Abstract
Complementary colors are color pairs which, when combined in the right proportions, produce white or black. Complementary actions refer here to forms of social interaction wherein individuals adapt their joint actions according to a common aim. Notably, complementary actions are incongruent actions. But being incongruent is not sufficient to be complementary (i.e., to complete the action of another person). Successful complementary interactions are founded on the abilities: (i) to simulate another person’s movements, (ii) to predict another person’s future action/s, (iii) to produce an appropriate incongruent response which differ, while interacting, with observed ones, and (iv) to complete the social interaction by integrating the predicted effects of one’s own action with those of another person. This definition clearly alludes to the functional importance of complementary actions in the perception–action cycle and prompts us to scrutinize what is taking place behind the scenes. Preliminary data on this topic have been provided by recent cutting-edge studies utilizing different research methods. This mini-review aims to provide an up-to-date overview of the processes and the specific activations underlying complementary actions.
Introduction
Motor resonance is defined as the subliminal activation of the motor system—and of the imitative response—while observing actions performed by others (reviewed in ). explained that: “when we observe actions performed by other individuals our motor system ‘resonates’ along with that of the observed agent” (pp. 38–39). Numerous neurophysiological studies have in fact demonstrated that a motor resonant mechanism is at work in the motor, premotor, and the posterior parietal cortices when individuals are instructed to observe goal-directed actions being executed by another or others (for review, see ; ; ). The discovery of mirror neurons in monkeys provided the physiological model for this perception–action coupling mechanism (). Located in the ventral premotor cortex (area F5) and the posterior parietal cortex, mirror neurons were found to fire both when a monkey carried out a goal-directed action as well as when it observed that same action being performed by another subject (). Motor resonance appears then to pre-activate the motor system of an observer in order to represent and interpret the movements of another person even before the “go” signal has been given and activation remains for the most part on an unconscious level ().
While actions that are observed and those that are being planned appear functionally equivalent (), it is unclear if the visual representation of an observed action inevitably leads to its motor representation. This is particularly true with regard to complementary (from Latin complementum; i.e., that fills up) actions, a specific class of movements which differ from -although interacting with- an observed action (; ). In the case, for example, that someone hands us a mug by its handle, we will automatically, without giving it a second thought, grab the mug using a whole-hand-grasp (the most appropriate grasping posture in this particular situation). The types of grasps adopted by the two interacting agents are incongruent, but they are nevertheless appropriate and complementary.
As a working definition, complementary actions refer here to any form of social interaction wherein two (or more) individuals coordinate and mutually complete their incongruent actions, rather than performing imitative behaviors. In this respect, we can define as complementary affordances all the action possibilities in which suitable motor programs aiming to bring a joint goal to completion are activated (such as grasping and offering a coin when seeing an open hand in sign of request). Depending on its posture and context, therefore, an extended open hand could lead to a donation, to a handshake or to an infinite number of other actions (). Activation of a complementary affordance is an important social tool, and it suggests that the automatic, rapid decoding of social cues influences intentional behavior in our everyday interactions, maximizing the efficiency of our responses. These examples illustrate the functional importance of complementary actions in the action–perception domain (), and they prompt us to examine the mechanisms involved in producing those responses.
Behavioral Studies of Complementary Actions
Since the direct matching between observed and performed actions is thought to occur automatically, when we observe an action which differs from our intended action we have to inhibit the tendency to imitate (). While the mechanism leading to automatic imitation is relatively well-studied (), it is less clear how this automatic tendency is brought under control.
Evidence that task representation plays a pivotal role in shaping our actions has been provided by a series of studies (,; ; ) in which participants were explicitly instructed to prepare imitative or complementary actions after viewing a virtual actor grasp a manipulandum using either a precision grip (PG; i.e., opposition between the index finger and thumb) or a whole-hand grasp (WHG; i.e., opposition of the thumb with the other fingers). As expected, participants were faster at preparing their response in imitative contexts if the action to be carried out was congruent with what they had observed. When, instead, they were expected to carry out complementary actions, they responded faster when their action was dissimilar to the one they had just observed. The task representation (imitative vs. complementary) seems then to overrule long-term stimulus-response associations, influencing the way that action–perception coupling takes place. Further evidence concerning this flexible perception–action coupling was produced by a 3D motion capture study () in which reaching and grasping parameters of congruent responses were found to improve in imitative contexts, and incongruent responses were facilitated in complementary contexts. Consistent with these findings, demonstrated that also the level of action coding can be modified (e.g., toward coding in terms of movements) depending on task requirements. Taken together, these data challenge the idea that action observation automatically leads to imitation in the observer and suggest that, depending on the context, observed actions can prime incongruent responses.
Recently, , ) showed that participants involved in face-to-face interactions can mutually adjust their movements in time and space even in the absence of instructions to either imitate or perform a complementary response. This demonstrates that priming does not strictly depend on task-constrains, and that humans might indeed be able to actively shift from imitative to complementary actions, thanks to neuro-cognitive processes that still needs to be clarified.
Neuroimaging Studies of Complementary Actions
Few studies have examined the neural circuitry behind joint actions, and in particular the human mirror neuron system’s (hMNS) involvement in complementary forms of social interaction. Might the hMNS provide a substrate for complementary actions? And if not, what role do other brain systems play?
In a pioneering experiment, the response of the hMNS was specifically investigated in imitative and complementary action contexts using functional magnetic resonance imaging (fMRI; ,). Signals were recorded while the participants prepared to grasp a manipulandum in one of two ways—with a WHG or a PG—after they viewed an actor carrying out that action. It was found that preparation for complementary actions resulted in an increased blood-oxygen-level-dependent (BOLD) signal in the right inferior frontal gyrus (IFG) and in the bilateral inferior parietal lobule (IPL), two core components of the mirror system (Figure 1). This finding can be explained in terms of different kinds of mirror neurons: strictly congruent mirror neurons, which respond to identical actions, both observed and performed ones, and broadly congruent mirror neurons, which respond to non-identical observed and performed actions and objects linked to them (). It is also possible that in the complementary condition, when participants observe an action drawing attention to an object eliciting a different action, an interplay takes place between mirror and canonical neurons with the latter responding both during the time the action is being executed and also while the objects linked to those behaviors are perceived (). The need to carry out a complementary action involving a different object might then imply a combination of mirror and canonical neurons coding for different types of actions at different times of the sequence. The hypothesis that different classes of mirror neurons serve to integrate observed and executed actions during complementary kinds of social interaction is certainly an appealing one.
FIGURE 1
A more integrated description of neural circuits underlying complementary actions was recently outlined by
Two anatomically separate networks have thus been delineated: one that would decipher observed and executed actions into a single common code (
Notably, the temporal course of the low- and high-level systems interaction has long been debated.
If output from control systems guide and modulates the mirror system, this would represent a top-down process. The STORM model (social top-down response modulation) suggests that the decision to imitate or to inhibit imitation initially draws on social signals and is most likely supported by a brain network including medial Prefrontal Cortex (mPFC) and temporoparietal junction (TPJ), two core areas of the so-called Mentalizing system, engaged when participants judge other people’s mental state (
Neurophysiologic Studies of Complementary Actions
Action observation automatically activates corresponding motor representations in an observer, and the stronger support for this process comes from single-pulse transcranial magnetic stimulation (spTMS) over the primary motor cortex (M1) and concomitant electromyography (EMG; e.g.,
FIGURE 2

The functional shift. A fundamental requirement for successful complementary actions is the capacity to smoothly and efficiently switch from observing another person’s gestures to planning a corresponding reciprocal action. TMS-induced MEPs were recorded from participants’ hand muscles in response to observing an actor grasping an object and then trying vainly to fulfill a task (e.g., pouring coffee) in a cup which was strategically placed out of her reach but in the video foreground, close to the observer’s right hand (
At this point a new important question arose: at what point does this functional switch occur? A new experiment was designed in which TMS was delivered at five different timepoints corresponding to five kinematic landmarks characterizing the observed action (
A Working Memory Hypothesis
A dual process seems then to underlie joint actions: a low-level motor resonance analyzes and stores information on observed actions, while a high-level system would flexibly integrate our and others’ motor intentions and select the most appropriate response and time course to achieve joint goals (
Complementary actions are the ideal way to test this topic. During complex social interaction/s, the individual needs to keep information relative to the observed action available while contemporaneously attempting to process a response. In this type of context, the mirror system may be involved in keeping action-related information on hold to enable other brain areas to extract the meaning of the action observed to achieve a joint goal. Notably, observing another person’s actions priming for an incongruent reaction can lead to a motor resonant response in the observer’s corresponding muscles as well as a simultaneous preparation in different effectors necessary for achieving a complementary response (
Conclusion
The research outlined here shows that motor resonance elicited by action observation is modulated depending on its context: when an observed gesture is socially relevant (i.e., there is an implicit or explicit request) anticipatory complementary activations follow. The assumption that observing an action automatically triggers the inclination to imitate probably arose because most studies did not explicitly challenge the automaticity or flexibility of the visuomotor transformation process. The data outlined here have contributed to shedding light on the functioning of the human motor system in social contexts and on the types of social behavior frequently occurring in real-world settings. From a wider perspective, we can theorize that defining the conditions and the modalities by which motor resonant responses to action observation can be modulated may prove to have specific translational implications leading to the development of novel neuro-rehabilitation protocols for patients with localized lesions to cortical motor areas (e.g., ischemic stroke) and for pathologies such as autism (
Conflict of Interest Statement
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Statements
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
action observation, perception–action coupling, social interactions, motor resonance, transcranial magnetic stimulation
Citation
Sartori L and Betti S (2015) Complementary actions. Front. Psychol. 6:557. doi: 10.3389/fpsyg.2015.00557
Received
27 January 2015
Accepted
17 April 2015
Published
01 May 2015
Volume
6 - 2015
Edited by
Claudia Gianelli, University of Potsdam, Germany
Reviewed by
Lucia M. Sacheli, Sapienza University of Rome, Italy; Lincoln J. Colling, Australian Catholic University, Australia
Copyright
© 2015 Sartori and Betti.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Luisa Sartori, Dipartimento di Psicologia Generale, Università di Padova, Via Venezia 8, 35131 Padova, Italy, luisa.sartori@unipd.it
This article was submitted to Cognition, a section of the journal Frontiers in Psychology.
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