Effector- and target-independent representation of observed actions: evidence from incidental repetition priming

Neural dynamics of grip and goal integration during the processing of others' actions with objects: An ERP study

Recent behavioural evidence suggests that when processing others’ actions, motor acts and goal-related information both contribute to action recognition. Yet the neuronal mechanisms underlying the dynamic integration of the two action dimensions remain unclear. This study aims to elucidate the ERP components underlying the processing and integration of grip and goal-related information. The electrophysiological activity of 28 adults was recorded during the processing of object-directed action photographs (e.g., writing with pencil) containing either grip violations (e.g. upright pencil grasped with atypical-grip), goal violations (e.g., upside-down pencil grasped with typical-grip), both grip and goal violations (e.g., upside-down pencil grasped with atypical-grip), or no violations. Participants judged whether actions were overall typical or not according to object typical use. Brain activity was sensitive to the congruency between grip and goal information on the N400, reflecting the semantic integration between the two dimensions. On earlier components, brain activity was affected by grip and goal typicality independently. Critically, goal typicality but not grip typicality affected brain activity on the N300, supporting an earlier role of goal-related representations in action recognition. Findings provide new insights on the neural temporal dynamics of the integration of motor acts and goal-related information during the processing of others’ actions.

Probing Representations of Gymnastics Movements: A Visual Priming Study

In this study, we designed a visual short-term priming paradigm to investigate the mechanisms underlying the priming of movements and to probe movement representations in motor experts and matched controls. We employed static visual stimuli that implied or not human whole-body movements, that is, gymnastics movements and static positions. Twelve elite female gymnasts and twelve matched controls performed a speeded two-choice response time task. The participants were presented with congruent and incongruent prime-target pairs and had to decide whether the target stimulus represented a gymnastics movement or a static position. First, a visual priming effect was observed in the two groups. Second, a stimulus-response rote association could not easily account for our results. Novel primes never presented as targets could also prime the targets. Third, by manipulating three levels of prime-target relations in moving congruent pairs, we demonstrated that the more similar prime-target pairs, the greater the facilitation in target. Lastly, gymnastics motor expertise impacted on priming effects.

The role of attention in human motor resonance

Observation of others' actions evokes in primary motor cortex and spinal circuits of observers a subliminal motor resonance response, which reflects the motor program encoding observed actions. We investigated the role of attention in human motor resonance with four experimental conditions, explored in different subject groups: in the first explicit condition, subjects were asked to observe a rhythmic hand flexion-extension movement performed live in front of them. In two other conditions subjects had to monitor the activity of a LED light mounted on the oscillating hand. The hand was clearly visible but it was not the focus of subjects' attention: in the semi-implicit condition hand movement was relevant to task completion, while in the implicit condition it was irrelevant. In a fourth, baseline, condition subjects observed the rhythmic oscillation of a metal platform. Motor resonance was measured with the H-reflex technique as the excitability modulation of cortico-spinal motorneurons driving a hand flexor muscle. As expected, a normal resonant response developed in the explicit condition, and no resonant response in the baseline condition. Resonant responses also developed in both semi-implicit and implicit conditions and, surprisingly, were not different from each other, indicating that viewing an action is, per se, a powerful stimulus for the action observation network, even when it is not the primary focus of subjects' attention and even when irrelevant to the task. However, the amplitude of these responses was much reduced compared to the explicit condition, and the phase-lock between the time courses of observed movement and resonant motor program was lost. In conclusion, different parameters of the response were differently affected by subtraction of attentional resources with respect to the explicit condition: time course and muscle selection were preserved while the activation of motor circuits resulted in much reduced amplitude and lost its kinematic specificity.

Activity in superior parietal cortex during training by observation predicts asymmetric learning levels across hands

A dominant concept in motor cognition associates action observation with motor control. Previous studies have shown that passive action observation can result in significant performance gains in humans. Nevertheless, it is unclear whether the neural mechanism subserving such learning codes abstract aspects of the action (e.g. goal) or low level aspects such as effector identity. Eighteen healthy subjects learned to perform sequences of finger movements by passively observing right or left hand performing the same sequences in egocentric view. Using functional magnetic resonance imaging we show that during passive observation, activity in the superior parietal lobule (SPL) contralateral to the identity of the observed hand (right\left), predicts subsequent performance gains in individual subjects. Behaviorally, left hand observation resulted in positively correlated performance gains of the two hands. Conversely right hand observation yielded negative correlation - individuals with high performance gains in one hand exhibited low gains in the other. Such behavioral asymmetry is reflected by activity in contralateral SPL during short-term training in the absence of overt physical practice and demonstrates the role of observed hand identity in learning. These results shed new light on the coding level in SPL and have implications for optimizing motor skill learning.

Press to grasp: how action dynamics shape object categorization

Action and object are deeply linked to each other. Not only can viewing an object influence an ongoing action, but motor representations of action can also influence visual categorization of objects. It is tempting to assume that this influence is effector-specific. However, there is indirect evidence suggesting that this influence may be related to the action goal and not just to the effector involved in achieving it. This paper aimed, for the first time, to tackle this issue directly. Participants were asked to categorize different objects in terms of the effector (e.g. hand or foot) typically used to act upon them. The task was delivered before and after a training session in which participants were instructed either just to press a pedal with their foot or to perform the same foot action with the goal of guiding an avatar's hand to grasp a small ball. Results showed that pressing a pedal to grasp a ball influenced how participants correctly identified graspable objects as hand-related ones, making their responses more uncertain than before the training. Just pressing a pedal did not have any similar effect. This is evidence that the influence of action on object categorization can be goal-related rather than effector-specific.

Perturbing the action observation network during perception and categorization of actions' goals and grips: state-dependency and virtual lesion TMS effects

Watching others grasping and using objects activates an action observation network (AON), including inferior frontal (IFC), anterior intraparietal (AIP), and somatosensory cortices (S1). Yet, causal evidence of the differential involvement of such AON sensorimotor nodes in representing high- and low-level action components (i.e., end-goals and grip type) is meager. To address this issue, we used transcranial magnetic stimulation-adaptation (TMS-A) during 2 novel action perception tasks. Participants were shown adapting movies displaying a demonstrator performing goal-directed actions with a tool, using either power or precision grips. They were then asked to match the end-goal (Goal-recognition task) or the grip (Grip-recognition task) of actions shown in test pictures to the adapting movies. TMS was administered over IFC, AIP, or S1 during presentation of test pictures. Virtual lesion-like effects were found in the Grip-recognition task where IFC stimulation induced a general performance decrease, suggesting a critical role of IFC in perceiving grips. In the Goal-recognition task, IFC and S1 stimulation differently affected the processing of "adapted" and "nonadapted" goals. These "state-dependent" effects suggest that the overall goal of seen actions is encoded into functionally distinct and spatially overlapping neural populations in IFC-S1 and such encoding is critical for recognizing and understanding end-goals.

When far becomes near: shared environments activate action simulation

It has been proposed that one means of understanding a person's current behaviour and predicting future actions is by simulating their actions. That is, when another person's actions are observed, similar motor processes are activated in the observer. For example, after observing a reach over an obstacle, a person's subsequent reach trajectory is more curved, reflecting motor priming. Importantly, such motor states are only activated if the observed action is in near (peripersonal) space. However, we demonstrate that when individuals share action environments, simulation of another person's obstacle avoiding reach path takes place even when the action is in far (extrapersonal) space. We propose that action simulation is influenced by factors such as ownership. When an "owned" object is a potential future obstacle, even when it is viewed beyond current action space, simulations are evoked, and these leave a more stable memory capable of influencing future behaviour.

Observed effector-independent motor learning by observing

A compelling idea in cognitive neuroscience links motor control and action observation. Recent work supports the idea that a link exists not just between action observation and action planning, but between observation and motor learning. Several studies support the idea that cortical regions that underlie active motor learning also play a role in motor learning by observing. The goal of the present study was to test whether motor learning by observing is effector dependent (as in active motor learning) or effector independent (as in studies of action observation and mirror neurons). Right-handed human subjects observed a video depicting another individual learning to reach to visual targets in a force field (FF). The video showed reaching in a clockwise FF (CWFF) or a counter-clockwise FF (CCWFF), and depicted an individual reaching with the right or left arm. After observation, all subjects were asked to reach in a CWFF, using their right arm. As in our prior studies, subjects who observed a CWFF prior to the CWFF test performed better than subjects who observed a CCWFF. We show here that this effect was seen both when observers watched others reach using their right arm, and when observers watched others learning to reach using the left arm. These results suggest that information about novel forces learned through observation is represented in an effector-independent coordinate frame, and are consistent with the idea that the brain links not only observation and movement, but motor learning as well, through abstract representations of actions.

Swallowing intentional off-state in aging and Alzheimer's disease: preliminary study

Frontal cortical activation is elicited when subjects have been instructed not to initiate a sensorimotor task. The goal of this preliminary fMRI study was to examine BOLD response to a "Do Not Swallow" instruction (an intentional "off-state") in the context of other swallowing tasks in 3 groups of participants (healthy young, healthy old, and early Alzheimer's disease (AD)). Overall, the older group had larger, bilaterally active clusters in the cortex, including the dorsomedial prefrontal cortex during the intentional swallowing off-state; this region is commonly active in response inhibition studies. Disease-related differences were evident where the AD group had significantly greater BOLD response in the insula/operculum than the old. These findings have significant clinical implications for control of swallowing across the age span and in neurodegenerative disease. Greater activation in the insula/operculum for the AD group supports previous studies where this region is associated with initiating swallowing. The AD group may have required more effort to "turn off" swallowing centers to reach the intentional swallowing off-state.

Incidental action observation modulates muscle activity

Similar circuits in the brain are engaged during the performance and observation of identical actions. Such engagement manifests in priming effects, where observation of an action leads to faster production of that action and slower production of an action involving a different movement of the same effector (e.g. observed finger flexion vs. produced finger extension), or a completely different effector (e.g. observed hand action vs. produced leg action). Here, we asked whether priming occurs for actions involving identical muscle groups where the degree of muscle contraction in observed actions was the same or different to that underlying an instructed response and whether patterns of muscle activation were also affected. Participants held an unseen rubber ball between their forefinger and thumb and responded to colour cues instructing a hard or a soft squeeze, whilst EMG activity from the first dorsal interosseous and the abductor pollicis brevis was recorded. The colour cues were superimposed on videos depicting a hard or soft squeeze of an identical rubber ball. Thus, there were two congruent (observe hard, produce hard; observe soft, produce soft) and two incongruent (observe hard, produce soft; observe soft, produce hard) conditions. Results showed that reaction time was slowed and EMG activity was modulated in the direction of the difference between observed and instructed squeezing movements. Hence, neural circuits underlying action observation are sensitive not only to differences in the actual muscle groups underlying observed actions but also to different extents of activation of the same muscle groups.

Priming of reach trajectory when observing actions: hand-centred effects

When another person's actions are observed it appears that these actions are simulated, such that similar motor processes are triggered in the observer. Much evidence suggests that such simulation concerns the achievement of behavioural goals, such as grasping a particular object, and is less concerned with the specific nature of the action, such as the path the hand takes to reach the goal object. We demonstrate that when observing another person reach around an obstacle, an observer's subsequent reach has an increased curved trajectory, reflecting motor priming of reach path. This priming of reach trajectory via action observation can take place under a variety of circumstances: with or without a shared goal, and when the action is seen from a variety of perspectives. However, of most importance, the reach path priming effect is only evoked if the obstacle avoided by another person is within the action (peripersonal) space of the observer.

Through the looking glass: counter-mirror activation following incompatible sensorimotor learning

The mirror system, comprising cortical areas that allow the actions of others to be represented in the observer's own motor system, is thought to be crucial for the development of social cognition in humans. Despite the importance of the human mirror system, little is known about its origins. We investigated the role of sensorimotor experience in the development of the mirror system. Functional magnetic resonance imaging was used to measure neural responses to observed hand and foot actions following one of two types of training. During training, participants in the Compatible (control) group made mirror responses to observed actions (hand responses were made to hand stimuli and foot responses to foot stimuli), whereas the Incompatible group made counter-mirror responses (hand to foot and foot to hand). Comparison of these groups revealed that, after training to respond in a counter-mirror fashion, the relative action observation properties of the mirror system were reversed; areas that showed greater responses to observation of hand actions in the Compatible group responded more strongly to observation of foot actions in the Incompatible group. These results suggest that, rather than being innate or the product of unimodal visual or motor experience, the mirror properties of the mirror system are acquired through sensorimotor learning.