The Necessity of the Human Ventromedial Prefrontal Cortex for Prosocial Motivation

Research Background and Motivation

The ventromedial prefrontal cortex (vmPFC) plays a crucial role in decision-making processes. Functional neuroimaging studies indicate that the vmPFC is key in handling rewards and efforts and is also associated with prosocial behavior. However, the indispensability of the vmPFC in these functions remains unknown. Many lesion studies on the vmPFC are case studies or small group studies involving less than ten patients. Given the typical variability in individuals’ social behavior, effort, and reward processing, a larger sample size is essential for reliable conclusions. Therefore, this study aims to explore the causal role of the vmPFC in prosocial behavior by comparing patients with rare focal vmPFC lesions (n=25), patients with lesions in other areas (n=15), and healthy controls (n=40).

Paper Source

This research paper is co-authored by Patricia L. Lockwood, Jo Cutler, Daniel Drew, Ayat Abdurahman, Deva Sanjeeva Jeyaretna, Matthew A. J. Apps, Masud Husain, and Sanjay G. Manohar, from the University of Birmingham’s Centre for Human Brain Health, Institute for Mental Health, University of Oxford’s Department of Experimental Psychology, University of Cambridge’s Department of Psychology, Oxford University’s Clinical Neurosciences Department, and John Radcliffe Hospital’s Neurology Department. The paper was published in the journal Nature Human Behaviour in 2024.

Detailed Research Process

Research Design

This study employed an effort-based decision-making task to compare the prosocial behaviors of 25 patients with vmPFC lesions with two control groups (other lesion patients and healthy controls). Each participant chose to rest or work for a reward in each trial, which could benefit themselves or an anonymous participant (prosocial condition). Using computational neuroscience methods, the study fitted multiple computational models to precisely quantify the integration of effort and reward in decision-making.

Experimental Steps

1. Participant Selection: A total of 80 participants were included, divided into vmPFC lesion group, other lesion group, and healthy control group. All participants were assessed by a neurologist before the task began.
2. Task Design: Participants performed a physical test with a handheld dynamometer to measure their maximum voluntary contraction (MVC) and chose between resting or working in each trial. The work option required varying levels of effort (30%-70% MVC) to obtain different rewards (2-10 points).
3. Prosocial Condition: Trials were divided into two conditions: self-benefit (reward for the participant) and other-benefit (reward for another anonymous participant). Each participant completed 75 interleaved trials (75 per condition) across three blocks, with a one-minute rest between blocks.
4. Computational Modeling: The study used various models to fit participants’ choices to precisely quantify effort discounting (κ parameter) and decision consistency (β parameter) and compared the models’ fit.

Data Processing and Analysis

A generalized linear mixed-effects model (GLMM) was used to analyze the data, assessing the impact of vmPFC damage on prosocial behavior, effort, and reward processing. Voxel-based lesion-symptom mapping (VLSM) analysis was employed to reveal specific vmPFC sub-regions involved in prosocial behavior, effort, and reward processing.

Research Results

VmPFC Damage Reduces Prosocial Behavior

Compared to the control groups, vmPFC lesion patients exhibited significantly less prosocial behavior when helping others. This finding was validated across multiple behavioral and computational parameters, showing vmPFC lesion patients earned fewer points and displayed lower willingness to work and exerted less force when the rewards benefited others. VLSM analysis further showed that different sub-regions of the vmPFC had different influences on prosocial behavior, with medial vmPFC lesions associated with antisocial behavior, while lateral vmPFC lesions relatively increased prosocial behavior.

Sensitivity to Effort and Reward

VmPFC damage also reduced participants’ sensitivity to effort, but sensitivity to reward was limited to specific sub-regions. Specifically, vmPFC lesion patients had a significantly lower willingness to work under low-effort conditions compared to healthy controls, but the difference was not significant under high-effort conditions. Furthermore, VLSM analysis indicated that regions involved in effort and reward processing overlapped with those for prosocial behavior but also had independent specific areas.

Computational Model Analysis

Computational model analysis revealed that the 2κ2β parabolic model best explained participants’ choices. This model indicated that vmPFC damage increased the discounting of prosocial rewards, meaning vmPFC lesion patients had higher discounting for others’ rewards, while there was no significant change in discounting for self-rewards.

Conclusion and Significance

By combining a large sample size and multiple methods, this study provides evidence for the multi-faceted causal role of the vmPFC in prosocial behavior, effort, and reward processing. The results suggest that different vmPFC sub-regions have varying roles in prosocial behavior—medial vmPFC lesions are associated with antisocial behavior, while lateral vmPFC lesions relatively increase prosocial behavior. These findings are crucial for understanding the fundamental functions of the vmPFC in decision-making and offer new insights for clinical interventions. Future research can further explore the specific functions of different sub-regions in social behavior, effort, and reward processing, as well as their collaboration with other brain areas.

Research Highlights

1. Large Sample Size: Compared to previous case studies and small group studies, this study has a larger sample size, increasing the reliability of the results.
2. Combination of Multiple Methods: Combining behavioral analysis, computational modeling, and VLSM analysis comprehensively revealed the role of the vmPFC in prosocial behavior.
3. Sub-Regional Differences: This study is the first to clarify the different roles of vmPFC sub-regions in prosocial behavior, providing new directions for future research.

This study not only deepens our understanding of the vmPFC’s role in prosocial behavior but also serves as a vital reference for clinical practice, furthering advancements in the field of neuroscience.