote neural plastic changes and behavioral improvements in a way similar to movement execution. Moreover, a growing body of evidence in healthy adults proposed that the combination of AO and MI with each other [6] or with central and peripheral noninvasive stimulations might have a greater impact on brain plasticity and motor learning than when these techniques are applied alone [7][8][9][10].
In line with this emerging hypothesis, this special issue was published. Authors from 11 countries across Europe, Asia, America, and Australia submitted scientific papers in the format of research article (13), clinical study (2), and review article (2) proposing interesting new insights or reviewing the literature on this topic in different research fields such as neurophysiology, human neuroscience, rehabilitation, and sport neuroscience. The result is a collection of 17 articles showing an increasingly widespread interest in studying the neural mechanisms underlying AO and MI and in applying them in combination with movement execution or other stimulation methodologies.
Four neurophysiological studies examined the combination of MI and AO with peripheral and central stimulations and motor practice to access whether and how these combined techniques evoked changes to the central nervous system activity and improvements in behavioral tasks. In particular, E. Traverse at al. investigated how MI associated with somatosensory electrical stimulation (SS) modulated corticospinal and spinal excitability with respect to MI and SS applied alone. The study by E. Saruco et al. addressed the timing-dependent effects of MI combined with anodal transcranial direct current stimulation (a-tDCS) on improving the performance during a postural task. A critical view on the efficacy of AO associated with a-tDCS in providing advantages to motor learning was raised by the results presented by D. Apšvalka et al. who investigated whether a-tDCS applied over the primary motor cortex during observational practice facilitated the acquisition and retention of a keypress sequence learning task compared to a sham treatment. Doubts on the efficacy of observational practice, when compared to physical practice, were expressed in the neurophysiological and behavioral investigation by N. Alhajri et al., which evaluated the degree of mu suppression in those conditions. AO and MI also represent valuable tools to investigate how brain activity changes as a function of age, pathological conditions, or motor expertise. A. Mouthon et al. Durand et al. evaluated the efficacy of personalized observation, execution, and mental imagery (POEM) therapy, a new approach designed to integrate sensorimotor and languagebased strategies to treat verb anomia. Although AO and MI are cognitive stimulation methodologies mostly applied during neurorehabilitation, their efficacy was also shown in the treatment of orthopedic patients. U. Marusic et al. administered an AO + MI intervention combined with conventional rehabilitation techniques to verify its effectiveness in older adults after total hip arthroplasty.
In conclusion, this special issue is aimed at providing a fresh state of the art about new means to evoke neural plasticity and behavioral improvements in healthy adults and sportsmen and suggests innovative therapeutic approaches in addition to pharmacological and conventional treatments during rehabilitation. This collection of papers, together with the existing literature, prompts the use of AO and MI in combination with other stimulation techniques and/or motor practice as a valuable research tool for investigating brain physiology in healthy and pathological conditions and as a fruitful intervention methodology to cope with behavioral and cognitive deficits. Although further studies are necessary to elucidate the brain mechanisms underlying these combined stimulation techniques and to address the criticisms also discussed in this special issue in order to improve and rationalize such a tool, the potentiality of these methods is promising for both clinical and sport performance applications. We hope that this special issue will encourage scientists from different domains to deeply investigate how to boost AO and MI effectiveness with the final aim of discovering new tools for rehabilitation and for performance enhancements in sports.

Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.