Abstract—A major limitation of the approaches used in most of the studies performed so far for the characterization of the brain responses during social interaction is that only one of the participating brains is measured each time. The “interaction” between cooperating, competing or communicating brains is thus not measured directly, but inferred by independent observations aggregated by cognitive models and assumptions that link behavior and neural activation. In this paper, we use the simultaneous neuroelectric recording of several subjects engaged in cooperative games (EEG hyperscanning). This EEG hyperscanning allow us to observe and model directly the neural signature of human interactions in order to understand the cerebral processes generating and generated by social cooperation or competition. We used a paradigm called Prisoner’s dilemma derived from the game theory. Results collected in a population of 22 subjects suggested that the most consistently activated structure in social interaction paradigms is the medial prefrontal cortex, which is found to be active in all the conflict situations analyzed. The role of the anterior cingulated cortex (ACC) assumes a main character being a discriminant factor for the “defect” attitude of the entire population examined. This observation is compatible with the role that the Theory of Mind assigns to the ACC. I. INTRODUCTION In the last ten years, the field of neuroscience, with the powerful brain scanning devices like functional Magnetic Resonance Imaging (fMRI) or high resolution EEG (HREEG) has been able to provide important insights on the neural basis for memory, decision-making and other cognitive functions at the basis of social interaction. The standard experimental paradigm consists in measuring the brain activity in individual subjects (using fMRI, EEG or other imaging methods) during the performance of an identical sensory, cognitive or motor task. Similar experimental paradigms have been applied for studying the Manuscript submitted April 16, 2007. This work was supported in part by the Minister for Foreign Affairs, Division for the Scientific and Technologic Development, in the framework of a bilateral project between Italy and China (Tsinghua University) and by the European Union, through the MAIA project, the European IST Programme FET Project FP6-003758. Babiloni F., Astolfi L., Colosimo A., Department of Human Physiology and Pharmacology, University “Sapienza”, Rome, Italy. E-mail: Fabio.Babiloni@uniroma1.it . Cincotti F., Mattia D., Tocci A., IRCCS "Fondazione Santa Lucia", Rome, Italy. Marciani MG., Department of Neuroscience, University “Tor Vergata”, Rome, Italy. Salinari S., Department of Computer and Systems Science, University “Sapienza”, Rome, Italy. Gao S., Department of Biomedical Engineering, Tsinghua University, Beijng, China. De Vico Fallani F., Interdepartmental Research Centre for Models and Information Analysis in Biomedical Systems, University “Sapienza”, Rome, Italy. brain activity during social interactions. In the investigation of the neural basis of social interaction, the use of game theory has been proved useful. In fact, game theory allows a formal definition of social situation in which the players may profit or loose by cooperating or competing [1]. Researchers have begun to investigate what happens in the brain of subjects when they are involved in games like the prisoner’s dilemma in which you have to decide whether to cooperate with an opponent, or defect [2]. The aim is to understand the modification of brains activity during interactions, namely if the cortical activity located in the anterior cingulated areas (ACC) and in other cortical sites hypothesized in the current Theory of Mind (TOM) held during competition or defection. Once estimated the functional connectivity from the high resolution EEG recordings in different Regions Of Interest (ROIs) for each subject, the issue of the extraction of significant descriptors of such connectivity networks was faced with the use of the graph theory. In particular, we evaluated the topological- weighted properties of the cortical networks by calculating mathematical features including, global and local efficiency and strength measures. An issue in such analysis is that these different indexes returned precious information regarding the different organization between the cooperative versus “non-cooperative” subjects populations. II. METHODS A. Experimental design The Prisoner’s Dilemma is a game with two players and two possible moves: cooperate or defect. If the two players cooperate, they both have small wins (COP). If one player cooperates and the other defects, the cooperator has a big loss and the defector has a big win. If both players defect, they both have small losses (DFT). Tit for Tat (TFT) cooperates in the previous round and imitates its opponent’s defection in the next move. The aim of the game is to reach the highest score. Subjects interacted one in front to each other, seated beside. A screen displaying the information necessary to the execution of the games and generating the timing of the tasks will be disposed in front of all the subjects. They generated their choice during the task performance (decision to cooperate or to defect) through a keyboard and the computer will also record each subject’s response and generated a mark on the subject’s EEG traces for successive off-line analysis. The general timeline of each trial is as follow: the trial starts with the presentation of the Cortical Activity and Connectivity of Human Brain during the Prisoner’s Dilemma: an EEG Hyperscanning Study Babiloni F., Astolfi L., Cincotti F., Mattia D., Tocci A., Tarantino A., Marciani MG., Salinari S., Gao S., Colosimo A. and De Vico Fallani F. Proceedings of the 29th Annual International Conference of the IEEE EMBS Cité Internationale, Lyon, France August 23-26, 2007. SaB03.5 1-4244-0788-5/07/$20.00 ©2007 IEEE 4953