Practice modality of motor sequences impacts the neural signature of motor imagery.

Motor imagery is conceptualized as an internal simulation that uses motor-related parts of the brain as its substrate. Many studies have investigated this sharing of common neural resources between the two modalities of motor imagery and motor execution. They have shown overlapping but not identical activation patterns that thereby result in a modality-specific neural signature. However, it is not clear how far this neural signature depends on whether the imagined action has previously been practiced physically or only imagined. The present study aims to disentangle whether the neural imprint of an imagined manual pointing sequence within cortical and subcortical motor areas is determined by the nature of this prior practice modality. Each participant practiced two sequences physically, practiced two other sequences mentally, and did a behavioural pre-test without any further practice on a third pair of sequences. After a two-week practice intervention, participants underwent fMRI scans while imagining all six sequences. Behavioural data demonstrated practice-related effects as well as very good compliance with instructions. Functional MRI data confirmed the previously known motor imagery network. Crucially, we found that mental and physical practice left a modality-specific footprint during mental motor imagery. In particular, activation within the right posterior cerebellum was stronger when the imagined sequence had previously been practiced physically. We conclude that cerebellar activity is shaped specifically by the nature of the prior practice modality.

Sensory features of mental images in the framework of human actions.

What determines the sensory impression of a self-generated motor image? Motor imagery is a process in which subjects imagine executing a body movement with a strong kinesthetic and/or visual component from a first-person perspective. Both sensory modalities can be combined flexibly to form a motor image. 90 participants of varying ages had to freely generate motor images from a large set of movements. They were asked to rate their kinesthetic as well as their visual impression, the perceived vividness, and their personal experience with the imagined movement. Data were subjected to correlational analyses, linear regressions, and representation similarity analyses. Results showed that both action characteristics and experience drove the sensory impression of motor images with a strong individual component. We conclude that imagining actions that impose varying demands can be considered as reexperiencing actions by using one's own sensorimotor representations that represent not only individual experience but also action demands.

Subjective vividness of motor imagery has a neural signature in human premotor and parietal cortex.

Motor imagery (MI) is the process in which subjects imagine executing a body movement with a strong kinesthetic component from a first-person perspective. The individual capacity to elicit such mental images is not universal but varies within and between subjects. Neuroimaging studies have shown that these inter-as well as intra-individual differences in imagery quality mediate the amplitude of neural activity during MI on a group level. However, these analyses were not sensitive to forms of representation that may not map onto a simple modulation of overall amplitude. Therefore, the present study asked how far the subjective impression of motor imagery vividness is reflected by a spatial neural code, and how patterns of neural activation in different motor regions relate to specific imagery impressions. During fMRI scanning, 20 volunteers imagined three different types of right-hand actions. After each imagery trial, subjects were asked to evaluate the perceived vividness of their imagery. A correlation analysis compared the rating differences and neural dissimilarity values of the rating groups separately for each region of interest. Results showed a significant positive correlation in the left vPMC and right IPL, indicating that these regions particularly reflect perceived imagery vividness in that similar rated trials evoke more similar neural patterns. A decoding analysis revealed that the vividness of the motor image related systematically to the action specificity of neural activation patterns in left vPMC and right SPL. Imagined actions accompanied by higher vividness ratings were significantly more distinguishable within these areas. Altogether, results showed that spatial patterns of neural activity within the human motor cortices reflect the individual vividness of imagined actions. Hence, the findings reveal a link between the subjective impression of motor imagery vividness and objective physiological markers.

Imagined and Executed Actions in the Human Motor System: Testing Neural Similarity Between Execution and Imagery of Actions with a Multivariate Approach.

Simulation theory proposes motor imagery (MI) to be a simulation based on representations also used for motor execution (ME). Nonetheless, it is unclear how far they use the same neural code. We use multivariate pattern analysis (MVPA) and representational similarity analysis (RSA) to describe the neural representations associated with MI and ME within the frontoparietal motor network. During functional magnetic resonance imaging scanning, 20 volunteers imagined or executed 3 different types of right-hand actions. Results of MVPA showed that these actions as well as their modality (MI or ME) could be decoded significantly above chance from the spatial patterns of BOLD signals in premotor and posterior parietal cortices. This was also true for cross-modal decoding. Furthermore, representational dissimilarity matrices of frontal and parietal areas showed that MI and ME representations formed separate clusters, but that the representational organization of action types within these clusters was identical. For most ROIs, this pattern of results best fits with a model that assumes a low-to-moderate degree of similarity between the neural patterns associated with MI and ME. Thus, neural representations of MI and ME are neither the same nor totally distinct but exhibit a similar structural geometry with respect to different types of action.

Motor imagery of locomotion with an additional load: actual load experience does not affect differences between physical and mental durations.

Motor imagery relies strongly on motor representations. Currently, it is widely accepted that both the imagery and execution of actions share the same neural representations (Jeannerod, Neuroimage 14:S103-S109, 2001). Comparing mental with actual movement durations opens a window through which to examine motor representations and how they relate to cognitive motor processes. The present experiment examined mental durations reported by participants standing upright who imagined walking either with or without an additional load while actually carrying or not carrying that same load. Results showed a robust effect of longer durations when imagining the additional load during mental walking, whereas physical walking with an additional load did not extend movement durations accordingly. However, experiencing an actual load during imagery did not influence mental durations substantially. This dissociation of load-related effects can be interpreted as being due to an interaction of motor processes and their cognitive representation along with a reduction in neural activity in vestibular and somatosensory areas during imagery of locomotion. It is argued that this effect might be specific to locomotion and not generalize to a broader range of movements.

How equivalent are the action execution, imagery, and observation of intransitive movements? Revisiting the concept of somatotopy during action simulation.

Jeannerod (2001) hypothesized that action execution, imagery, and observation are functionally equivalent. This led to the major prediction that these motor states are based on the same action-specific and even effector-specific motor representations. The present study examined whether hand and foot movements are represented in a somatotopic manner during action execution, imagery, and action observation. The experiment contained ten conditions: three execution conditions, three imagery conditions, three observation conditions, and one baseline condition. In the nine experimental conditions, participants had to execute, observe, or imagine right-hand extension/flexion movements or right-foot extension/flexion movements. The fMRI results showed a somatotopic organization within the contralateral premotor and primary motor cortex during motor imagery and motor execution. However, there was no clear somatotopic organization of action observation in the given regions of interest within the contralateral hemisphere, although observation of these movements activated these areas significantly.

Strength gains by motor imagery with different ratios of physical to mental practice.

The purpose of this training study was to determine the magnitude of strength gains following a high-intensity resistance training (i.e., improvement of neuromuscular coordination) that can be achieved by imagery of the respective muscle contraction imagined maximal isometric contraction (IMC training). Prior to the experimental intervention, subjects completed a 4-week standardized strength training program. 3 groups with different combinations of real maximum voluntary contraction (MVC) and mental (IMC) strength training (M75, M50, M25; numbers indicate percentages of mental trials) were compared to a MVC-only training group (M0) and a control condition without strength training (CO). Training sessions (altogether 12) consisted of four sets of two maximal 5-s isometric contractions with 10 s rest between sets of either MVC or IMC training. Task-specific effects of IMC training were tested in four strength exercises commonly used in practical settings (bench pressing, leg pressing, triceps extension, and calf raising). Maximum isometric voluntary contraction force (MVC) was measured before and after the experimental training intervention and again 1 week after cessation of the program. IMC groups (M25, M50, M75) showed slightly smaller increases in MVC (3.0% to 4.2%) than M0 (5.1%), but significantly stronger improvements than CO (-0.2%). Compared to further strength gains in M0 after 1 week (9.4% altogether), IMC groups showed no "delayed" improvement, but the attained training effects remained stable. It is concluded that high-intensity strength training sessions can be partly replaced by IMC training sessions without any considerable reduction of strength gains.

Your mind's hand: motor imagery of pointing movements with different accuracy.

Jeannerod (2001) postulated that motor control and motor simulation states are functionally equivalent. If this is the case, the specifically relevant task parameters in online motor control should also be represented in motor imagery. We tested whether the different spatial accuracy demands of manual pointing movements are reflected on a neural level in motor imagery. During functional magnetic resonance imaging (fMRI) scanning, 23 participants imagined hand movements that differed systematically in terms of pointing accuracy needs (i.e., none, low, high). In a low-accuracy condition, two big squares were presented visually prior to the imagery phase. These squares had to be pointed at alternately on a mental level. In the high-accuracy condition, two little squares had to be hit. As expected on the basis of speed-accuracy trade-off principles, results showed that participants required more time when accuracy of the imagined movements increased. The fMRI results showed a stepwise increase in activation in the anterior cerebellum and the anterior part of the superior parietal lobe (SPL) with rising accuracy needs. Moreover, we found increased activation of the anterior part of the SPL and of the dorsal premotor cortex (dPMC) when imagery included a square (i.e., in the low- and high-accuracy conditions) compared to the no-square condition. These areas have also been discussed in relation to online motor control, suggesting that specific task parameters relevant in the domain of motor control are also coded in motor imagery. We suggest that the functional equivalence of action states is due mostly to internal estimations of the expected sensory feedback in both motor control and motor imagery.

Neural correlates of attentional focusing during finger movements: A fMRI study.

One finding in recent motor control and learning research is that an external focus (i.e., attending to environmental aspects) improves performance, whereas an internal focus (i.e., controlling bodily movements) impedes it. Despite being replicated in behavioral studies, the neurophysiological basis of this phenomenon remains largely unknown. The present authors separate global attention to actions into an external and an internal focus. Using a between-participants design, participants were either trained to attend to moving their fingers (internal focus) or to the keys to be hit (external focus) during learning a finger sequence. Subsequently, they applied functional magnetic resonance imaging under focus (internal/external), dual task, and move-only conditions. Results revealed higher activation in primary somatosensory and motor cortex for an external compared to an internal focus. The authors conclude that external participants focused on the task-related environment (i.e., the keys) to enhance tactile input to somatosensory areas that closely connect to motor areas.

Motor imagery and its implications for understanding the motor system.

In the neurosciences, motor imagery (MIm) has not just been a topic of basic research. It has also attracted attention in applied research as a therapeutic tool. MIm is conceptualized as an internal simulation of motor acts that generates images on the basis of motor representations. Therefore, MIm is associated with neural activation of the cortical and subcortical motor system. The resulting concept of functional equivalence between MIm and execution opens a window to study the organization of motor processes and, more generally, to understand the neural plasticity of the motor system.

How are actions physically implemented?

This chapter focuses on the interdisciplinary discussion between cognitive psychologists and neuroscientists on how actions, the results of decision processes, are implemented. After surveying the approaches used in action implementation research, we analyze the contributions of these different approaches in more detail. Topics covered include expertise research in sports science, knowledge structures, neuroscientific research on motor imagery and decision making, computational models in motor control, robotics, and brain-machine interfaces. This forms the basis for discussing central issues for interdisciplinary research on action implementation from different viewpoints. In essence, most findings show the need to abandon serial frameworks of information processing suggesting a step-by-step pattern from perception, evaluation, and selection to execution. Instead, an outlook on new approaches is given, opening a route for future research in this field.

Cognitive motor processes: the role of motor imagery in the study of motor representations.

Motor imagery is viewed as a window to cognitive motor processes and particularly to motor control. Mental simulation theory [Jeannerod, M., 2001. Neural simulation of action: a unifying mechanism for motor cognition. NeuroImage 14, 103-109] stresses that cognitive motor processes such as motor imagery and action observation share the same representations as motor execution. This article presents an overview of motor imagery studies in cognitive psychology and neuroscience that support and extend predictions from mental simulation theory. In general, behavioral data as well as fMRI and TMS data demonstrate that motor areas in the brain play an important role in motor imagery. After discussing results on a close overlap between mental and actual performance durations, the review focuses specifically on studies reporting an activation of primary motor cortex during motor imagery. This focus is extended to studies on motor imagery in patients. Motor imagery is also analyzed in more applied fields such as mental training procedures in patients and athletes. These findings support the notion that mental training procedures can be applied as a therapeutic tool in rehabilitation and in applications for power training.

Neural activation in cognitive motor processes: comparing motor imagery and observation of gymnastic movements.

The simulation concept suggested by Jeannerod (Neuroimage 14:S103-S109, 2001) defines the S-states of action observation and mental simulation of action as action-related mental states lacking overt execution. Within this framework, similarities and neural overlap between S-states and overt execution are interpreted as providing the common basis for the motor representations implemented within the motor system. The present brain imaging study compared activation overlap and differential activation during mental simulation (motor imagery) with that while observing gymnastic movements. The fMRI conjunction analysis revealed overlapping activation for both S-states in primary motor cortex, premotor cortex, and the supplementary motor area as well as in the intraparietal sulcus, cerebellar hemispheres, and parts of the basal ganglia. A direct contrast between the motor imagery and observation conditions revealed stronger activation for imagery in the posterior insula and the anterior cingulate gyrus. The hippocampus, the superior parietal lobe, and the cerebellar areas were differentially activated in the observation condition. In general, these data corroborate the concept of action-related S-states because of the high overlap in core motor as well as in motor-related areas. We argue that differential activity between S-states relates to task-specific and modal information processing.