The mirror-neuron system and handedness: a "right" world?

Effector-specific improvements in action prediction in left-handed individuals after short-term physical practice

Research has established the influence of short-term physical practice for enhancing action prediction in right-handed (RH) individuals. In addition to benefits of physical practice for these later assessed perceptual-cognitive skills, effector-specific interference has been shown through action-incongruent secondary tasks (motor interference tasks). Here we investigated this experience-driven facilitation of action predictions and effector-specific interference in left-handed (LH) novices, before and after practicing a dart throwing task. Participants watched either RH (n = 19) or LH (n = 24) videos of temporally occluded dart throws, across a control condition and three secondary-task conditions: tone-monitoring, RH or LH force monitoring. These conditions were completed before and after physical practice throwing with the LH. Significantly greater improvement in prediction accuracy was shown post-practice for the LH- versus RH-video group. Consistent with previous work, effector-specific interference was shown, exclusive to the LH-video group. Only when doing the LH force monitoring task did the LH-video group show secondary task interference in prediction accuracy. These data support the idea that short-term physical practice resulted in the development of an effector-specific motor representation. The results are also consistent with other work in RH individuals (showing RH motor interference) and hence rule out the interpretation that these effector specific effects are due to the disruption of more generalized motor processes, thought to be lateralized to the left-hemisphere of the brain.

Does the Degree and/or Direction of Handedness in Children With Developmental Coordination Disorder Influence Motor and Cognitive Performance? A Pilot Study

Research has shown that nonright-handedness in children with developmental coordination disorder (DCD) is associated with poorer motor and cognitive performance. This study investigated the influence of degree and direction of handedness on performance using the Home Handedness Questionnaire, the Hit-the-Dot test, the Movement Assessment Battery for Children, and the digital Trail-Making Test. Eighteen children with DCD and 21 typically developing children aged 8–12 years participated in this study. The distribution of degree and direction of handedness in the group of DCD children were not different from that found in the typically developing group. In the Hit-the-Dot test, typically developing children significantly performed better than children with DCD, no matter which hand was dominant or to which degree. A significant inconsistent-handed advantage in the subdomain balance was found for children with DCD. Inconsistent handedness also seems to be an advantage for children with DCD on the digital Trail-Making Test performance. The relationship between the subcategories of the Movement Assessment Battery for Children and the digital Trail-Making Test part B is stronger for consistent than for inconsistent handedness. Our findings suggest that children with DCD and inconsistent handedness might benefit from greater crosstalk across hemispheres. In addition, these predispositions can be reinforced or discouraged throughout development and via occupational therapy.

Motor resonance in monkey parietal and premotor cortex during action observation: Influence of viewing perspective and effector identity

Observing others performing motor acts like grasping has been shown to elicit neural responses in the observer`s parieto-frontal motor network, which typically becomes active when the observer would perform these actions him/herself. While some human studies suggested strongest motor resonance during observation of first person or egocentric perspectives compared to third person or allocentric perspectives, other research either report the opposite or did not find any viewpoint-related preferences in parieto-premotor cortices. Furthermore, it has been suggested that these motor resonance effects are lateralized in the parietal cortex depending on the viewpoint and identity of the observed effector (left vs right hand). Other studies, however, do not find such straightforward hand identity dependent motor resonance effects. In addition to these conflicting findings in human studies, to date, little is known about the modulatory role of viewing perspective and effector identity (left or right hand) on motor resonance effects in monkey parieto-premotor cortices. Here, we investigated the extent to which different viewpoints of observed conspecific hand actions yield motor resonance in rhesus monkeys using fMRI. Observing first person, lateral and third person viewpoints of conspecific hand actions yielded significant activations throughout the so-called action observation network, including STS, parietal and frontal cortices. Although region-of-interest analysis of parietal and premotor motor/mirror neuron regions AIP, PFG and F5, showed robust responses in these regions during action observation in general, a clear preference for egocentric or allocentric perspectives was not evident. Moreover, except for lateralized effects due to visual field biases, motor resonance in the monkey brain during grasping observation did not reflect hand identity dependent coding.

The effect of action observation/execution on mirror neuron system recruitment: an fMRI study in healthy individuals

Action observation and execution activate regions that are part of the motor and mirror neuron systems (MNS). Using functional magnetic resonance (fMRI), we defined the presence and extent of MNS activation during three different motor tasks with the dominant, right-upper limb in healthy individuals. The influence of the modality of task administration (execution, observation, observation and execution) was also investigated. fMRI scans during the execution (E) of a motor task, the observation (O) of a video showing the same task performed by another person and the simultaneous observation and execution (OE) of the task were obtained from three groups of healthy subjects (15 subjects per group) randomized to perform: a simple motor (SM) task, a complex motor (CM) task and a finalistic motor (FM) task. Manual dexterity was assessed using the 9-hole peg test and maximum finger tapping frequency. MNS activation was higher during FM than SM or CM tasks, independently from the modality of administration (E, O, or OE). Inferior frontal gyrus recruitment was more significant during SM than CM tasks in the E and O conditions. Compared to SM and FM, CM task resulted in increased recruitment of brain regions involved in complex motor task performance. Compared to O and E, OE resulted in the recruitment of additional, specific, brain areas in the cerebellum, temporal and parietal lobes. The modality of administration and the type of task modulated MNS recruitment during motor acts. This might have practical implications for the set-up of individualized motor rehabilitation strategies.

Hemispheric asymmetries in the processing of body sides: A study with ambiguous human silhouettes

When required to indicate the perceived orientation of pictures of human silhouettes with ambiguous front/back orientation and handedness, both right- and left-handed participants perceive the figures more frequently as right-handed than as left-handed, which seems to indicate an attentional bias towards the right arm of human bodies. Given that past research exploiting the divided visual field paradigm indicated a processing advantage for contralateral body parts in both hemispheres, we tested whether human silhouettes with ambiguous handedness presented in the right visual field would be interpreted more frequently as right-handed compared with those presented in the left visual field. We confirmed the expected lateralised embodiment of ambiguous human bodies, in line with previous studies showing that right and left limbs are processed faster and/or more accurately when presented in the right and left hemifield, respectively.

Motor experience influences object knowledge

An object's perceived readiness-for-action (e.g., its size, the degree of rotation from its canonical position, the user's viewpoint) can influence semantic knowledge retrieval. Yet, the organization of object knowledge may also be affected by body-specific sensorimotor experiences. Here, we investigated whether people's history of performing motor actions with their hands influences the knowledge they store and retrieve about graspable objects. We compared object representations between healthy right- and left-handers (Experiment 1), and between unilateral stroke patients, whose motor experience was changed by impairment of either their right or left hand (Experiment 2). Participants saw pictures of graspable everyday items with the handles oriented toward either the left or right hand, and they generated the type of grasp they would employ (i.e., clench or pinch) when using each object, responding orally. In both experiments, hand dominance and object orientation interacted to predict response times. In Experiment 1, judgments were fastest when objects were oriented toward the right hand in right-handers, but not in left-handers. In Experiment 2, judgments were fastest when objects were oriented toward the left hand in patients who had lost the use of their right hand, even though these patients were right-handed prior to brain injury. Results suggest that at least some aspects of object knowledge are determined by motor experience, and can be changed by new patterns of motor experience. People with different bodily characteristics, who interact with objects in systematically different ways, form correspondingly different neurocognitive representations of the same common objects. (PsycINFO Database Record

Areas Recruited during Action Understanding Are Not Modulated by Auditory or Sign Language Experience

The observation of other people’s actions recruits a network of areas including the inferior frontal gyrus (IFG), the inferior parietal lobule (IPL), and posterior middle temporal gyrus (pMTG). These regions have been shown to be activated through both visual and auditory inputs. Intriguingly, previous studies found no engagement of IFG and IPL for deaf participants during non-linguistic action observation, leading to the proposal that auditory experience or sign language usage might shape the functionality of these areas. To understand which variables induce plastic changes in areas recruited during the processing of other people’s actions, we examined the effects of tasks (action understanding and passive viewing) and effectors (arm actions vs. leg actions), as well as sign language experience in a group of 12 congenitally deaf signers and 13 hearing participants. In Experiment 1, we found a stronger activation during an action recognition task in comparison to a low-level visual control task in IFG, IPL and pMTG in both deaf signers and hearing individuals, but no effect of auditory or sign language experience. In Experiment 2, we replicated the results of the first experiment using a passive viewing task. Together, our results provide robust evidence demonstrating that the response obtained in IFG, IPL, and pMTG during action recognition and passive viewing is not affected by auditory or sign language experience, adding further support for the supra-modal nature of these regions.

The left side of motor resonance

Motor resonance is defined as the internal activation of an observer's motor system, specifically attuned to the perceived movement. In social contexts, however, different patterns of observed and executed muscular activation are frequently required. This is the case, for instance, of seeing a key offered with a precision grip and received by opening the hand. Novel evidence suggests that compatibility effects in motor resonance can be altered by social response preparation. What is not known is how handedness modulates this effect. The present study aimed at determining how a left- and a right-handed actor grasping an object and then asking for a complementary response influences corticospinal activation in left- and right-handers instructed to observe the scene. Transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs) were thus recorded from the dominant hands of left- and right-handers. Interestingly, requests posed by the right-handed actor induced a motor activation in the participants' respective dominant hands, suggesting that left-handers tend to mirror right-handers with their most efficient hand. Whereas requests posed by the left-handed actor activated the anatomically corresponding muscles (i.e., left hand) in all the participants, right-handers included. Motor resonance effects classically reported in the literature were confirmed when observing simple grasping actions performed by the right-handed actor. These findings indicate that handedness influences both congruent motor resonance and complementary motor preparation to observed actions.

When Left Is Not Right

According to the body-specificity hypothesis, hearing action words creates body-specific mental simulations of the actions. Handedness should, therefore, affect mental simulations. Given that pictures of actions also evoke mental simulations and often accompany words to be learned, would pictures that mismatch the mental simulation of words negatively affect learning? We investigated effects of pictures with a left-handed, right-handed, or bimanual perspective on left- and right-handers’ learning of object-manipulation words in an artificial language. Right-handers recalled fewer definitions of words learned with a corresponding left-handed-perspective picture than with a right-handed-perspective picture. For left-handers, there was no effect of perspective. These findings suggest that mismatches between pictures and mental simulations evoked by hearing action words can negatively affect right-handers’ learning. Left-handers, who encounter the right-handed perspective frequently, could presumably overcome the lack of motor experience with visual experience and, therefore, not be influenced by picture perspective.

Motor resonance in left- and right-handers: evidence for effector-independent motor representations

The idea of motor resonance was born at the time that it was demonstrated that cortical and spinal pathways of the motor system are specifically activated during both action-observation and execution. What is not known is if the human action observation-execution matching system simulates actions through motor representations specifically attuned to the laterality of the observed effectors (i.e., effector-dependent representations) or through abstract motor representations unconnected to the observed effector (i.e., effector-independent representations). To answer that question we need to know how the information necessary for motor resonance is represented or integrated within the representation of an effector. Transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs) were thus recorded from the dominant and non-dominant hands of left- and right-handed participants while they observed a left- or a right-handed model grasping an object. The anatomical correspondence between the effector being observed and the observer's effector classically reported in the literature was confirmed by the MEP response in the dominant hand of participants observing models with their same hand preference. This effect was found in both left- as well as in right-handers. When a broader spectrum of options, such as actions performed by a model with a different hand preference, was instead considered, that correspondence disappeared. Motor resonance was noted in the observer's dominant effector regardless of the laterality of the hand being observed. This would indicate that there is a more sophisticated mechanism which works to convert someone else's pattern of movement into the observer's optimal motor commands and that effector-independent representations specifically modulate motor resonance.

The lateralization of motor cortex activation to action-words

What determines the laterality of activation in motor cortex for words whose meaning is related to bodily actions? It has been suggested that the neuronal representation of the meaning of action-words is shaped by individual experience. However, core language functions are left-lateralized in the majority of both right- and left-handers. It is still an open question to what degree connections between left-hemispheric core language areas and right-hemispheric motor areas can play a role in semantics. We investigated laterality of brain activation using fMRI in right- and left-handed participants in response to visually presented hand-related action-words, namely uni- and bi-manual actions (such as "throw" and "clap"). These stimulus groups were matched with respect to general (hand-) action-relatedness, but differed with respect to whether they are usually performed with the dominant hand or both hands. We may expect generally more left-hemispheric motor cortex activation for hand-related words in both handedness groups, with possibly more bilateral activation for left-handers compared to right-handers. In our study, both participant groups activated motor cortex bilaterally for bi-manual words. Interestingly, both groups also showed a left-lateralized activation pattern to uni-manual words. We argue that this reflects the effect of left-hemispheric language dominance on the formation of semantic brain circuits on the basis of Hebbian correlation learning.

ALE meta-analysis of action observation and imitation in the human brain

Over the last decade, many neuroimaging studies have assessed the human brain networks underlying action observation and imitation using a variety of tasks and paradigms. Nevertheless, questions concerning which areas consistently contribute to these networks irrespective of the particular experimental design and how such processing may be lateralized remain unresolved. The current study aimed at identifying cortical areas consistently involved in action observation and imitation by combining activation likelihood estimation (ALE) meta-analysis with probabilistic cytoarchitectonic maps. Meta-analysis of 139 functional magnetic resonance and positron emission tomography experiments revealed a bilateral network for both action observation and imitation. Additional subanalyses for different effectors within each network revealed highly comparable activation patterns to the overall analyses on observation and imitation, respectively, indicating an independence of these findings from potential confounds. Conjunction analysis of action observation and imitation meta-analyses revealed a bilateral network within frontal premotor, parietal, and temporo-occipital cortex. The most consistently rostral inferior parietal area was PFt, providing evidence for a possible homology of this region to macaque area PF. The observation and imitation networks differed particularly with respect to the involvement of Broca's area: whereas both networks involved a caudo-dorsal part of BA 44, activation during observation was most consistent in a more rostro-dorsal location, i.e., dorsal BA 45, while activation during imitation was most consistent in a more ventro-caudal aspect, i.e., caudal BA 44. The present meta-analysis thus summarizes and amends previous descriptions of the human brain networks related to action observation and imitation.

Hand preference influences neural correlates of action observation

It has been argued that we map observed actions onto our own motor system. Here we added to this issue by investigating whether hand preference influences the neural correlates of action observation of simple, essentially meaningless hand actions. Such an influence would argue for an intricate neural coupling between action production and action observation, which goes beyond effects of motor repertoire or explicit motor training, as has been suggested before. Indeed, parts of the human motor system exhibited a close coupling between action production and action observation. Ventral premotor and inferior and superior parietal cortices showed differential activation for left- and right-handers that was similar during action production as well as during action observation. This suggests that mapping observed actions onto the observer's own motor system is a core feature of action observation - at least for actions that do not have a clear goal or meaning. Basic differences in the way we act upon the world are not only reflected in neural correlates of action production, but can also influence the brain basis of action observation.