Your Brain Reacting to Social Injustice

What's the science?

How do we perceive injustice? Many neuroimaging studies have looked at how we perceive the violation of social norms by analyzing brain activity while participants play a computer game. For example, participants might have the option to punish one player who is acting unfairly (e.g. stealing) towards another. Further, different hormones, like oxytocin, influence our social behaviour, suggesting they can play a role in our perception of injustice. This week in The Journal of Neuroscience, Stallen and colleagues performed a new set of experiments using brain imaging to analyze the perception of injustice.

How did they do it?

First, oxytocin was administered to half of the participants. Next, all participants underwent an fMRI brain scan, while playing three computer games: 1) Participants played against an opponent, called a ‘taker’. The taker had the opportunity to steal up to 100 chips away from the participant, and the participant could then punish the taker by giving up up to 100 of their own chips. For each chip they gave up 3 would be taken from the taker, (injustice happening to them) 2) Participants received 200 chips and observed a taker stealing chips from another player, and could then punish the taker (using up to 100 chips, 3 taken from the taker for each chip given up), (observing social injustice, punishing as a third party) and 3) Participants observed a taker stealing chips from another player, and could compensate the disadvantaged player using up to 100 chips (the disadvantaged player was given 3 chips per chip given up) (observing social injustice, compensating as a third party). Participants knew they would receive real monetary compensation after the games according to their performance, and all games were anonymous.

What did they find?

Participants who received oxytocin were more likely to dole out small punishments, frequently, to a taker who took chips from the participant or another player, versus participants who did not receive oxytocin. When the authors compared trials in which a participant doled out punishment to the taker versus compensating the disadvantaged player, there was greater activity in the ventral striatum -- a brain region involved in processing rewards. The decision to administer punishment was associated with activity in the anterior insula -- a brain region involved in “gut feelings” and decision making involving risk. Activity in the amygdala, a brain region associated with affective arousal, was correlated with the severity of punishment administered but only in experiment #2, when participants observed a taker behaving unfairly towards someone else.

What's the impact?

This is the first study to assess the perception of social justice in situations where an individual is experiencing injustice firsthand compared to observing injustice as a third party. This study suggests two distinct brain mechanisms might be at play during these unjust situations.

A word of caution: Different brain regions are activated in many different situations. Just because a brain region is known to be activated during reward, for example, does not necessarily mean that brain region will always be active during reward processing.

M. Stallen et al., Neurobiological Mechanisms of Responding to Injustice. Journal of Neuroscience. (2018). Access the original scientific publication here.

The Basal Forebrain Influences the Brain’s Default Mode

What's the science?

The ‘default mode network’ is a unique network of brain regions that is dysregulated in many clinical conditions, like epilepsy and depression. Typically, these regions are most active while a person is resting or reflecting internally. Brain activity in the default mode network at a particular frequency (gamma frequency) is high during rest.The basal forebrain is a brain region thought to contribute to arousal and attention - for example, it accelerates learning and boosts neural signals in response to sensory stimulation. This week in PNAS, Nair and colleagues assessed the relationship between gamma frequency power (strength) in the basal forebrain and the default mode network, to understand how these regions might influence one another. 

How did they do it?

They studied the basal forebrain and default mode network in rats using recordings from electrodes in these regions, specifically measuring the gamma power. During the recordings, the rats were placed in their home cages where they groomed themselves and were quiet but awake, or they were placed in a large new area, where they tend to engage in active, exploratory behaviour. Next, they recorded the firing (activity) of basal forebrain neurons and tested whether it was correlated with fluctuations in gamma power, in order to understand whether the gamma frequency was generated locally. Finally, they tested whether patterns of gamma frequency activity in the basal forebrain predicted activity in the default mode network or vice versa.

What did they find?

They found that gamma power was high while the mice were in their home cages, and low while the mice explored an area. This change in gamma power was not due to movement. They also found that the neuronal firing of basal forebrain neurons was related to gamma power, indicating that these neurons were responsible for generating the signal. Finally, they found that gamma power in the basal forebrain predicted gamma power in the default mode network.

Hugo Gambo, Eeg gamma, image by BrainPost, CC BY-SA 3.0

What's the impact?

This is the first study to observe a link between the the default mode network, active during rest, and the basal forebrain, traditionally understood to increase arousal. Basal forebrain neurons are also activated during rest along with the default mode network. This study suggests that the default mode network is influenced by the basal forebrain, which could be a new clinical target for disorders in which the default mode network is affected.

Nair et al., Basal forebrain contributes to default mode network regulation. PNAS. (2018). Access the original scientific publication here.

Hunger Affects Brain Activation in Response to Food Across the Lifespan

What's the science?

In adults, brain activity in response to food cues has been shown to predict overeating and weight gain. We do not yet understand the relationship between the brain’s response to food and overeating across the lifespan, including in children, who are especially vulnerable to food cues. Recently in Neuroimage, Charbonnier and colleagues show how certain brain regions activate in response to food depending on hunger level in different age groups.

How did they do it?

They recruited children, teens, adults and elderly participants who were scanned twice using functional MRI, once in a hungry state after fasting all night and once in a full (‘sated’) state after being fed.  Before the scan, participants rated how much they liked various foods. In the scanner, participants performed a food-viewing task where they viewed images of high and low calorie foods. The viewing of non-food images was a control task. Brain activity was compared while participants viewed high versus low calorie foods, in the hungry or full state, across different age groups.

What did they find?

Brain activation in the hungry state was greater across the lifespan when viewing high calorie foods (compared to low calorie foods) in two regions of the prefrontal cortex: the dorsolateral prefrontal cortex (involved in controlling actions) and the dorsomedial prefrontal cortex (involved in processing reward and value). Hunger state alone did not affect brain activation when viewing food. Age also did not affect brain activation for high compared to low calorie foods, even though younger participants rated liking high calorie food more.

What's the impact?

This is the first study to look at the effects of hunger and different food cues on brain activation in different age groups. The activation of the dorsolateral prefrontal cortex could reflect inhibition of eating, whereas the dorsomedial prefrontal cortex could be activating to process the reward value of the food, however this needs to be investigated further. It is important to understand brain mechanisms for eating behaviors across the lifespan in order to develop strategies to prevent obesity.

Reach out to study author Dr. Daniel Crabtree on Twitter @DanielCrabtree9

L. Charbonnier et al., Effects of hunger state on the brain responses to food cues across the life span. Neuroimage. 171, 246–255 (2018). Access the original scientific publication here.