Differences in Social Decision-Making between Proposers and Responders during the Ultimatum Game: An EEG Study

Abstract

The Ultimatum Game (UG) is a typical paradigm to investigate social decision-making. Although the behavior of humans in this task is already well established, the underlying brain processes remain poorly understood. Previous investigations using event-related potentials (ERPs) revealed three major components related to cognitive processes in participants engaged in the responder condition, the early ERP component P2, the feedback-related negativity (FRN) and a late positive wave (late positive component, LPC). However, the comparison of the ERP waveforms between the responder and proposer conditions has never been studied. Therefore, to investigate condition-related electrophysiological changes, we applied the UG paradigm and compared parameters of the P2, LPC and FRN components in twenty healthy participants. For the responder condition, we found a significantly decreased amplitude and delayed latency for the P2 component, whereas the mean amplitudes of the LPC and FRN increased compared to the proposer condition. Additionally, the proposer condition elicited an early component consisting of a negative deflection around 190 ms, in the upward slope of the P2, probably as a result of early conflict-related processing. Using independent component analysis (ICA), we extracted one functional component time-locked to this deflection, and with source reconstruction (LAURA) we found the anterior cingulate cortex (ACC) as one of the underlying sources. Overall, our findings indicate that intensity and time-course of neuronal systems engaged in the decision-making processes diverge between both UG conditions, suggesting differential cognitive processes. Understanding the electrophysiological bases of decision-making and social interactions in controls could be useful to further detect which steps are impaired in psychiatric patients in their ability to attribute mental states (such as beliefs, intents, or desires) to oneself and others. This ability is called mentalizing (also known as theory of mind).

Keywords: brain source reconstruction; decision-making; event-related potentials; independent component analysis; social cognition; ultimatum game.

Figure 1

Illustration of the Ultimatum Game. Tasks differed in response requirement. Each trial began with the instruction to press the spacebar (S). As soon as participants did, a central fixation cross appeared in the middle of the monitor (F). After an interval of 2 s, participants saw a message (O) indicating to either make an offer (A—Proposer) or accept or reject an offer (B—Responder), depending on the condition. This time point was considered as our t = 0 for the ERP analyses, and is labeled with a horizontal red line. The response to the offer (R) was displayed simultaneously when participants pressed the button indicating their decision. It presented as a smiling or frowning face in the proposer condition, whereas the smiley was neutral in the responder condition. This figure has been adapted from Horat et al. (2017).

Figure 2

Offers and acceptance rates. (A) Number of offers made by the participants (dark gray—participants in proposer condition) and by the computer (light gray—participants in responder condition). The bars represent the amount of times every offer (1–9 CHF) was made, with standard errors (±SE). The distribution of the values (1–9 CHF) offered by the computer player was similar to the distribution of offers by the human proposer (t = −2.08; p = 0.06). (B) Acceptance rates of the participants (dark gray—participants in responder condition) and the computer (light gray—participants in proposer condition). The bars represent the percentage of accepted offers for each offered amount with standard errors (±SE). The distribution of the acceptance rates was not significantly different for the two players in both conditions (proposer: Friedman = 23.096; p = 0.151; responder: Friedman = 8.903, p = 0.446).

Figure 3

(A) Electrode positions. Placement of the 128 electrodes on the head cap. The electrodes relevant for this study are indicated, namely FPz, AFz, Fz, FCz, Cz, CPz and Pz from top to bottom (anterior to central head). (B) Grand average waveforms of five single electrodes. Grand average waveforms for proposer (solid black line) and responder (dashed gray line) during the Ultimatum Game (UG) for all outcomes (both acceptance and refusal of the offer) at the midline electrodes FPz, AFz, Fz, FCz and Cz. (C) Grand average waveform. Grand average waveform for the electrode average of five electrodes (FPz, AFz, Fz, FCz and Cz) of the proposer (solid black line) and responder (dashed gray line) conditions. The labels show the main components N2, P2, FRN and LPC. The band around the LPC component represents the complete length of the component, whereas the shaded area stands for the middle 50% that were taken for the analysis. In the responder condition, note the absence of a N2 component, the delayed latency and lowered amplitude for the P2, and the higher mean amplitude for the FRN and LPC. (D) Source reconstruction. Source reconstruction of the time-window of the N2 component (180–200 ms). A significant higher activation (p < 0.05) around the anterior cingulate cortex (ACC) was found for the proposer condition.

Figure 4

Independent Component Analysis (ICA). We focused on the IC predominately activated for the period of interest for the N2 component, around 180–200 ms after stimulus onset. (A) Represents the ERP waveform and the most important IC of the proposer condition, (B) of the responder condition, respectively. Note the lower variance of the IC for the responder than the proposer condition (2.2% and 5%, respectively), and the inversed topography of the brain map.