Iterated Prisoner’s Dilemma in neuroscience

of their reputation and can hence affect the subsequent players’ behaviors in the last iterations of the game, contrary to standard game theory’s predictions. Thefore, in a repeated game both the signals related to reward magnitude and to re-sponse timing revealed by the fMRI come into play during social decision-mak-ing.

to the reward system⁵² which is usually related to individuals’ will to refuse im-mediate reward in order to get higher long-term reward.

The matrix below represents the amount of money which players could be awarded based on their social interactions. Players scanned through fMRI were identified by Player 2 , while non-scanned players by Player 1.

Cooperate Defect

Cooperate $2(2) $3(0)

Defect $0(3) $1(1)

Figure 3.2 Payoff matrix Prisoner’s Dilemma

The main aim of the first experiment was to distinguish the neural areas of acti-vation in case of cooperation and non-cooperation behaviors between players; in case of cooperation, the different areas activated in a social and non-social con-text were distinguished. The aim of the second experiment was instead to put into evidence the different responses given by the player when playing with a human or a computer partner. As in the neural experiments previously analyzed, the players were keener to cooperate with a human partner than a computer, even

52 More details about the neural reward system will be provided in the subchapter 3.5.

Player 1

Player 2

if the probabilities of this last type of partner were derived from the strategies played in the first experiment by the human unconstrained partner. In both ex-periments mutual cooperation was played the highest number of times in case of a human partner. Moreover, it was observed that when cooperation was reached by players, the probability that even in the following iterations cooperation was played by both players was higher with respect to the probability of defection by one or both players.

Regarding the correspondent neural activity in player’s brain, experimentalists analyzed both BOLD reaction in the player’s brain to the outcome observed and during his decision-making. In fact, each stage game of the Iterated game con-sisted of 12 seconds in which players could decide whether to cooperate or defect followed by 9 seconds in which the outcome obtain was showed to the players.

As regard the BOLD reaction to the outcome, experimenters observed in player 2 that BOLD response to cooperative outcome was higher than BOLD response to the other outcomes. The major brain areas of activation for the (Cooperate, Cooperate) outcome were identified in the anteroventral striatum, the anterior cingulate cortex and the orbitofrontal cortex.All these brain areas are related to the reward processing, analyzed thereafter; hence, the activation of these areas was coherent with the player strategy to cooperate in order to obtain a long-term advantage to the detriment of the immediate gain that he would benefit by choos-ing to defect, betraychoos-ing the trust of the partner. The difference between the ante-roventral striatum and the orbitofrontal cortex is that the first brain structure is

associated only to the cooperation with a human counterpart while the second can be associated to both a human or a computer counterpart. As regard the an-teroventral striatum, since the ventral striatum is usually associated to reward, it has been observed in neuroscience experiments that the increase of this brain area activity is associated to the reward of the economic agent relatively to the reward provided to its partner and not to its own reward in absolute terms. To demon-strate that the benefit derived from choosing the cooperative outcome was prin-cipally due to the willingness to reciprocate the altruism expressed by the partner in the previous round and not simply for receiving a payoff of 2$, researchers verified what happened in the first experiment in case a sum of 2$ was provided to the same player in a non-social context that did not presuppose the establish-ment of a relationship of cooperation based on reciprocal altruism. In this case, the anteroventral striatum, the anterior cingulate cortex and the orbitofrontal cor-tex did not present the activation observed in case of mutual cooperation, proving the fact that the benefit received by player in a social context is not the same to receive an equal monetary amount in a non-social perspective.

As regard the BOLD reaction during the decision of a player to cooperate after having observed the other partner cooperation move, experimentalists observed the activation of the rostral anterior cingulate cortex. As already observed in the experiment about the ultimatum game, the anterior cingulate cortex is associated to the emotional-cognitive conflict and hence, it can be attributed once again to the conflict between receiving an immediate reward or delay a higher reward.

Moreover, the higher the activation of the ventromedial prefrontal frontal cortex

during players’ decision to resist the immediate temptation of defecting in order to get a higher long-term reward was coherent with recent neural studies accord-ing to which ventromedial prefrontal frontal cortex is associated to long term rewards and punishment. Another brain structure interested in this process was the right post-central gyrus, located on the primary somatosensory cortex and involved in reciprocal cooperation. Overall, the activation of all these brain re-gions is associated to an increase in cooperation based on reciprocal altruism by players.