• social facilitation
• licking behaviour - Go noGo task
• Anterior Cyngulate Cortex
• pupillometry
• electrophysiology
• mirror neurons

The observation of actions made by a conspecific can lead, in some cases, to the repetition or the intensification of the same actions. This process, named “social facilitation”, seems to be at the basis of social interaction and communication among the members of the same group or species, increasing their survival and fitness. Indeed, this behaviour is ingrained in a lot of social animals such as ants, fishes, birds, rats, and both non-human and human primates. Interestingly, in all these species, foraging has been found to be one of the most “shared” behaviour. A possible explanation of the social facilitation process is that a facilitative response is achieved by means of a “resonance” mechanism in which the brain of the observer is activated specifically by observing (or listening) actions. This idea is based on the existence of mirror neurons, a class of neurons that modulate their activity both during the execution and the observation of the same action. They have been firstly described in the ventral premotor area (F5) in macaque monkeys, but successively they have been described also in other brain areas. Moreover, areas with mirror properties were later reported in humans through studies involving functional magnetic resonance imaging (fMRI).
Recently, this special class of neurons have been described in rats: neurons responding both to the execution and observation of a grasping action have been found in the motor cortex (M1) while emotional mirror neuros, responding to self- or other pain experience, have been reported in the anterior cingulate cortex (ACC). ACC in human and non-human primates contributes to the ability to recognize intentions and emotions in others, since its recruitment is modulated by positive or negative outcomes of nearby conspecifics. In humans, ACC is recruited both during the direct experience and witnessing of pain in others, but it is also important in cognitive processes including decision making, attention and motivation, all processes necessary for social cognition. Moreover, recent studies pointed out the role of ACC in the development of the psychopathological features of autism spectrum disorder (ASD) contributing to a rigid and stereotyped behaviour. All these evidences propose the cingulate cortex as an information hub for the social network, albeit the underlying neurobiological basis involved in signalling of social paradigms are still poorly understood.
Mouse models offer many advantages in preclinical research, especially because their genome is similar to the human genome (99%) and easy to manipulate, allowing a great variety of molecular tools, making mouse to be one of the best animal models to study both health and disease. Moreover, mice are also suitable models to study social behaviour: they present a wide repertoire of social behaviours that range from parenting and communal nesting their pups, juvenile play to sexual and aggressive behaviours as adults.
Considering these aspects, in the present study, we used a mouse model to investigate the neurophysiological bases of social facilitation. As a first step, we settled a novel social facilitation test for mice, and we found that the observation of spontaneous licking made by a highly motivated demonstrator strongly facilitated the drinking behaviour in a head-fixed observer. This finding, in an evolutionary perspective, suggests the existence, even in mice, of a system involved in the understanding of others’ behaviours with an important social meaning. As a second step, we investigated the modulation induced by licking observation on two additional neurophysiological readouts: (i) the measure of pupil diameter and (ii) single units’ activity. For this purpose, we settled a different foraging task, teaching animals to intersperse licking bouts with waiting periods, better suitable for the measure of both pupillometry and electrophysiological recordings in subjects observing the task. Demonstrator and observer mice learned to reach the spout to lick the reward, only after few seconds of stay in a waiting zone. After the training phase, observer animals were implanted with a microwires chronic array in their ACC. Our results showed an increase in pupil diameter both during self-licking and licking observation, suggesting an increase of the arousal and attentiveness in these conditions. Moreover, single unit activity recorded from ACC neurons showed a modulation of firing rate both during licking execution and observation. In particular, among 132 recorded neurons, the majority of neurons (n = 88) responded to self-licking, about 18,2% of which were significantly active also during observation of a licking behaviour, but not in control conditions. Moreover, we were also able to distinguish between glutamatergic pyramidal (PNs, 77,3%) and inhibitory fast-spiking (FSNs, 22,7%) neurons. We found no differences in terms of fraction of motor or visuomotor neurons, between FSNs and PNs population. By the way, FSNs and PNs retained the same electrophysiological properties of the same class of neurons recorded in the premotor and motor cortices: FSNs showed a great intensity of activation respect to PNs, and a trend suggested that they also retained an earlier onset and a greater duration of activation. According to literature, both FSNs and PNs showed an earlier onset and peak-time as well as greater duration of the response in observation than in execution.
Given these preliminary results, we can speculate that the observation of a drinking conspecific, increases attention and motivation in the observer, increasing firing rate of (both pyramidal and fast spiking) neurons in the ACC during action observation. Those neurons, active also during licking execution, could represent the neural correlates underlying action understanding and social facilitation, making a trait d’union between observation and reinforced execution. Our results can be considered the first clear-cut evidence of the presence of mirror neurons in mice, potentially involved in all behaviours with an essential social meaning.