Motor interference in interactive contexts

When Corticospinal Inhibition Favors an Efficient Motor Response

Many daily activities involve responding to the actions of other people. However, the functional relationship between the motor preparation and execution phases still needs to be clarified. With the combination of different and complementary experimental techniques (i.e., motor excitability measures, reaction times, electromyography, and dyadic 3-D kinematics), we investigated the behavioral and neurophysiological signatures characterizing different stages of a motor response in contexts calling for an interactive action. Participants were requested to perform an action (i.e., stirring coffee or lifting a coffee cup) following a co-experimenter's request gesture. Another condition, in which a non-interactive gesture was used, was also included. Greater corticospinal inhibition was found when participants prepared their motor response after observing an interactive request, compared to a non-interactive gesture. This, in turn, was associated with faster and more efficient action execution in kinematic terms (i.e., a social motor priming effect). Our results provide new insights on the inhibitory and facilitatory drives guiding social motor response generation. Altogether, the integration of behavioral and neurophysiological indexes allowed us to demonstrate that a more efficient action execution followed a greater corticospinal inhibition. These indexes provide a full picture of motor activity at both planning and execution stages.

The relevance of signal timing in human-robot collaborative manipulation

To achieve a seamless human-robot collaboration, it is crucial that robots express their intentions without perturbating or interrupting the task that a human partner is performing at that moment. Although it has not received much attention so far, this issue is important when robots assist humans in physical and manipulation tasks. The main question addressed here is whether there is a more appropriate time to inform a human partner that a robot is requesting to pass them an object. This question is posed in a reference scenario where human individuals are involved in a continuous pick-and-place task that cannot be interrupted. Our findings showed that providing a cue at the beginning of a reach-to-grasp movement could severely interfere with the ongoing human action, increasing the number of errors made by humans, slowing down and degrading the smoothness of their arm movement, and deflecting their gaze. These disruptive interferences strongly decreased, until they disappeared, when the robot provided the cue to the human partners shortly after the participants picked up an object, identifying this as the best signaling timing. The results of this work showed how the signaling timing may have a decisive influence on the performances of the human-robot teamwork and contribute to understanding the mechanisms underpinning the phenomenon of cognitive-motor interference in humans.

The Shape of Water: How Tai Chi and Mental Imagery Effect the Kinematics of a Reach-to-Grasp Movement

The aim of the present study was to investigate the effect of Tai Chi (TC) and mental imagery (MI) on motor performance. MI is the ability of representing different types of images and it can be improved through constant practice (e.g., of TC). The majority of previous literature has mainly investigated the impact of this mental factor by means of qualitative indexes, whereas studies considering more rigorous measures such as kinematic parameters are rare. In this vein, little is known about how MI can affect reach-to-grasp, one of the most studied models in kinematic research. The present study attempts to fill that gap by investigating the relationship between MI and motor performance in TC, a practice that largely promotes the adoption of mental training. One TC master, four instructors, ten apprentices and fifteen untrained participants were requested to reach toward and grasp an object while mentally representing one out of five different images related to water with an increasing degree of dynamicity and expansion (i.e., still water, flowing water, wave, whirlpool, and opening water flower). Kinematic profiles of movements were recorded by means of six infra-red cameras using a 3-D motion analysis system. We tested whether: (i) focusing on MI during the task would help in optimizing motor efficiency, and (ii) expertise in TC would be reflected in higher flexibility during the task. The results indicate that kinematics is highly sensitive to MI and TC practice. In particular, our main finding suggests a statistically significant general improvement in motor efficiency for the TC group and a beneficial effect for all the participants when focusing on the most expansive image (i.e., opening water flower). Moreover, regression analysis indicates that MI and TC practice make online control more flexible in an experience-based way. These results have important implications for the use of mental imagery and TC in the retraining of motor function in people with physical disabilities.

Social Motor Priming: when offline interference facilitates motor execution

Many daily activities involve synchronizing with other people's actions. Previous literature has revealed that a slowdown of performance occurs whenever the action to be carried out is different to the one observed (i.e., visuomotor interference). However, action execution can be facilitated by observing a different action if it calls for an interactive gesture (i.e., social motor priming). The aim of this study is to investigate the costs and benefits of spontaneously processing a social response and then executing the same or a different action. Participants performed two different types of grips, which could be either congruent or not with the socially appropriate response and with the observed action. In particular, participants performed a precision grip (PG; thumb-index fingers opposition) or a whole-hand grasp (WHG; fingers-palm opposition) after observing videos showing an actor performing a PG and addressing them (interactive condition) or not (non-interactive condition). Crucially, in the interactive condition, the most appropriate response was a WHG, but in 50 percent of trials participants were asked to perform a PG. This procedure allowed us to measure both the facilitator effect of performing an action appropriate to the social context (WHG)-but different with respect to the observed one (PG)-and the cost of inhibiting it. These effects were measured by means of 3-D kinematical analysis of movement. Results show that, in terms of reaction time and movement time, the interactive request facilitated (i.e., speeded) the socially appropriate action (WHG), whereas interfered with (i.e., delayed) a different action (PG), although observed actions were always PGs. This interference also manifested with an increase of maximum grip aperture, which seemingly reflects the concurrent representation of the socially appropriate response. Overall, these findings extend previous research by revealing that physically incongruent action representations can be integrated into a single action plan even during an offline task and without any training.

Grasping Neurons in the Ventral Premotor Cortex of Macaques Are Modulated by Social Goals

Although it is established that F5 neurons can distinguish between nonsocial goals such as bringing food to the mouth for eating or placing it in a container, it is not clear whether they discriminate between social and nonsocial goals. Here, we recorded single-unit activity in the ventral premotor cortex of two female macaques and used a simple reach-to-grasp motor task in which a monkey grasped an object with a precision grip in three conditions, which only differed in terms of their final goal, that is, a subsequent motor act that was either social (placing in the experimenter's hand ["Hand" condition]) or nonsocial (placing in a container ["Container" condition] or bringing to the mouth for eating ["Mouth" condition]). We found that, during the execution of the grasping motor act, the response of a sizable proportion of F5 motor neurons was modulated by the final goal of the action, with some having a preference for the social goal condition. Our results reveal that the representation of goal-directed actions in ventral premotor cortex is influenced by contextual information not only extracted from physical cues but also from cues endowed with biological or social value. Our study suggests that the activity of grasping neurons in the premotor cortex is modulated by social context.

Movement Notation Revisited: Syntax of the Common Morphokinetic Alphabet (CMA) System

Advances in the study of non-verbal behavior and communication have generated a need for movement transcription systems capable of incorporating continuous developments in visual and computer technology. Our research team has been working on the construction of a common morphokinetic alphabet (CMA) for the systematic observation of daily life activities. The project, which was launched several years ago, was designed to create a system for describing and analyzing body motion expression, physical activity, and physical appearance. In this paper, we describe an idiosyncratic application of Noam Chomsky’s phrase marker grammar to the morphokinetic phrase, the objective being to establish the grammatical rules and basic order of the symbol string according to a relational tree formed by the breakdown of the syntactic components identified as structuring the visual description of movement. Criteria for using the CMA as a coding system and a free transcription system are proposed.

Visuo-motor interference with a virtual partner is equally present in cooperative and competitive interactions

Automatic imitation of observed actions is thought to be a powerful mechanism, one that may mediate the reward value of interpersonal interactions, but that could also generate visuo-motor interference when interactions involve complementary movements. Since interpersonal coordination seems to be crucial both when cooperating and competing with others, the questions arises as to whether imitation-and thus visuo-motor interference-occurs in both scenarios. To address this issue, we asked human participants to engage in high- or low-interactive (Interactive or Cued condition, respectively), cooperative or competitive, joint reach-to-grasps with a virtual partner. More specifically, interactions occurred in: (i) a Cued condition, where participants simply adapted their movement timing to synchronize with (during cooperation) or anticipate (during competition) the virtual partner's grasp; (ii) an Interactive condition requiring the same adaptation, as well as a real-time selection of their action according to the virtual character's movement. To simulate a realistic human-human interaction, the virtual character would change its movement speed in consecutive trials according to participants' behaviour. Results demonstrate that visuo-motor interference-as indexed by movement kinematics (higher maximum wrist height during complementary compared to imitative power grips)-emerge in both cooperative and competitive motor interactions only when predictions about the partner's movements are needed to perform one's own action (interactive condition). These results support the idea that simulative imitation is heavily present when individuals need to match their behaviours closely.

A direct effect of perception on action when grasping a cup

Affordances represent features of an object that trigger specific actions. Here we tested whether the presence and orientation of a handle on a cup could bias grasping movements towards it in conditions where subjects were explicitly told to ignore the handle. We quantified the grip aperture profile of twelve healthy participants instructed to grasp a cup from its body while it either had no handle, a handle pointing towards, or away from the grasping hand (3 ‘move’ conditions, with large grip aperture). To ensure the smaller grip aperture afforded by the handle was implicitly processed, we interspersed trials in which participants had to grasp the cup from its handle or a handle not attached to a cup with a small grip aperture. We found that grip aperture was smaller in the presence of a handle in the ‘move’ conditions, independently of its orientation. Our finding, of an effect of the handle during the execution of a grasp action, extends previous evidence of such an influence measured during motor preparation using simple reaction times. It suggests that the specific action elicited by an object’s attribute can affect movement performance in a sustained manner throughout movement execution.