Brain mechanisms preventing automatic imitation

Almost 20 years ago a group of neuroscientists discovered that some neurons within a region of the monkey brain necessary for movement production, fired both when the monkey performed an action and when the monkey observed a similar action performed by someone else. In order to explain this phenomenon the researchers hypothesized that the observer simulates the observed action in his own motor system, as if he/she performed the action himself; according to this hypothesis the simulation served the understanding of others actions. Even if it is not clear yet if simulation is necessary to understand others actions, many scientific evidences converge on the fact that also in humans observing others actions activates networks of the brain active when performing actions. However, how is it possible to reconcile the fact that one is still able to react to observed actions in non-imitative ways, even if observing an action activates the motor programs to execute that very same action? Indeed, imagine you are a goalkeeper trying to catch the ball in a penalty kick: when the kicker hits the ball your motor system should be activated as if you were performing yourself the action observed. However it would be extremely unproductive if you would imitate the action observed instead of jumping towards the ball to catch it. With the present project I want to understand which brain dynamics allow us to react to observed actions in non-imitative ways without the motor simulation influencing the performance. In order to shed light on this problem I will exploit magnetoencephalography (MEG), a technique capable of recording time-course activity of brain networks with millisecond precision. During the MEG sessions divided in three different experiments, participants will watch two actions while they will be required either to passively observe them (exp. 1), or to react to them with two (exp. 2) actions similar or completely different from those observed. Indeed, the similarity between the pool of actions observed and the pool of actions that participants can perform, modulates the effects of the motor simulation on the performance. Simultaneously with MEG, in the third experiment I will try to modulate the activity of relevant brain networks shown to be involved in setting the appropriate potential actions to be prepared for the required task. This is a pioneering and exciting aspect of this last experiment since it provides the possibility to record MEG activity while simultaneously using transcranial electrical stimulation to influence brain regions of interest, which it has been shown to be possible only recently.

We know from the early '90 that our motor system responds not only to movement execution-related inputs, but also to more complex inputs, like the observation of another's action, putatively thanks to "mirror neurons"; according to the main hypothesis, whenever we observe an action, our motor system is activated as if we were silently replicating that action in our own motor system. My project dealt with the issue of why do not we imitate all the time since our motor system is activated as if we were about to perform the action observed, and, more specifically, why do not we imitate while preparing to react to others actions. Thanks to the experiments conducted, I highlighted a plausible mechanism thanks to which we are able to efficiently control our motor system even in conditions in which, like a goalkeeper for example, we have to react to others' actions with different movements compared to those observed. This mechanism is called action pre-selection by inhibition. When preparing actions similar to those observed, a phenomenon called "automatic imitation" takes place, in which the mere observation of an action similar to those prepared by the observer, triggers the execution of that very action. However this does not happen when the actions prepared are different from those observed. The main result of my project shows that this block of input from the brain to the muscles, takes place in the primary sensorimotor cortices. Indeed what we observed is a typical index related to inhibitory processes, when observing an action while preparing to react to this stimulus with different actions compared to the one observed. These results have interesting implications with respect to the plausibility of the simulation mechanisms hypothesised by the mirror neurons discoverers. Indeed my result might fill a theoretical gap present in the literature, i.e., where in the nervous system does the simulation of other's actions stop to give our will control to our motor system?