Design
A self-controlled neuroimaging study.
Time and setting
Experiments were performed in the National Key Laboratory of Cognitive Neuroscience and Learning at Beijing Normal University, China in September 2009.
Subjects
A total of 23 healthy right-handed (Edinburgh Handedness Inventory[22]) Chinese undergraduate and graduate students from Beijing Normal University (8 males, 15 females; 18–27 years of age; 21.7 ± 2.1 years) volunteered to participate in this experiment.
Inclusion criteria
Right-handed subjects were included. None of the participants had a history of neurological disorders, surgery or serious physical illness. The participants provided written informed consent after a detailed description of the study.
Exclusion criteria
Left-handed subjects, subjects with neurological disorders, surgery or serious physical illness were excluded. Methods
Functional brain image processing and functional magnetic resonance imaging acquisition during decision-making in the ultimatum and dictator games
Scanning was performed using a 3.0 T functional magnetic resonance imaging scanner at the National Key Laboratory of Cognitive Neuroscience and Learning at Beijing Normal University, China. Functional images were acquired using T2*-weighted echo-planar imaging sequence (repetition time = 2 seconds, echo time = 30 ms, flip angle = 90°, field of view = 200 mm × 200 mm, matrix = 64 × 64, number of slices for a whole brain = 30, slice thickness = 4 mm, gap = 0 mm, resolution = 3.1 × 3.1 × 4.0 mm3). T1-weighted anatomy images were acquired using SPGR sequence (repetition time = 2 530 ms, echo time = 3.39 ms, flip angle = 7°, matrix = 256 × 256, number of slices for the whole brain = 128, slice thickness = 1.33 mm, resolution = 1.00 × 1.00 × 1.33 mm3).
Trial procedure of functional magnetic resonance imaging stimulus design
The trial began with the presentation of a cross (‘+’) for 2, 4 or 6 seconds (selected randomly) on a screen inside the functional magnetic resonance imaging scanner. Two alternative proposals between unfair and fair offers (Figure 3) were presented for 6 seconds.
During this time, subjects were required to choose one of the two proposals by pressing the left or right button. There were two types of proposals in each trial: ultimatum game proposals and dictator game proposals. The type of proposal was indicated by the sentence at the top of the two alternative proposals. If the sentence at the top was phrased ‘Xiaosun must accept your offer’, it was a dictator game proposal. A blank jitter was then presented for 2, 4 or 6 seconds, selected randomly. At the end of each trial, the subject’s ‘partner’ (actually a computer), responded by accepting or rejecting the offers, and feedback reporting their response was presented for 4 seconds. In ultimatum game trials, a rejection by the responder meant that both proposers and responders received ¥0, and accepting meant that the reward was allocated as proposed. In dictator game trials, the responder could only accept the proposer’s offers with no opportunity to reject them.
Functional magnetic resonance imaging experimental procedure
The experiment was divided into three parts: an instruction phase, a scanning phase during which participants performed the task, and a post-scan debriefing phase.
In the instruction phase, the participants were familiarized with the ultimatum and dictator game tasks, and performed a number of practice trials on a laptop computer. Participants were told that in the ultimatum game/dictator game, they would play one game each with several different players. They were told that their offers with each player would not be revealed to the other players and, therefore, would not affect subsequent players’ responses.
In the scanning phase, to better compare unfair offers with fair offers, participants were asked to make a forced choice between the unfair proposal and the fair proposal (¥5: ¥5) (Table 3).
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Participants completed 48 trials (24 ultimatum game trials, 24 dictator game trials) in two runs. Each run lasted approximately 8 minutes. Trials were presented in a pseudo-random order. The timeline for a single trial of the ultimatum game or dictator game is presented in Figure 1. In ultimatum game trials, if the participant chose the unfair proposal (¥8: ¥2) they received “reject” responses at a ratio of 7/12 (randomly selected). In the ultimatum game, the ratios of response rejection were 4/12, and 0 for proposals of ¥7: ¥3 and ¥5: ¥5, respectively. The ratio for rejecting and accepting responses was based on that used in a previous study[23]. To minimize the potential effects of habit-related bias, the probability of fair and unfair proposals appearing on the left and right sides of the screen was counterbalanced. In the post-scan debriefing phase, participants were debriefed and given RMB¥50 (approximately USD$7) for participation.
Functional magnetic resonance imaging data processing
The imaging data were preprocessed and analyzed using SPM5 (Wellcome Department of Cognitive Neurology, London, United Kingdom). Image preprocessing included: slice scan time correction, head motion correction, spatial normalization to Montreal Neurological Institute space, and smoothing with an 8-mm full-width-at-half-maximum Gaussian kernel. Events were modeled with a general linear model time-locked to the onset of the proposals.
Statistical analysis
Paired-sample t-tests were performed in a voxel-by-voxel manner with SPM5 (Wellcome Department of Cognitive Neurology) between the two groups, based on the difference between averaged brain activation signals measured between the proposal of fair offers in the ultimatum game and the dictator game. Findings (voxels in the whole-brain showing significant differences) were considered statistically significant at a height threshold of P < 0.004, and an extension threshold of 10 voxels.
