Invisible Influence: How Brain Injuries Increase Impulsivity and Social Influence

This study investigated how different parts of the brain help us make decisions alone or when influenced by others. They found that damage to the upper part of the medial prefrontal cortex (dmPFC) makes people more vulnerable to the influence of others, especially if the other person is impulsive. Damage to the lower part of the medial prefrontal cortex (vmPFC) makes people prefer immediate rewards, showing greater impulsivity. This suggests that different parts of the medial prefrontal cortex have different roles: one helps us resist social pressure and another helps us control impulsivity. These findings help us better understand how the brain guides us in social and economic decisions.

The medial prefrontal cortex (mPFC) is a region of the brain involved in making decisions and dealing with social information, such as understanding what others think or want. For a long time, scientists believed that different parts of this area had different functions: the upper part (called dorsomedial, or dmPFC) was more involved in thinking about others, while the lower part (ventromedial, or vmPFC) was more involved in thinking about oneself.

However, many of these findings come from studies that observe the brain in action, but they cannot prove cause and effect. This has generated debate, because it is still not known exactly what each part of the mPFC does, especially when it comes to social behavior or economic decisions.

One of the most discussed ideas is that there is a kind of “gradient” within the mPFC: the lower part helps you think about yourself, and the upper part helps you think about others. But even this idea has been questioned.

For example, the ventral part of the mPFC, which theoretically would only help with thinking about oneself, also seems to be involved in understanding other people’s preferences, making decisions for them, and even mixing one’s own preferences with those of others.

On the other hand, the dorsal part (dmPFC), which is supposed to be more social, also appears in situations where the person is evaluating their own choices. This suggests that neither of these regions is exclusive to social or individual information.

To clarify these functions, it would be ideal to have studies with causal evidence, such as with people who have suffered specific brain injuries, and with large sample sizes. One way to study social and economic decisions at the same time is to use a task called “delegated intertemporal choice.”

In it, participants have to choose between receiving a smaller amount immediately or a larger amount later. This measures how impulsive or patient each person is.

Interestingly, studies show that a person’s preferences in this regard can change just by knowing what other people prefer. When someone has to make these choices on behalf of someone else (even a stranger), they tend to adjust their decisions to be more like the other person’s. This shows how we are influenced socially, a phenomenon known as social influence or “social contagion.”

Neuroimaging studies suggest that the medial prefrontal cortex, especially its dorsal part, is activated when people follow the majority opinion. In other words, this part of the brain appears to be important for social conformity.

Other research shows that both the dorsal medial prefrontal cortex and the ventral medial prefrontal cortex are involved in economic and social decisions.

One theory is that the brain changes the way it represents the values ​​of choices when it is influenced by others, and this change occurs in the medial prefrontal cortex. However, even with these observations, we still do not know whether the medial prefrontal cortex is really necessary for these influences to occur, since most studies only observe correlations, not cause and effect.

In addition, the medial prefrontal cortex also appears to be involved in decisions that have nothing to do with others, for example, choosing between receiving a reward now or waiting for a bigger one.

There are studies showing that its activity, and its connection with other areas of the brain, is related to this preference.

Some studies of people who had lesions in this region have shown that they became more impulsive, but these studies were conducted with very few people. And in brain imaging, the ventral regions of the medial prefrontal cortex are sometimes difficult to see accurately because of technical limitations. So it’s important to use studies with more people and more robust approaches to figure out the true role of the medial prefrontal cortex in these functions.

In this particular study, the researchers took a relatively large group of people who had lesions specifically in the medial prefrontal cortex (33 people) and compared them with two other groups: a group with lesions in other parts of the brain (17 people) and a healthy control group without lesions (71 people).

Lesion locations in the mPFC and control lesion groups. (a) Participants in the mPFC lesion group (N = 33) had focal damage to the mPFC, with lesions extending into the lateral sections (area 13) of the bilateral mPFC and including subregions of the medial surface (areas 9, 14, 25, and 32). (b) Participants in the control lesion group (N = 17) also had damage caused primarily by subarachnoid hemorrhage, but in areas outside the mPFC.

All participants completed a task to measure how impulsive or patient they were in their decision-making. They were then presented with the choices of two other people (actually simulated choices by the researchers): one more impulsive and one more patient. The participants then repeated the task to see if they changed their own preferences after learning about the others. To measure this precisely, the scientists used advanced mathematical models and analyzed the results alongside images of the brain lesions.

The results showed that people with lesions in the medial prefrontal cortex were more influenced by the preferences of others, especially when those others were impulsive. This was not as strong in the other groups. In other words, the medial prefrontal cortex appears to be important for resisting social influence.

More specifically, lesions in the dorsal part (dmPFC) were linked to greater susceptibility to influence in impulsive decisions. In contrast, lesions in the ventral part (vmPFC) were associated with a greater general tendency towards impulsiveness in one's own choices, even before any social influence.

Lesions to the mPFC increase temporal impulsivity without affecting preference uncertainty. (a) Illustration of the preference-uncertainty (KU) model. In the KU model, people’s temporal discounting preferences are represented by a probability distribution. The mean (km) of this distribution indicates temporal impulsivity, while the standard deviation (ku) reflects the level of preference uncertainty. (b) Comparison of participants’ temporal impulsivity (km) and preference uncertainty (ku), derived from the preference-uncertainty (KU) model, between groups revealed that lesions to the mPFC increased temporal impulsivity, but not preference uncertainty, compared with healthy controls.

In summary, the study showed that different parts of the medial prefrontal cortex have distinct roles: the dmPFC is involved in how we are influenced socially, while the vmPFC is more involved in controlling impulsivity. This study provides strong evidence that these areas do indeed cause these effects, which helps to better understand how the brain handles social and economic decisions.

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Dorsomedial and ventromedial prefrontal cortex lesions differentially impact social influence and temporal discounting

Zhilin Su, Mona M. Garvert, Lei Zhang, Todd A. Vogel, Jo Cutler,

Masud Husain, Sanjay G. Manohar, and Patricia L. Lockwood 

PLoS Biol 23(4): e3003079.

https://doi.org/10.1371/journal.pbio.3003079

Abstract:

The medial prefrontal cortex (mPFC) has long been associated with economic and social decision-making in neuroimaging studies. Several debates question whether different ventral mPFC (vmPFC) and dorsal mPFC (dmPFC) regions have specific functions or whether there is a gradient supporting social and nonsocial cognition. Here, we tested an unusually large sample of rare participants with focal damage to the mPFC (N = 33), individuals with lesions elsewhere (N = 17), and healthy controls (N = 71) (total N = 121). Participants completed a temporal discounting task to estimate their baseline discounting preferences before learning the preferences of two other people, one who was more temporally impulsive and one more patient. We used Bayesian computational models to estimate baseline discounting and susceptibility to social influence after learning others’ economic preferences. mPFC damage increased susceptibility to impulsive social influence compared to healthy controls and increased overall susceptibility to social influence compared to those with lesions elsewhere. Importantly, voxel-based lesion-symptom mapping (VLSM) of computational parameters showed that this heightened susceptibility to social influence was attributed specifically to damage to the dmPFC (area 9; permutation-based threshold-free cluster enhancement (TFCE) p < 0.025). In contrast, lesions in the vmPFC (areas 13 and 25) and ventral striatum were associated with a preference for seeking more immediate rewards (permutation-based TFCE p < 0.05). We show that the dmPFC is causally implicated in susceptibility to social influence, with distinct ventral portions of mPFC involved in temporal discounting. These findings provide causal evidence for sub-regions of the mPFC underpinning fundamental social and cognitive processes.