Modulation of social context on the activity of motor and mirror neurons in the ventral premotor cortex (F5c) of Macaque monkey

Several studies show that motor neurons' and mirror neurons' (MN) discharge in the monkey ventral premotor cortex (F5) and in the inferior parietal lobule (IPL) is modulated by the action's goal and by the context in which the action has been executed (Fogassi et al., 2005; Bonini et al. 2009). In this Ph.D thesis, I investigated if biological information, such as the presence of a container, and social cues, such as social interactions with another experimenter or with the observer, can differently affect the activity of motor and mirror neurons in F5 convexity during a visual and motor task. Moreover, we also investigated if the passive or active observation of an action could have a different effect on the visual discharge of mirror neurons. During passive observation the monkey is required to simply observe an action without any interaction with the observed agent. Conversely, during active observation the monkey is required to observe an action in order to plan a subsequent action coordinated with the observed agent. We carried out electrophysiological recordings in the ventral premotor cortex of two macaques (Macaca mulatta), with a sixteen channels multielectrode system. The neural activity was recorded in the visual task (VT) and in the motor task (MT). The visual task was characterized by 4 conditions in which the monkey observed an experimenter grasping an object/piece of food followed by one out of four possible action goals: 1. Container (grasping to place into a container), 2. Hand (grasping to place into another experimenter's hand), 3. Monkey (grasping to give to the monkey), 4. Mouth (grasping to eat). The motor task was characterized by 3 conditions Container, Hand and Mouth of the VT, but in this case, the monkey performed the grasping action. We found that, motor neurons and mirror neurons could discriminate the same motor act according to the final goal of the action and that, both during the motor and the visual tasks, motor neurons' and MNs' discharge are modulated by the social context, in particular, by the social goal of the action (condition Hand and Monkey for the visual task, and condition Hand for the motor task). Our results seem to support the idea that motor and mirror neurons could have a key role not only in the encoding of other's intentions, but also in the encoding of others' intention in relation to social goals. This would allow the individual to create an internal representation of the action performed by others, that integrates different aspects not only of the biological context but also of the social context. Moreover, unexpectedly, we found that the number of MNs which prefers the Monkey condition in the VT is virtually absent. It's also interesting to note that, in this condition, the MNs spiking activity, during the observation of the action, tends to decrease in relation with the increase of the time occurring from the experimenter's hand contact with the target object to the monkey's hand release from its handle in order to grasp the object offered by the experimenter. These results, together with those obtained from the Local Field Potential (LFP) analysis, suggest that the active action observation has an inhibitory effect on the MNs visual discharge in F5. The mechanism, by which this attenuation would occur, could be attributed to the events related to the observation of the action performed by the experimenter in the Monkey condition. In fact, in this phase, the monkey has to prepare the action but, at the same time, it has to maintain suppressed the action until the experimenter starts his movement toward the monkey. During this phase it is possible that the motor neurons (and probably also some F5 mirror neurons) can induce an interference or inhibitory effect on the visual discharge of mirror neurons, activated during the observation of the grasping action. These results seem to contrast previous hypothesis about the functional role of mirror neurons, which postulate that the activity of mirror neurons during the action observation reflects a motor preparation or an action anticipation (Kilner et al., 2004; Fontana et al., 2012).