Mirror Neurons in the Macaque Monkey: A Critical Review.

In a number of experimental studies carried out in Italy in the early 1990s, it was observed that a set of neurons from the macaque monkey premotor cortex is activated both when a specific action is performed and when the same action is simply observed. These neurons were given the evocative name of "mirror neurons". In this article, the conclusions from these studies and subsequent elaborations are reviewed in order to examine their soundness in theoretical terms.

A human homologue of monkey F5c.

Area F5c is a monkey premotor area housing mirror neurons which responds more strongly to grasping observation when the actor is visible than when only the actor's hand is visible. Here we used this characteristic fMRI signature of F5c in seven imaging experiments - one in macaque monkeys and six in humans - to identify the human homologue of monkey F5c. By presenting the two grasping actions (actor, hand) and varying the low level visual characteristics, we localized a putative human homologue of area F5c (phF5c) in the inferior part of precentral sulcus, bilaterally. In contrast to monkey F5c, phF5c is asymmetric, with a right-sided bias, and is activated more strongly during the observation of the later stages of grasping when the hand is close to the object. The latter characteristic might be related to the emergence, in humans, of the capacity to precisely copy motor acts performed by others, and thus imitation.

Grasping synergies: a motor-control approach to the mirror neuron mechanism.

The discovery of mirror neurons revived interest in motor theories of perception, fostering a number of new studies as well as controversies. In particular, the degree of motor specificity with which others' actions are simulated is highly debated. Human corticospinal excitability studies support the conjecture that a mirror mechanism encodes object-directed goals or low-level kinematic features of others' reaching and grasping actions. These interpretations lead to different experimental predictions and implications for the functional role of the simulation of others' actions. We propose that the representational granularity of the mirror mechanism cannot be any different from that of the motor system during action execution. Hence, drawing from motor control models, we propose that the building blocks of the mirror mechanism are the relatively few motor synergies explaining the variety of hand functions. The recognition of these synergies, from action observation, can be potentially very robust to visual noise and thus demonstrate a clear advantage of using motor knowledge for classifying others' action.

Action experience, more than observation, influences mu rhythm desynchronization.

Since the discovery of mirror neurons in premotor and parietal areas of the macaque monkey, the idea that action and perception may share the same neural code has been of central interest in social, developmental, and cognitive neurosciences. A fundamental question concerns how a putative human mirror neuron system may be tuned to the motor experiences of the individual. The current study tested the hypothesis that prior motor experience modulated the sensorimotor mu and beta rhythms. Specifically, we hypothesized that these sensorimotor rhythms would be more desynchronized after active motor experience compared to passive observation experience. To test our hypothesis, we collected EEG from adult participants during the observation of a relatively novel action: an experimenter used a claw-like tool to pick up a toy. Prior to EEG collection, we trained one group of adults to perform this action with the tool (performers). A second group comprised trained video coders, who only had experience observing the action (observers). Both the performers and the observers had no prior motor and visual experience with the action. A third group of novices was also tested. Performers exhibited the greatest mu rhythm desynchronization in the 8-13 Hz band, particularly in the right hemisphere compared to observers and novices. This study is the first to contrast active tool-use experience and observation experience in the mu rhythm and to show modulation with relatively shorter amounts of experience than prior mirror neuron expertise studies. These findings are discussed with respect to its broader implication as a neural signature for a mechanism of early social learning.

Mirror adaptation in sensory-motor simultaneity.

BACKGROUND: When one watches a sports game, one may feel her/his own muscles moving in synchrony with the player's. Such parallels between observed actions of others and one's own has been well supported in the latest progress in neuroscience, and coined "mirror system." It is likely that due to such phenomena, we are able to learn motor skills just by observing an expert's performance. Yet it is unknown whether such indirect learning occurs only at higher cognitive levels, or also at basic sensorimotor levels where sensorimotor delay is compensated and the timing of sensory feedback is constantly calibrated. METHODOLOGY/PRINCIPAL FINDINGS: Here, we show that the subject's passive observation of an actor manipulating a computer mouse with delayed auditory feedback led to shifts in subjective simultaneity of self mouse manipulation and auditory stimulus in the observing subjects. Likewise, self adaptation to the delayed feedback modulated the simultaneity judgment of the other subjects manipulating a mouse and an auditory stimulus. Meanwhile, subjective simultaneity of a simple visual disc and the auditory stimulus (flash test) was not affected by observation of an actor nor self-adaptation. CONCLUSIONS/SIGNIFICANCE: The lack of shift in the flash test for both conditions indicates that the recalibration transfer is specific to the action domain, and is not due to a general sensory adaptation. This points to the involvement of a system for the temporal monitoring of actions, one that processes both one's own actions and those of others.

Interpreting actions: the goal behind mirror neuron function.

Crucial to our everyday social functioning is an ability to interpret the behaviors of others. This process involves a rapid understanding of what a given action is not only in a physical sense (e.g., a precision grip around the stem of a wine glass) but also in a semantic sense (e.g., an invitation to "cheers"). The functional properties of fronto-parietal mirror neurons (MNs), which respond to both observed and executed actions, have been a topic of much debate in the cognitive neuroscience literature. The controversy surrounds the role of the "mirror neuron system" in action understanding: do MNs allow us to comprehend others' actions by allowing us to internally represent their behaviors or do they simply activate a direct motor representation of the perceived act without recourse to its meaning? This review outlines evidence from both human and primate literatures, indicating the importance of end-goals in action representations within the motor system and their predominance in influencing action plans. We integrate this evidence with recent views regarding the complex and dynamic nature of the mirror neuron system and its ability to respond to broad motor outcomes.

Misconceptions about mirror-induced motor cortex activation.

Observation of self-produced hand movements through a mirror, creating an illusion of the opposite hand moving, was recently reported to induce ipsilateral motor cortex activation, that is, motor cortex activation for the hand in rest. The reported work goes far beyond earlier work on motor cortex activation induced by action observation, by implying a complete reversal of contralateral and ipsilateral motor cortex activation under mirror view conditions. Such a reversal would represent an unprecedented degree of neural plasticity. We considered such a reversal physiologically implausible and conducted a study with an improved design. The results refute the reversal of contralateral and ipsilateral motor cortex activation under mirrored viewing conditions as methodologically unsound. The investigation confirmed, however, more subtle expressions of motor cortical activity induced by self-produced movements observed through a mirror.