Transcranial direct current stimulation of the medial prefrontal cortex modulates the propensity to help in costly helping behavior

doi:10.1016/j.neulet.2018.03.027

Neuroscience Letters

Volume 674, 1 May 2018, Pages 54-59

https://doi.org/10.1016/j.neulet.2018.03.027 Get rights and content

Highlights

•
TDCS was used to demonstrate the causal relationship between the cortical excitability of mPFC and costly helping decision-making.
•
Subjects received anodal stimulation are more prosocial than the subjects received cathodal stimulation when the probability was high.
•
The study offers the first evidence of the cortical excitability of mPFC affecting the decision making in costly helping behavior.

Abstract

Social decision-making engages traditional decision-making processes (e.g. valuation), as well as social cognition processes (e.g. inferring the affective and mental states of another person). Neuroimaging and neuro-stimulation studies have suggested the involvement of the medial prefrontal cortex (mPFC) in a variety of social decision-making tasks. Yet no study has investigated the effect of the cortical excitability of mPFC in the decision-making of costly helping behavior. Here, we used tDCS to demonstrate the causal relationship between the cortical excitability of mPFC and costly helping decision-making. Subjects assigned to the anodal, cathodal and sham groups were required to decide whether they would like to cost their own money to relieve another subject (a confederate actually) from painful electrical shocks with a certain probability of success. Results showed that the subjects receiving anodal stimulation acted more prosaically than the subjects receiving cathodal stimulation. And this effect was only significant when the probability of success was high. We proposed that tDCS induced modulation of the cortical excitability, targeting the mPFC, can affect the prosocial propensity in costly helping behavior, and the possible underlying mechanisms were discussed.

Introduction

Helping behavior refers to voluntary actions intended to help or benefit other individuals or groups, which is a typical prosocial behavior [1]. Based on our daily experience, the offer of help is not always guaranteed, even when help is explicitly requested. People may decide to help or not to help in different circumstances, especially, when helping others is not in the interests of themselves and even be costly for them [2]. Helping behavior, especially costly helping behavior is actually, a social decision-making process [[3], [4], [5]]. Individuals would value the pros and cons to make a decision that affects both of the self and others.

Social decision-making is a complex process which must engage the process of social cognition (e.g. inferring the affective and mental states of another person), as well as the traditional decision-making process (e.g. valuation). In the case of costly helping decision-making, specifically, on the one hand, the decision depends on whether you would empathize or mentalize with the person in need. According to the “empathy-altruism hypothesis”, helping behavior may be initiated when we feel empathy for the person, that is, feeling and understanding what that person is experiencing [6]. On the other hand, the decision also depends on the assessment of the situation, such as the cost and the probability of success of helping [7]. People are unlikely to decide to help if they believe their help is doomed to fail or the cost is beyond their range.

Therefore, decision-making of costly helping would involve both of the traditional decision-making brain network and the social cognition brain network [8,9]. It is noteworthy that one brain region plays a key role in both of the networks: the medial prefrontal cortex (mPFC). Previous studies systematically suggested that in the decision-making related process the activity of mPFC is responsible for the generating and computation of subjective values, regardless of reward types [[10], [11], [12], [13]]. It was also consistently suggested that the mPFC plays a central role in understanding other’s feelings, emotions and thoughts, especially during the affective theory of mind (ToM) [14]. Taken together, it was reasonable to assume that the cortical excitability of mPFC might be crucial in costly helping decision-making.

Transcranial direct current stimulation (tDCS) modulates the cortical excitability by passing a direct current between an anodal and a cathodal electrode. Anodal stimulation causes depolarization of the resting membrane potential, making the neurons under the electrode site more likely to fire, whereas cathodal stimulation leads to hyperpolarization which results in the decrease of neuronal excitability [15,16]. This technique can provide insight into the role of cortical regions in cognitive functions by modulating neural excitability and observing the corresponding effects of the stimulation on the targeted [17]. Previous studies proved that using tDCS targeting in mPFC may modulate subjects’ behavior in social decision-making tasks(for a review, see [16]).

However, how the excitability of mPFC may influence the costly helping behavior has not been investigated yet. The aim of the current study was to demonstrate the causal involvement of mPFC in the decision-making of costly helping behavior. The target electrode of tDCS was placed over the mPFC and the reference electrode was placed over visual cortex based on previous studies [[18], [19], [20]]. Two subjects with different roles would participant in the experiment each time. One was the “decider” while the other one was the “pain-taker”. The pain-taker would receive a painful electrical shock in each given trial. Before the shock applied, the decider would have the chance to help the pain-taker. An offer with two aspects of information would be presented to the decider: 1) the money they need to spend if they choose to help; 2) the probability of successful helping. Based on the previous studies, the activations in the mPFC were suggested to provide the basis for one's “emotional bond” with other persons [21] and greater activity in mPFC was associated with higher levels of self-reported experienced empathy as well as with daily helping behavior [22]. And anodal tDCS stimulation applied on mPFC has also been suggested to increase the trustworthiness and altruism in social decision-making [23]. We, therefore, hypothesized that the anodal stimulation to mPFC would increase the propensity to help in this task and the cathodal stimulation to mPFC would show a reverse effect.

Section snippets

Subjects

Sixty right-handed subjects (20.8 ± 0.33y (mean ± s.e)) with no history of neurological disorders, brain injuries or developmental disabilities participated in the experiment and got reimbursed for their participation. All subjects were screened for possible contraindications to tDCS and were gave written informed consent after they fully understood the task. All research procedures were approved by the Medical Ethical Committee of College of Psychology and Sociology at Shenzhen University

Results

For the percentage of acceptance, after controlling for the individual difference in empathic traits, we observed a significant interaction of probability × stimulation (F (4, 106) = 3.569, p = 0.012, η_p² = 0.119). Pairwise comparison revealed that when probability was 10% the difference between different stimulation groups were insignificant (anodal: 17.6 ± 3.5%; cathodal: 21.1 ± 3.5%; sham: 20.0 ± 3.5%, anodal vs cathodal, p = 0.397; anodal vs sham, p = 0.919; cathodal vs sham, p = 0.477).

Discussion

The present study used tDCS to demonstrate the causal relationship between the cortical excitability of mPFC and costly helping behavior. The results suggested that subjects received anodal stimulation chose to help others other more than subjects received cathodal stimulation, but only when the probability of success was high. When the probability was low or median, the effect of stimulation was not significantly different between groups. A trend was observed that the effect of stimulation was

Conflict of interest statement

The authors declare that they have no conflicts of interest for this study.

Acknowledgements

This study was funded by the National Natural Science Foundation of China (no. 31500877, 31400963, 31571129) and Humanity and Social Science Youth Foundation of Ministry of Education of China (16YJC190021).

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Research articleTranscranial direct current stimulation of the medial prefrontal cortex modulates the propensity to help in costly helping behavior

Highlights

Abstract

Introduction

Section snippets

Subjects

Results

Discussion

Conflict of interest statement

Acknowledgements

Personal. Individ. Differ.

Brain Cogn.

Brain Stimul.

Brain Stimul.

Behav. Brain Res.

The Roots of Prosocial Behavior in Children

High-frequency rTMS on DLPFC increases prosocial attitude in case of decision to support people

Soc. Neurosci.

Response time in economic games reflects different types of decision conflict for prosocial and proself individuals

Proc. Natl. Acad. Sci. U. S. A

Evidence for altruism: toward a pluralism of prosocial motives

Psychol. Inq.

Social evaluation and the empathy-altruism hypothesis

J. Pers. Soc. Psychol.

The role of social cognition in decision making

Philos. Trans. R. Soc. Lond. B Biol. Sci.

How social cognition can inform social decision making

Front. Neurosci.

A mechanistic account of value computation in the human brain

Proc. Natl. Acad. Sci. U. S. A

How the brain integrates costs and benefits during decision making

Proc. Natl. Acad. Sci. U. S. A

Hierarchical competitions subserving multi-attribute choice

Nat. Neurosci.

Selective alteration of human value decisions with medial frontal tDCS is predicted by changes in attractor dynamics

Sci. Rep.

Neural processing associated with cognitive and affective Theory of Mind in adolescents and adults

Soc. Cogn. Affect. Neurosci.

Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation

J. Physiol.

The stimulated social brain: effects of transcranial direct current stimulation on social cognition

Ann. N. Y. Acad. Sci.

Anodal transcranial direct current stimulation of right temporoparietal area inhibits self-recognition

Cogn. Affect. Behav. Neurosci.

Enhancing the activity of the DLPFC with tDCS alters risk preference without changing interpersonal trust

Front. Neurosci.

Research article
Transcranial direct current stimulation of the medial prefrontal cortex modulates the propensity to help in costly helping behavior