Abstract:

Judgments about whether an action is morally right or wrong typically depend on our capacity to infer the actor’s beliefs and the outcomes of the action. Prior neuroimaging studies have found that mental state (e.g., beliefs, intentions) attributions for moral judgment involve a complex neural network including the temporoparietal junction (TPJ). However, neuroimaging studies cannot demonstrate a direct causal relationship between the activity of this brain region and its role in mental state attribution for moral judgment. In the current study, we aimed to provide evidence of a direct link between the neural and behavioral results through the application of transcranial direct current stimulation (tDCS) over the bilateral TPJ of our participants. In the experiment 1, each participant was required to complete two similar tasks of moral judgment before and after receiving tDCS. We studied whether tDCS over the right TPJ and left TPJ altered participants’ mental state attribution for moral judgment. The results indicated that improving the activity of the right TPJ and restraining the activity of the left TPJ decreased the role of actor’s beliefs in moral judgments and led to an increase in the dependence of the participants’ moral judgments on the action’s consequences. In the experiment 2, the participants were randomly assigned to one of three stimulation treatments (right anodal/left cathodal tDCS, left anodal/right cathodal tDCS, or sham stimulation). We found that enhancing the excitability of the left TPJ as well as inhibiting the right TPJ using tDCS could strengthen the role of beliefs and intentions in moral judgment, extending the function of the left TPJ in moral judgment. The effect of enhancing the role of beliefs in moral judgment highly depended on contextual settings, demonstrating that the degree of the right TPJ and left TPJ's role in the integration of beliefs in moral judgment could be various across different scenario types. These results were robust in the case of scenarios involving business relationships. We also found that moral judgment measured by the condemnation ratings of participants were unaffected in all four moral conditions when applying anodal stimulation over the right TPJ and cathodal stimulation over the left TPJ, which is consistent with previous tDCS findings that enhancing the excitability of the right TPJ does not affect the attribution of mental states in judgment tasks. We also found that the participants exhibited reduced reaction times both in the cases of intentional harms and attempted harms after receiving right cathodal/left anodal tDCS over the TPJ. These findings inform and extend the current neural models of moral judgment and moral development typically in developing people and in individuals with neurodevelopmental disorders such as autism.

Key words: moral judgment, intention, consequence, right and left temporo-parietal junction, transcranial direct current stimulation