The Moral Brain Hypothesis: Neuro-Psychological Foundations of Moral Values and Norms
July 19, 2018 | 14:00 - 15:30 | BLU-003
Moral codes of conducts are a hallmark of human civilization: moral norms have been developed by every known human society to regulate social interactions. The emergence and maintenance of these norms has been proposed to reflect the evolution in the human brain of neurocognitive processes beneficial for cohabiting in large social groups, a perspective fully captured by the Social Brain Hypothesis. To date, however, there is still little knowledge of what these processes are and how they interact with other-purposes processes, such as those involved in (non-moral) value-based decisions. In the present thesis I propose to close this gap by asking: are there neurocognitive processes that specifically represent moral values (rather than universally moral and material values)? How and where are moral and material value representations integrated to inform decisions? To address these questions, I conducted three studies where I combined behavioral measures of moral choice and preferences with neuroimaging (i.e., functional magnetic resonance imaging, fMRI) and brain-stimulation techniques (i.e., transcranial direct current stimulation, tDCS) to investigate the neural mechanisms involved in a) estimating the right course of action in morally ambiguous situations (i.e., where morally right and wrong behaviors depend on the agent’s moral preferences), and b) implementing behavioral control functions that instantiate the behaviors prescribed by a moral norm shared by a group. In a first fMRI study, I developed a novel experimental paradigm where I concurrently elicited subjective values in different choice contexts, a moral context and a financial context, in order to disclose a) if moral subjective values could be reliably estimated and used to explain individual moral preferences, b) if and where these subjective values are represented in the brain, and c) if subjective moral values are estimated by domain-general valuation processes or if these rely (to some extent) on moral-specific valuation processes. My results revealed that moral subjective values could be solidly estimated adopting traditional computational models of choice processes. However, I found no evidence for a common neuro-computational process that estimated both moral and financial subjective values. Instead, the evidence suggested that computations of moral subjective values rely on domain-specific neural functions performed predominantly in the right temporo-parietal junction (rTPJ). The second study used tDCS to alter the excitability of the right dorso-lateral prefrontal cortex (dlPFC) during acquisition of several behavioral measures, which allowed me to directly test a causal role for this brain area in arbitrating between self-interested motives and moral motives – in this specific case honesty. The data obtained in this context revealed that increasing excitability of the right dlPFC lead to a significant increase of honest behavior. No effect of tDCS was observed in a variety of other control tasks, suggesting a specific involvement of this brain region in regulating internal conflicts between selfish and moral motives. Finally, the third study of the present thesis combined online tDCS and fMRI to examine at the neural-network level the causal brain activity responsible for implementing fairness, the moral norm that regulates how resources should be shared among members of a society. Intriguingly this approach allows me investigate how one brain area – the right DLPFC – may change how it influences whole networks in a context-dependent manner. Replicating a previous own study, behaviorally I showed that increasing/decreasing the excitability of lateral prefrontal cortex (LPFC) lead to an increase/decrease of compliance with the fairness norm in the presence of a credible punishment threat and to decreased/increased compliance in the absence of such punishment threat. These behavioral effects were mirrored by changes at the neural level. My fMRI results revealed that anodal stimulation of the right LPFC resulted in increased amygdala activity and increased connectivity between the stimulated area and the orbitofrontal cortex (OFC). In contrast, cathodal tDCS to the right LPFC induced changes in functional activity in the anterior cingulate cortex (ACC), left LPFC, and bilateral inferior parietal lobules (IPL). Jointly, these findings support the hypothesis that the LPFC drives fairness-norm compliance by regulating the salience of inputs from neural areas processing social emotions – responsive to the threat of sanctions for norm violations – and cognition – strategically arbitrating when it is potentially safe to violate the fairness norm. In my thesis I used morality, one of the most important tools regulating social interactions, to add evidence in support of the Social Brain Hypothesis. This hypothesis proposes that the cortical enlargement of the human brain mainly reflected the evolution of neuro-cognitive processes beneficial for co-habiting in large social groups. Taken together, my findings revealed some of these social-specific processes: study one identified neural activity specifically involved in the estimation of subjective moral values, compared to subjective financial material values; study two revealed evidence demonstrating a causal role for the right LPFC in arbitrating conflicts between competing moral and material values to determine moral behavior; finally, study three revealed that the LPFC controls moral behavior by selectively modulating the behavioral relevance of social emotions and cognition depending on the context in which moral decisions are taken.