“To accept or to reject? It depends on who proposes it”. An fMRI study on the Ultimatum Game.

Introduction: In the Ultimatum Game (UG), a proposer decides in which proportion to split a fixed amount of money (e.g., 10 €) with a responder. Both get their share only if the responder accepts the offer. Despite what would be predicted by expected utility models, typically some offers (most frequently the lowest ones) are rejected, being considered unfair [3]. Brain imaging and TMS studies [2, 4-5, 7] have investigated the brain circuits involved in different aspects of the UG. In the present study, we tested the hypothesis that the identity of the proposers, specifically, their economic status, i) affects acceptance rates; ii) modulates brain activity of the responders in the UG protocol. Methods: Twenty-one healthy right-handed volunteers (all females; mean age 22.8 ± 3.1) took part in this study. During the fMRI scan, each volunteer was asked to play 54 trials (split in 3 runs) of UG. All human proposers were presented as females and could be either A) old-age retirees living on a small pension, or B) well-to-do businesswomen/professionals. Their identity was introduced by a brief description including first name, age and social status (e.g., Maria, 84, with minimum pension). Offers by a computer were the control condition. The amount of money to share was € 10 in each trial, with three types of offers. that we defined as: Unfair (1 or 2 €), Fair (4 or 5 €), Mid-value (3 €). On each trial, participants saw the description of the proposer (8 s), then they were presented with an offer (8 s). Both proposer and offer type were presented in random order. Finally, participants accepted or rejected the offer by pressing a button. Functional imaging was performed on a 3T Philips Achieva scanner. Thirty axial slices were acquired (TR=2000 ms; FOV=240x240 mm; in-plane matrix=80x80; voxel size=3.0x3.0x4.0 mm). Data analysis was carried out using a General Linear Model (GLM) as implemented in SPM5; significance level was set at α < 0.05 corrected for multiple comparisons, as assessed by AlphaSim (http://afni.nimh.nih.gov/afni/doc/manual/AlphaSim). Results: Behavioral data: Acceptance rates were significantly higher for proposer A than for the other proposers, for both Unfair and Mid-value offers (p<0.001), but not for Fair offers. fMRI data: Unfair offers, compared to Fair offers, elicited signal increases in anterior mid-cingulate cortex (aMCC) and right prefrontal cortex (PFC); smaller (subthreshold) clusters were present also in left PFC and, bilaterally, in anterior insula (Fig.1). The contrast Fair vs. Unfair offers did not elicit suprathreshold activity. When we compared proposer B vs A in the trials in which offers were subsequently accepted, we found a significant signal increase in the right insula (Fig. 2). When offers were subsequently rejected, a comparison of proposer A vs B revealed an active cluster in precuneus/posterior cingulate cortex (PCC) (Fig. 3). The opposite contrasts did not reveal any significant cluster. Conclusions: We show here that the economic status of the proposers selectively affects i) acceptance rates, and ii) brain activity; namely, in case of accepted offers, proposer B caused a selective activation in posterior insula. This area is known to have complex functions, among which a role in pain perception and modulation [6] and in attention to negative emotions [8]. This activation might suggest that accepting the offers from proposer B bears some similarity with an unpleasant experience. In case of subsequently rejected offers, proposer A caused a selective activation in precuneus/PCC. This is a very complex region, part of the so-called "default mode network", which is active during the conscious resting state, and inhibited during non-self related tasks [1]. 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