利他是指个体损耗自身资源使他人获益的行为。利他行为塑造了人类进化的历史,广泛渗透于社会和政治生活。利他行为被高度重视,且容易受到不平等情境影响。目前对不平等情境下利他的神经机制知之甚少。为了阐明左侧背外侧前额叶皮层在不平等情境下利他中的作用,对左侧背外侧前额叶皮层施加经颅直流电刺激改变脑区活动,探究其对优势和劣势不平等情境下利他行为影响。具体而言,对左侧背外侧前额叶皮层施加1.5 mA（毫安）阳极刺激,然后让被试完成独裁者博弈任务以测量利他行为。实验结果显示,阳极tDCS刺激左侧背外侧前额叶皮层显著影响被试在优势不平等情境下的利他倾向,降低被试对自身损耗的关注;但是对劣势不平等情境下的利他倾向无显著影响。这一结果表明,左侧背外侧前额叶皮层在优势不平等情境下利他中发挥重要作用,促使个体超越个人利益,提高利他倾向。
Abstract
Altruism, which is defined as behavior in which individuals deplete their own resources to benefit others, has played a pivotal role in human evolution and is deeply embedded in social and political life. Although highly valued, altruistic behavior is susceptible to influence by inequity situations. Specifically, individuals are more willing to act altruistically when they possess a higher initial endowment than their partners (an advantageous inequity situation) compared to when they have a lower initial endowment (a disadvantageous inequity situation). Neuroimaging studies suggest that advantageous and disadvantageous inequity engage distinct neurocognitive mechanisms. The left dorsolateral prefrontal cortex (lDLPFC), a brain region critical for executive function and impulse control, is hypothesized to play a role in altruistic decision-making. However, whether the lDLPFC differentially modulates altruism in advantageous versus disadvantageous inequity contexts remains unclear.
To address this question, we employed transcranial direct current stimulation (tDCS) to modulate the activity of lDLPFC and examined its effects on altruistic behavior under both advantageous and disadvantageous inequity conditions. Using a 2 × 2 mixed design, we manipulated stimulation condition (between-subjects factor: anodal vs. sham tDCS) and the inequity condition (within-subjects factor: advantageous vs. disadvantageous). A total of 70 healthy participants were recruited in the experiment (35 female, Mage = 20.79 ± 1.84 years). Participants were randomly assigned to receive either anodal tDCS (1.5 mA applied to the lDLPFC for 20 minutes) or sham tDCS (stimulation terminated after 30 seconds). Following the stimulation, participants completed a modified Dictator Game task to assess altruistic propensities. In this task, participants allocated monetary payoffs between themselves and anonymous partners across multiple trials, with no time constraints imposed.
In advantageous inequity trials, participants' payoffs exceeded those of their partners (e.g., $50 vs. $20), while in disadvantageous inequity trials, participants' payoffs were lower (e.g., $20 vs. $50). Altruistic choices involved participants sacrificing a portion of their own payoff (e.g.,$4) to increase their partner's payoff (e.g.,$13). Trials were presented in separate, counterbalanced blocks. Using mixed-effects logistic regression (lme4 package in R), we analyzed the impact of lDLPFC stimulation on altruistic behavior.
The results demonstrated that anodal tDCS over the lDLPFC significantly enhanced altruistic tendencies in advantageous inequity conditions, reducing participants' focus on personal losses. In advantageous inequity conditions, participants' altruistic behavior decreased as the cost of altruism increased (β = -1.630, p < .001, Bonferroni-corrected) and increased as the benefit of altruism increased (β = .991, p < .001, Bonferroni-corrected). The interaction between tDCS stimulation and the cost of altruism was significant (β = .564, p < .01, Bonferroni-corrected). Anodal tDCS over the left DLPFC attenuated the impact of altruistic costs on altruism in advantageous inequity conditions, indicating that anodal stimulation of the left DLPFC reduced participants' focus on self-interest in such contexts. In contrast, no significant effect was observed in disadvantageous inequity conditions. In disadvantageous inequity conditions, participants' altruistic behavior also decreased as the cost of altruism increased (β = -2.403, p < .001, Bonferroni-corrected) and increased as the benefit of altruism increased (β = .871, p < .001, Bonferroni-corrected). However, the interaction between tDCS stimulation and the cost of altruism was not significant (β = .279, p > .05), nor was the interaction between tDCS stimulation and the benefit of altruism (β = -.218, p > .05). These findings highlight the critical role of the lDLPFC in promoting altruism under advantageous inequity, enabling individuals to transcend self-interest and prioritize the welfare of others.
In summary, this study investigated the impact of tDCS applied to the dorsolateral prefrontal cortex (DLPFC) on altruistic behavior under conditions of inequality. Anodal tDCS stimulation of the left DLPFC was found to reduce participants' sensitivity to altruistic costs in situations of advantageous inequality. However, it did not influence their sensitivity to altruistic costs in disadvantageous inequity condition. These findings suggest that the left DLPFC plays a specific role in promoting altruistic behavior in contexts of advantageous inequality, while its involvement in altruism appears to differ in situation of disadvantageous inequity.
关键词
不平等 /
利他 /
左侧背外侧前额叶皮层 /
经颅直流电刺激
Key words
inequity /
altruism /
left dorsolateral prefrontal cortex /
transcranial direct current stimulation
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基金
*本研究得到广东省哲学社会科学规划基金项目（GD24CYJ58）和广东省基础与应用基础研究基金项目（2024A1515011519）的资助
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