Article summary with The role of the motor system in action understanding and communication: Evidence from human infants and non-primates by Salo a.o. - 2018

What is the mirror neuron system?

The mirror neuron system refers to a group of specialized neurons that mirror the actions and behaviors of others. Mirror neurons are found in the ventral premotor cortex and inferior parietal lobe and show a shared neural circuitry in the sensorimotor brain regions for both execution and observation of actions.

What is the EEG mu rhythm?

The mu rhythm consists of synchronized patterns of electrical activity involving large numbers of neurons in the part of the brain that control voluntary movement. The EEG mu rhythm provides one method of measuring activity of the mirror neuron system and measuring sensorimotor activity. The mu rhythm reflects oscillatory activity within the alpha band and exhibits changes in amplitude reflecting activation of the sensorimotor system. The amplitude of a signal decreases as compared to baseline activity, with peak suppression over sensorimotor areas. This suppression of activity is likely due to an increase in desynchronized neuronal activity associated with the processing of motoric information. 

What does the EEG mu rhythm of infants tell us about their action understanding?

The infant mu rhythm is sensitive to all coherent motion (not necessarily human actions or actions with which they have experience). Younger infants will attribute goals to both human and nonhuman agents, whereas older infants only do so for human and human-like agents, but not inanimate objects. This early pattern of non-discriminant mu rhythm desynchronization suggests that young infants are recruiting their motor system in trying to decipher the meaning or purpose behind all motion as they learn to discriminate that which is similar to their own developing motor repertoire and that which is not.

How could mirroring support action understanding?

There are several hypotheses to explain how the mechanism of how mirroring supports action understanding may work:

• Direct-matching hypothesis. This hypothesis suggests that internal representation of perceived actions is supported through activation of a one-to-one overlapping system with one´s own actions. Through a simulation process with resonant activation of the motor system, the visual description of an action is mapped on to the corresponding motor representation. Such motor activation indicates that an observed action is described into a motor format which belongs to the observer´s motor repertoire.
• Ideomotor theory. According to this theory, resonant motor activity during observation of actions supports goal interpretation through facilitating an expectation of the outcome. Repeated co-occurrence of an action and its effect creates a bi-directional link between the activation of one´s motor system and a representation of the effect. When watching another person perform the same or a similar action, the motor program for that action is engaged, which in turn activates the effect representation. The effect representation then facilitates anticipatory behaviors relevant to that effect.
• Associative learning. Contingency and contiguity of action-perception experiences create and shape the function of the motor system during action observation.

How can gestures be a link between action understanding and language?

Gestures support a transition from an earlier ability for interpreting concrete actions with visible consequences, to the more difficult process of interpreting spoken words which are completely abstract and arbitrary and have no visible consequence in the physical world. Action, gesture, and language may be overlapping cognitive systems. The ability to communicate through language builds from the ability to understand the intentions and goals driving one´s own and others´ actions.

How could gestures have served as an evolutionary step between action and language?

Our prehistoric ancestor learned that they could direct others´ attention through their physical movements. These physical movements became more refined and specific and used not only for directing attention but also to convey specific information. We would now refer to these actions as communicative gestures. As the use of gestures to communicate expanded, the gestures became paired with vocalizations which enlarged the number and specificity of ideas one could convey. Vocalizations then also became increasingly fine-tuned and eventually became representations and means for communication.

What is the gesture as intermediary hypothesis?

According to this hypothesis, infants extend their understanding of actions to communicative gestures, which in turn supports their understanding of spoken language. The same process for understanding the referential, intentional, and goal-directed nature of actions is applied to understanding gestures and word learning. Gestures support a transition from the earlier ability for interpreting concrete object-directed actions to the more difficult process of interpreting spoken words. Once an infant understands the goal-directedness of actions, it can extend that property to more abstract gestures, and in turn to words.

What is the mirror system hypothesis for the evolution of language?

The mirror system hypothesis for the evolution of language states that the mirror neuron system lays the foundation for the link between actions, action understanding, gestures, and language. Insofar as the ability to represent and interpret others´ actions supports the understanding of communicative gestures and spoken language, the mirror neuron system may serve as a means for that representation – it is a direct link between communicator and receiver.