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ABSTRACT: One of the main characteristics of Autism Spectrum Disorder (ASD) are problems with social interaction and communication. Here, we explored ASD-related alterations in 'reading' body language of other humans. Accuracy and reaction times were assessed from two observational tasks involving the recognition of 'biological motion' and 'emotions' from point-light displays (PLDs). Eye movements were recorded during the completion of the tests. Results indicated that typically developed-participants were more accurate than ASD-subjects in recognizing biological motion or emotions from PLDs. No accuracy differences were revealed on two control-tasks (involving the indication of color-changes in the moving point-lights). Group differences in reaction times existed on all tasks, but effect sizes were higher for the biological and emotion recognition tasks. Biological motion recognition abilities were related to a person's ability to recognize emotions from PLDs. However, ASD-related atypicalities in emotion recognition could not entirely be attributed to more basic deficits in biological motion recognition, suggesting an additional ASD-specific deficit in recognizing the emotional dimension of the point light displays. Eye movements were assessed during the completion of tasks and results indicated that ASD-participants generally produced more saccades and shorter fixation-durations compared to the control-group. However, especially for emotion recognition, these altered eye movements were associated with reductions in task-performance.
PLoS ONE 01/2012; 7(9):e44473. · 4.09 Impact Factor
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ABSTRACT: During movement observation, corticomotor excitability of the observer's primary motor cortex (M1) is modulated according to the force requirements of the observed action. Here, we explored the time course of observation-induced force encoding. Force-related changes in M1-excitability were assessed by delivering transcranial magnetic stimulations at distinct temporal phases of an observed reach-grasp-lift action. Temporal changes in force-related electromyographic activity were also assessed during active movement execution. In observation conditions in which a heavy object was lifted, M1-excitability was higher compared to conditions in which a light object was lifted. Both during observation and execution, differential force encoding tended to gradually increase from the grasping phase until the late lift phase. Surprisingly, however, during observation, force encoding was already present at the early reach phase: a time point at which no visual cues on the object's weight were available to the observer. As the observer was aware that the same weight condition was presented repeatedly, this finding may indicate that prior predictions concerning the upcoming weight condition are reflected by M1 excitability. Overall, findings may provide indications that the observer's motor system represents motor predictions as well as muscular requirements to infer the observed movement goal.
Psychological Research 09/2011; 76(4):503-13. · 2.47 Impact Factor
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ABSTRACT: Seeing or hearing manual actions activates the mirror neuron system, that is, specialized neurons within motor areas which fire when an action is performed but also when it is passively perceived. Using TMS, it was shown that motor cortex of typically developed subjects becomes facilitated not only from seeing others' actions, but also from merely hearing action-related sounds. In the present study, TMS was used for the first time to explore the "auditory" and "visual" responsiveness of motor cortex in individuals with congenital blindness or deafness. TMS was applied over left primary motor cortex (M1) to measure cortico-motor facilitation while subjects passively perceived manual actions (either visually or aurally). Although largely unexpected, congenitally blind or deaf subjects displayed substantially lower resonant motor facilitation upon action perception compared to seeing/hearing control subjects. Moreover, muscle-specific changes in cortico-motor excitability within M1 appeared to be absent in individuals with profound blindness or deafness. Overall, these findings strongly argue against the hypothesis that an increased reliance on the remaining sensory modality in blind or deaf subjects is accompanied by an increased responsiveness of the "auditory" or "visual" perceptual-motor "mirror" system, respectively. Moreover, the apparent lack of resonant motor facilitation for the blind and deaf subjects may challenge the hypothesis of a unitary mirror system underlying human action recognition and may suggest that action perception in blind and deaf subjects engages a mode of action processing that is different from the human action recognition system recruited in typically developed subjects.
Journal of Cognitive Neuroscience 05/2011; 23(5):1080-7. · 5.18 Impact Factor
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ABSTRACT: Folk psychology advocates the existence of gender differences in socio-cognitive functions such as 'reading' the mental states of others or discerning subtle differences in body-language. A female advantage has been demonstrated for emotion recognition from facial expressions, but virtually nothing is known about gender differences in recognizing bodily stimuli or body language. The aim of the present study was to investigate potential gender differences in a series of tasks, involving the recognition of distinct features from point light displays (PLDs) depicting bodily movements of a male and female actor. Although recognition scores were considerably high at the overall group level, female participants were more accurate than males in recognizing the depicted actions from PLDs. Response times were significantly higher for males compared to females on PLD recognition tasks involving (i) the general recognition of 'biological motion' versus 'non-biological' (or 'scrambled' motion); or (ii) the recognition of the 'emotional state' of the PLD-figures. No gender differences were revealed for a control test (involving the identification of a color change in one of the dots) and for recognizing the gender of the PLD-figure. In addition, previous findings of a female advantage on a facial emotion recognition test (the 'Reading the Mind in the Eyes Test' (Baron-Cohen, 2001)) were replicated in this study. Interestingly, a strong correlation was revealed between emotion recognition from bodily PLDs versus facial cues. This relationship indicates that inter-individual or gender-dependent differences in recognizing emotions are relatively generalized across facial and bodily emotion perception. Moreover, the tight correlation between a subject's ability to discern subtle emotional cues from PLDs and the subject's ability to basically discriminate biological from non-biological motion provides indications that differences in emotion recognition may - at least to some degree - be related to more basic differences in processing biological motion per se.
PLoS ONE 01/2011; 6(6):e20989. · 4.09 Impact Factor
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ABSTRACT: Observers are able to judge quite accurately the weights lifted by others. Only recently, neuroscience has focused on the role of the motor system to accomplish this task. In this respect, a previous transcranial magnetic stimulation (TMS) study showed that the muscular force requirements of an observed action are encoded by the primary motor cortex (M1). Overall, three distinct visual sources may provide information on the applied force of an observed lifting action, namely, (i) the perceived kinematics, (ii) the hand contraction state and finally (iii) intrinsic object properties. The principal aim of the present study was to disentangle these three visual sources and to explore their importance in mediating the encoding of muscular force requirements in the observer's motor system. A series of experiments are reported in which TMS was used to measure 'force-related' responses from the hand representation in left M1 while subjects observed distinct action-stimuli. Overall, results indicated that observation-induced activity in M1 reflects the level of observed force when kinematic cues of the lift (exp. 1) or cues on the hand contraction state (exp. 2) are available. Moreover, when kinematic cues and intrinsic object properties provide distinct information on the force requirements of an observed lifting action, results from experiment 3 indicated a strong preference for the use of kinematic features in mapping the force requirements of the observed action. In general, these findings support the hypothesis that the primary motor cortex contributes to action observation by mapping the muscle-related features of observed actions.
Neuropsychologia 04/2010; 48(7):2082-90. · 3.64 Impact Factor
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ABSTRACT: Several transcranial magnetic stimulation (TMS) studies have reported facilitation of the primary motor cortex (M1) during the mere observation of actions. This facilitation was shown to be highly congruent, in terms of somatotopy, with the observed action, even at the level of single muscles. With the present study, we investigated whether this muscle-specific facilitation of the observer's motor system reflects the degree of muscular force that is exerted in an observed action. Two separate TMS experiments are reported in which corticospinal excitability was measured in the hand area of M1 while subjects observed the lifting of objects of different weights. The type of action 'grasping-and-lifting-the-object' was always identical, but the grip force varied according to the object's weight. In accordance to previous findings, excitability of M1 was shown to modulate in a muscle-specific way, such that only the cortical representation areas in M1 that control the specific muscles used in the observed lifting action became increasingly facilitated. Moreover, muscle-specific M1 facilitation was shown to modulate to the force requirements of the observed actions, such that M1 excitability was considerably higher when observing heavy object lifting compared with light object lifting. Overall, these results indicate that different levels of observed grip force are mirrored onto the observer's motor system in a highly muscle-specific manner. The measured force-dependent modulations of corticospinal excitability in M1 are hypothesized to be functionally relevant for scaling the observed grip force in the observer's own motor system. In turn, this mechanism may contribute, at least partly, to the observer's ability to infer the weight of the lifted object.
European Journal of Neuroscience 03/2010; 31(6):1144-53. · 3.63 Impact Factor
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ABSTRACT: Several studies have demonstrated that the human motor cortex is activated by the mere observation of actions performed by others. In the present study, we explored whether the perception of 'impoverished motion stimuli', such as shadow animations, is sufficient to activate motor areas. To do so, transcranial magnetic stimulation (TMS) was applied over the hand area of the primary motor cortex (M1) while subjects observed shadow animations depicting finger motions. Data showed that resonant motor responses in M1 were only found when a biological effector was recognized from the observed shadow animation. Interestingly, M1 responses were similar for observing shadow or real motions. Therefore, the loss of 'pictorial' movement features in a shadow animation appeared to have no effect on motor resonance in M1. In summary, these findings suggest that the 'recognition' of biological motion from sparse visual input is both necessary and sufficient to recruit motor areas. This supports the hypothesis that the motor system is involved in recognizing the actions performed by others.
Neuroscience Letters 07/2009; 461(3):240-4. · 2.11 Impact Factor
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ABSTRACT: In the past few years, the use of motor imagery as an adjunct to other forms of training has been studied extensively. However, very little attention has been paid to how imagery could be used to greatest effect. It is well known that the provision of external cues has a beneficial effect on motor skill acquisition and performance during physical practice. Since physical execution and mental imagery share several common mechanisms, we hypothesized that motor imagery might be affected by external cues in a similar way. To examine this, we compared the motor imagery performance of three groups of 15 healthy participants who either physically performed or imagined performing a goal-directed cyclical wrist movement in the presence or the absence of visual and/or auditory external cues. As outcome measures, the participants' imagery vividness scores and eye movements were measured during all conditions. We found that visual movement-related cues improved the spatial accuracy of the participants' eye movements during imagery, while auditory cues specifically enhanced their temporal accuracy. Furthermore, both types of cues significantly improved the participants' imagery vividness. These findings indicate that subjects may imagine a movement in a better way when provided with external movement-related stimuli, which may possibly be useful with regard to the efficiency of mental practice in (clinical) training protocols.
Brain research 06/2009; 1278:50-8. · 2.46 Impact Factor
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ABSTRACT: Seeing or hearing manual actions activates the mirror neuron system, i.e., specialized neurons within motor areas which fire not only when an action is performed but also when it is passively perceived. Although it has been shown that mirror neurons respond to either action-specific vision or sound, it remains a topic of debate whether and how vision and sound interact during action perception. Here we used transcranial magnetic stimulation to explore multimodal interactions in the human motor system, namely at the level of the primary motor cortex (M1). Corticomotor excitability in M1 was measured while subjects perceived unimodal visual (V), unimodal auditory (A), or multimodal (V+A) stimuli of a simple hand action. In addition, incongruent multimodal stimuli were included, in which incongruent vision or sound was presented simultaneously with the auditory or visual action stimulus. A selective response increase was observed to the congruent multimodal stimulus as compared to the unimodal and incongruent multimodal stimuli. These findings speak in favour of 'shared' action representations in the human motor system that are evoked in a 'modality-dependent' way, i.e., they are elicited most robustly by the simultaneous presentation of congruent auditory and visual stimuli. Multimodality in the perception of hand movements bears functional similarities to speech perception, suggesting that multimodal convergence is a generic feature of the mirror system which applies to action perception in general.
Neuropsychologia 06/2009; 47(12):2593-9. · 3.64 Impact Factor
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ABSTRACT: Previous Transcranial Magnetic Stimulation (TMS) studies have shown that the observer's motor system is facilitated by the sole observation of motor actions. However, it has not been possible so far to decide whether the observer's motor system resonates primarily with the observed movement direction or the observed muscle activity, as both factors usually co-varied in these action observation studies. Here, we applied TMS to the wrist extensor and flexor during the observation of wrist motions such that the posture of the observer and the model in the video were either congruent or incongruent. Due to this manipulation, it was possible to disentangle whether the observer's primary motor cortex (M1) is facilitated in accordance to either the observed movement direction or the observed muscle activation. Findings revealed that M1 resonated predominantly according to muscle-specific rather than direction-specific parameters of observed movements. More specifically, muscle-specific facilitation was maximal during congruent postures and remained evident, even though to a lower extent, during incongruent postures in which muscle activation and movement direction parameters were discordant. Our findings support the hypothesis that M1 contributes to action observation, by representing the observed movement in intrinsic, muscle-related coordinates. This transformation from extrinsic to intrinsic coordinates might be an important prerequisite for action understanding and imitation. Additionally, our data offer a neurophysiological explanation for interference that emerges when an action is performed while an incongruent action is observed.
Cortex 12/2008; 45(10):1148-55. · 6.08 Impact Factor
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ABSTRACT: Previous studies using transcranial magnetic stimulation (TMS) have shown that during the observation of actions performed by others, the observer's primary motor cortex (M1) becomes facilitated in a highly muscle specific fashion. Here, we used TMS to explore the effect of posture, perspective and body side on muscle specific facilitation of left M1. Subjects viewed video's showing left and right hand extension (palm-down) movements from a first person or third person perspective with their hand posture either congruent (palm-down) or incongruent (palm-up) to the posture of the observed model. Data indicated that facilitation of left M1 was substantially different for observing actions executed with the right (contralateral) or left (ipsilateral) hand. For right hand actions, facilitation of left M1 was shown to be highly specific to the muscle used in the observed action ('intrinsic mapping'). During the observation of left hand stimuli, only half of the subjects displayed this muscle specific facilitation, whereas in the other half, M1 was facilitated according to the observed movement direction ('extrinsic mapping'). Absolute effect magnitude was particularly high when right hand actions were observed from a first person perspective, whereas, for left hand actions, the third person perspective was more efficient. The degree of postural congruency between body parts of the observer and observed model only mildly influenced M1 facilitation. Since action observation is increasingly considered in rehabilitation therapies, the present findings may help identifying the most effective conditions for stimulating the motor system during action observation.
Neuropsychologia 10/2008; 47(2):415-22. · 3.64 Impact Factor
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ABSTRACT: The purpose of this study was to evaluate the delayed effects of repetitive sensory stimulation with passive wrist movement on corticospinal excitability of the forearm and hand musculature. Motor evoked potential responses to single and double pulse transcranial magnetic stimulation were recorded from the flexor carpi radialis, extensor carpi radialis, and the first dorsal interosseous muscles of the right limb. Data were collected before and after a 1 hour session of passive wrist movement (intervention group, n = 11) or after a same period of rest (control group, n = 9). Motor evoked potential size and area were analyzed to evaluate corticospinal excitability and short interval intracortical inhibition and facilitation. Training with passive movement resulted in a prolonged increase in corticospinal excitability in the flexor carpi radialis and extensor carpi radialis (until at least 1 hour postintervention), but did not evoke significant changes in the levels of short interval intracortical inhibition and facilitation. No such effects were noted in the control group or first dorsal interosseous muscle. Prolonged proprioceptive stimulation with passive wrist movement induces a delayed increase in corticospinal excitability of the forearm muscles. Accordingly, this intervention may promote motor cortical reorganization in the targeted muscles. Results show induced effects from passive movement training that may prove useful for neurorehabilitation therapies.
Journal of clinical neurophysiology: official publication of the American Electroencephalographic Society 09/2008; 25(4):202-9. · 1.47 Impact Factor
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ABSTRACT: The present experiment addressed whether increases in corticospinal excitability following sensory stimulation with muscle tendon vibration are accompanied by reorganization of the forearm musculature representation within the primary motor cortex. Using transcranial magnetic stimulation, we mapped the corticomotor projection to the dominant flexor carpi radialis (FCR) and extensor carpi radialis brevis (ECR) muscle before and after interventional sensory stimulation obtained via muscle tendon vibration (80Hz, 60 min) to the dominant distal wrist flexor tendons. Following vibration, MEP amplitude at the optimal stimulation position, motor output area, as well as map volume, increased significantly for the ECR. None of these effects reached significance for the FCR. These results suggest that the antagonistic vibratory response (AVR), which is considered to be of cortical origin, induces a delayed facilitation of musculature that is antagonistic to the site of the directly activated Ia afferent pathways. This example demonstrates that peripheral sensory stimulation can induce lasting increases in corticospinal excitability in the absence of actual movements.
Behavioural Brain Research 07/2008; 190(1):41-9. · 3.42 Impact Factor
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ABSTRACT: Coordination in biological systems requires a continuous updating of the perception-action cycle. How the different sources of sensory information contribute uniquely to performance is still debated. Here, we directly compared the role of vision and kinesthesis by means of a tracking task in which the left wrist mimicked the passive motions imposed on the right wrist with a torque motor. The passive movements were perceived visually (alien hand) or kinesthetically (own hand), or a combination thereof (own hand, feel and see). Tracking occurred according to the same (isodirectional) or opposite (mirror-image) directions. Findings revealed that visual tracking was performed most successfully in the isodirectional and kinesthetic tracking in the mirror-image mode. Tracking was most successful when both sources of sensory information were present. These results suggest that vision and proprioception obey direction-dependent constraints that are consistent with extrinsic and intrinsic reference frames within which the perception-action cycle resides. Thus, each sensory modality contributes uniquely as a function of the spatial requirements of the tracking task, rather than one being superior over the other.
Behavioural Brain Research 04/2007; 178(2):229-34. · 3.42 Impact Factor
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ABSTRACT: Previous studies using transcranial magnetic stimulation (TMS) have shown that during the observation of actions performed by others, the observer’s primary motor cortex (M1) becomes facilitated in a highly muscle specific fashion. Here, we used TMS to explore the effect of posture, perspective and body side on muscle specific facilitation of left M1. Subjects viewed video’s showing left and right hand extension (palm-down) movements from a first person or third person perspective with their hand posture either congruent (palm-down) or incongruent (palm-up) to the posture of the observed model.Data indicated that facilitation of left M1 was substantially different for observing actions executed with the right (contralateral) or left (ipsilateral) hand. For right hand actions, facilitation of left M1 was shown to be highly specific to the muscle used in the observed action (‘intrinsic mapping’). During the observation of left hand stimuli, only half of the subjects displayed this muscle specific facilitation, whereas in the other half, M1 was facilitated according to the observed movement direction (‘extrinsic mapping’). Absolute effect magnitude was particularly high when right hand actions were observed from a first person perspective, whereas, for left hand actions, the third person perspective was more efficient. The degree of postural congruency between body parts of the observer and observed model only mildly influenced M1 facilitation. Since action observation is increasingly considered in rehabilitation therapies, the present findings may help identifying the most effective conditions for stimulating the motor system during action observation.
Neuropsychologia.
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Kaat Alaerts
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ABSTRACT: Neurowetenschappelijk onderzoek concentreerde zich het voorbije decenium steeds meer op de rol van het zogenaamde ‘spiegelsysteem’ in verschillende functies zoals het herkennen van acties, het begrijpen van acties en actie-imitatie. Spiegelneuronen werden initieel ontdekt in de premotorische cortex van de aap en hebben de opvallende eigenschap niet alleen te ontladen wanneer de aap een beweging uitvoert, maar ook wanneer deze een gelijkaardige beweging observeert. Om het humane spiegelsysteem te bestuderen werd in het verleden veelvuldig gebruik gemaakt van transcraniale magnetische stimulatie (TMS). Deze techniek is hiervoor uiterst geschikt, omdat toepassing van TMS toelaat om snelle en focale metingen van corticale activiteit binnen de primaire motorische cortex (M1) te verkrijgen. In het huidige doctoraatsproject wordt de TMS-techniek intensief gebruikt om te onderzoeken welke factoren van geobserveerde bewegingen belangrijk zijn om het humane spiegelsysteem te activeren. Verschillende parameters van waargenomen acties worden hiervoor gemanipuleerd om zo hun relatief belang na te gaan bij het mediëren van ‘resonerende’ activiteit binnen het motorische systeem van de waarnemer.
Wanneer een willekeurige actie wordt uitgevoerd moeten de hersenen de bewegingsrichting in de ruimte coderen (gespecifieerd in een extern referentie kader) om deze richtingsgerelateerde informatie vervolgens te vertalen naar specifieke activeitspatronen van verschillende spieren (gespecifieerd in een intrinsiek referentie kader). Aan de hand van ‘single cell recordings’ bij apen werd aangetoond dat M1 zowel richtings- als spier-coderende neuronen bevat. Het blijft echter grotendeels onduidelijk welke van deze twee referentiekaders gebruikt wordt tijdens het proces van observatie-naar-uitvoering ‘mappings’ in M1. Om deze vraag te beantwoorden hanteerden we in onze eerste studie een onderzoeksparadigma waarmee het mogelijk is te onderscheiden of de primaire motorische cortex van waarnemers resoneert in overeenstemming met de geobserveerde bewegingsrichting dan wel de geobserveerde spieractiviteit. Resultaten toonden aan dat in het spiegelsysteem, tenminste ter hoogte van M1, de spiervereisten van waargenomen acties gecodeerd worden, eerder dan de directionele eigenschappen.
Op basis van de resultaten van onze eerste studie (d.i. spierspecifieke activiteits-modulaties ter hoogte van M1 tijdens actie-observatie) èn de veronderstelling dat observatie-geïnduceerde reactivatie van motorische gebieden een positivie impact kan hebben op het herstel van de motorische functies na een beroerte, beoogden we met onze tweede studie te onderzoeken welke observatie ‘settings’ het meest efficiënt zijn om spierspecifieke responsen op te wekken ter hoogte van het motorisch systeem van de waarnemer. Hiervoor onderzochten we of verschillen in lichaamsconfiguraties tussen de waarnemer en het geobserveerd model een invloed hebben op observatie-geïnduceerde activiteit in M1. De resultaten van deze studie toonden aan dat activiteit binnen het spiegelsysteem substantieel afhangt van de lateraliteit van de waargenomen hand (d.i. observatie van linker of rechter hand), alsook van het perspectief van waaruit de actie wordt waargenomen (d.i. observatie vanuit het eerste of derde persoons-perspectief). Meer bepaald; activiteit in M1 bleek sterk te verhogen wanneer acties van de rechterhand geobserveerd werden vanuit een eerste persoons-perspectief, terwijl voor linkerhand acties het derde persoons-perspectief efficiënter bleek. Verschillen in houding (van de hand/arm) tussen de waarnemer en het geobserveerde model bleken echter weinig invloed te hebben op observatie-geïnduceerde activiteit in M1.
Algemeen toonden de resultaten van onze eerste twee studies aan dat M1-activiteit sterk bepaald wordt door spier-gerelateerde aspecten van geobserveerde bewegingen. Hieruit vloeit de vraag of het spiegelsysteem, en in het bijzonder M1, ook andere spier-specifieke parameters van de geobserveerde beweging codeert, zoals de gegenereerde spierkracht. Deze hypothese werd getest in onze derde en vierde studie van het project. De resultaten toonden aan dat activiteits-modulaties in M1 in sterke mate bepaalt worden door de spierkracht-vereisten van geobserveerde til-acties (d.i. het optillen van verschillend gewichten). Bovendien bleek het coderen van spierkracht-vereisten in M1 gemedieerd te worden door verschillende visuele aspecten van de geobserveerde til-acties, zoals het geassocieerd kinematische profiel van de actie, alsook de geobserveerd spanning/druk die geproduceerd wordt in de uitvoerende hand. Verrassend genoeg bleek object-gerelateerde informatie – zoals de geobserveerde vullingsgraad van het getilde object - maar weinig invloed te hebben op het coderen van spierkracht in M1.
In de laatste studie van het project werd de focus verschoven naar het onderzoeken van de manier waarop verschillende types van sensorische input het humane spiegelsysteem kunnen sturen. Hoewel werd aangetoond dat het spiegelsysteem geactiveerd kan worden door zowel visuele als auditieve actie-gerelateerde input, blijft het een onderwerp van discussie hoe beide modaliteiten met elkaar interageren ter hoogte van het spiegelsysteem. Om deze vraag te beantwoorden werd M1-activiteit gemeten terwijl proefpersonen unimodale visuele (V), unimodale auditieve (A) of multimodale (V+A) stimuli van een eenvoudige handbeweging observeren. Samengevat toonden de resultaten aan dat observatie-geïnduceerde activiteit in M1 aanzienlijk hoger is wanneer visuele en auditieve input van een waargenomen actie simultaan worden aangeboden. Deze resultaten suggereren de aanwezigheid van multimodale ‘audio-visuele’ actie-representaties ter hoogte van het humane motorische systeem. Mogelijk dragen deze representaties bij tot het creëren van een ‘audio-visueel-motor percept’ van waargenomen acties.
Samengevat kan gesteld worden dat dit doctoraatsproject een bijdrage levert tot de bestaande kennis over de rol van de primaire motorische cortex in het proces van observatie-tot-uitvoering ‘mappings’. Hoewel M1 traditioneel niet werd beschouwd als een effectief deel van het ‘kern’ spiegelsysteem, steunen onze gegevens het recente voorstel dat M1 mogelijks een functionele rol speelt binnen een `uitgebreid' spiegelnetwerk in de menselijke hersenen. Vermits actie observatie steeds vaker wordt overwogen in revalidatie-therapiëen, kunnen de huidige bevindingen bovendien bijdragen tot het identificeren van de meest efficiënte condities om het motorische systeem tijdens actie observatie te re-activeren. During the past decade, neuroscience has increasingly focused on the role of the so-called ‘mirror system’ in a variety of functions such as action recognition, action understanding, and imitation. Mirror neurons were initially discovered in the ventral premotor cortex of the macaque brain and have the striking property to discharge not only when the monkey performs an action, but also when it merely observes a similar action performed by another individual. To study the human mirror system at work during action perception, transcranial magnetic stimulation (TMS) has proven to be an excellent technique, as it can be used to obtain fast and focal measurements of cortical activity within the primary motor area (M1) of subjects observing actions performed by others. In the present doctoral project, the TMS technique is extensively used to investigate whether and how the mirror system is responsive to action observation. To do so, different parameters of perceived actions are manipulated to assess their relative importance in mediating ‘resonant’ activity in the observer’s motor system.
When a voluntary action is generated, the brain must encode the direction of the movement in space, specified in an external coordinate frame, and translate this directional information into a set of muscle activation patterns, specified in an intrinsic coordinate frame. From single cell recordings in monkeys, it was shown that M1 contains both direction-related and muscle-related neurons to a nearly equal extent. However, it remains largely unclear which of the two reference frames is used during the process of observation-to-execution mapping. To address this question, our first study adopted a paradigm from which it was possible to disentangle whether the observer’s primary motor cortex is facilitated in accordance to either the observed movement direction or the observed muscle activation. Overall, data indicated that the mirror system, at least at the level of M1, reflects the muscular requirements of observed actions, rather than the directional features.
Based on the findings of our first study (i.e., muscle specific activity modulations at the level of M1 during movement observation) and the notion that observation-induced reactivation of motor areas may have a positive impact on recovery of motor functions after stroke, the second study aimed to identify the most effective observation settings that mediate muscle specific facilitation of the motor system during action observation. More specifically, we explored the effects of varying body configurations between the observer and observed model on observation-induced activity in M1. Overall, it was shown that activity within the mirror system may depend substantially upon the laterality of the observed hand (i.e., left or right) and the perspective from which the action is observed (i.e., first or third person perspective). More specifically, facilitation of the observer’s M1 was shown to be particularly high when right hand actions were observed from a first person perspective, whereas, for left hand actions, the third person perspective was more efficient. On the other hand, the degree of postural congruency between body parts of the observer and observed model only mildly influenced the extent of M1 facilitation.
Overall, the results of these first two studies indicated that M1 activity is strongly driven by a muscle-specific mapping between the observed movement and the observer’s motor system. This raised the question whether the mirror system and, particularly M1, encodes also other parameters related to the muscular requirements of the observed movement, such as the produced force. This hypothesis was tested in the third and fourth study of the project. Overall, data indicated that observation-induced activity in M1 reflects the level of observed grip force when subjects observe the lifting of different object weights. Moreover, the encoding of grip force requirements in M1 appeared to be mediated by distinct visual cues of the observed lifting actions, such as the associated kinematical profile as well as the muscle tension/pressure produced in the acting hand. Surprisingly, object-related information on object weight (and consequently on exerted grip force) appeared to have only minor influences on the encoding of force within M1.
Finally, the focus shifted slightly in the last study in which we investigated how different types of sensory input drive the human mirror system. Although it has been shown that the mirror system responds to either action-specific vision or sound, it remains a topic of debate whether mirror activity reflects multisensory convergence of vision and sound during action perception. To address this question, M1 activity was measured while subjects perceived unimodal visual (V), unimodal auditory (A), or multimodal (V + A) stimuli of a simple hand action. In short, data indicated that perception-induced activity in M1 increases substantially from the simultaneous presentation of visual and auditory input about a perceived action. As such, these data speak in favour of multimodal ‘audio-visual’ action representations in the human motor system.
In summary, the present doctoral project provides novel insights on the process of observation-to-execution mapping within motor areas by measuring observation-induced activity modulations at the level of the primary motor cortex. More specifically, our data support the hypothesis that M1 may contribute to action observation, by representing the muscle-related features of observed actions.
The application of movement observation in motor rehabilitation is theoretically based on the finding that cortical motor circuits are activated in a similar way during movement observation as during execution. Increasing our insights into the visual features that actually mediate activity modulations in the motor system during movement perception may therefore help to identify the most effective conditions for stimulating the motor system during action observation. Doctor of Rehabilitation Sciences and Physiotherapy Afd. Beweg.contr. en Neuroplasticiteit Dept. Biomedische Kinesiologie Faculteit Bewegings- en Revalidatiewetenschappen Doctoral thesis Doctoraatsthesis
