To investigate the effect of incidental affect on trust-taking, we used a hybrid fMRI design, in which aversive affect was manipulated in a blocked fashion while social (trust) and NS (control) tasks were presented in an event-related fashion. Specifically, we varied aversive affect by creating an expectancy of weak or strong unpredictable electrical stimulation (the duration for both was 20 ms) that could occur at any time for the duration of an entire block. This expectancy was created by means of a block cue presented at the beginning of each block that informed participants about the game type (trust or control game) and the intensity of stimulation (weak or strong) for the current block (Fig. 1A). Stimulation intensity was communicated to participants in three ways: (i) via a verbal cue embedded in the 750-ms block cue [“strong” for treatment (“threat” condition) and “weak” for control (“no-threat” condition)]; (ii) via a predictable tactile reminder cue presented 700 ms after visual cue onset for a duration of 20 ms, which reflected the exact stimulation intensity of the current block; and (iii) via a specific background color that was consistently associated with either threat or no-threat blocks for each participant (color was counterbalanced across participants) and remained constant for the duration of a block. The number and time points of electrical stimulation events throughout the blocks were determined to be completely unpredictable to participants to augment the efficacy of the threat-of-shock treatment. For this purpose, the number of stimulation events was determined for each block by random draw from a γ distribution (shape parameter, 1; scale parameter, 1). Participants therefore experienced exactly one predictable reminder and, on average, one additional unpredictable electrical stimulation per block. The exact timing of these stimulation events was then determined at random time points between the offset of the cue display and onset of the resting screen drawing from a uniform distribution, with the constraint that at least 0.2 s separated successive electrical shocks. Timing and order of stimuli were randomized for each participant to maximize identification of the effects of aversive affect on the neural correlates of trust decisions using in-house software programmed in MATLAB.

Each block commenced with the set of cues described above that indicated the type of decision to be made (NS control or trust) and the level of stimulation (weak and strong) to be expected by participants for the rest of the block. After a brief and jittered interstimulus interval of 3 to 9 s, the first of three trials within a given block was displayed. In both the trust and the control game, participants were presented with a multiple-choice scenario, in which one of five amounts between 0 and 24 CHF could be transferred to player B or invested in an ambiguous lottery. In both games, participants faced an ambiguous back-transfer likelihood: In the trust game, participants were not informed about the probability of a beneficial back transfer and needed to infer this themselves for each interaction with a trustor. This was performed to avoid biasing participants’ decisions and to ensure that participants did not simply focus on explicit repayment probabilities, thereby maintaining the social nature of interactions in the trust game. Ambiguity was also present in the NS control game, which provided only a probability range within which a beneficial back transfer could occur. To encourage investments into the lottery, the probability range used in the control game fell between 40 and 60%. This range was based on previous research that systematically varied the amount of ambiguity, demonstrating significant decreases in investments with increasing levels of ambiguity (54). It was matched to the probability of receiving a beneficial back transfer in the trust game, as it included the likelihood of receiving a back-transfer amount equal to or larger than the investment in the trust game (0.46 in our prerecorded trust game data), as well as participants’ expectations about encountering a trustworthy trustee as identified by previous research (2). Participants always had the option to either invest all (24 CHF) or none (0 CHF) of their endowment. Moreover, each trial presented a novel choice scenario by (i) varying the intermediate options among the low (4, 6, or 8 CHF), medium (10, 12, or 14 CHF), and high (16, 18, or 20 CHF) categories of intermediate transfer amounts; (ii) varying the location of each choice option; and (iii) varying the location of the originally highlighted choice option. This variability was introduced to ensure that participants paid attention to all choice options on every trial and to avoid excessive use of heuristics. Intermediate amounts, location of choice options, and location of the initially highlighted choice option were fully counterbalanced across conditions. Participants selected their preferred option by moving a yellow dot that highlighted the currently selected choice option up and down by pressing two dedicated buttons on a standard MR-compatible four-button response box and confirming their choice by pressing a third button. At this point, the selected choice option was highlighted in red for the remaining duration of a trial. After a jittered intertrial interval (3 to 9 s), a new trial began. Please note that, to control for wealth effects, participants in our experiment did not receive trial-by-trial feedback about the financial outcome of their choices in both the trust and the NS control game. Using one-shot games with no feedback, we precluded learning- and outcome- related signals commonly observed in valuation regions [e.g., (55, 56)]. Participants completed 28 blocks (seven blocks per condition with an average length of 38.75 s) with three trials each in two runs. Thereafter, generic feedback was provided about the number of times player B and the lottery returned more or less than the participant’s investment, and participants completed an additional 28 blocks (data not reported here).

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