Serotonin is a neurotransmitter that has been correlated with numerous behaviours, including impulsive aggression, antisocial behaviour and violence. One reason for this correlation could be the effect that depleted serotonin levels could have on important parts of the brain when we perceive emotional stimuli.

To test the effects of serotonin depletion on the brain, Passamonti et al. (2012) asked one group of healthy volunteers to consume a drink that lacked tryptophan. A lack of tryptophan in the diet will reduce levels of serotonin available in the brain. In the control condition they were given a drink that would not alter their tryptophan levels in the brain. Tryptophan is an important amino acid that helps to build serotonin in the brain (the technical explanation is that serotonin is synthesized from tryptophan).

It might be important to note that there is no way of measuring serotonin levels in the brains of living animals (including humans). The researchers did measure the effects of teh drink and found that it was successful in reducing the levels of tryptophan, so it is assumed that serotonin levels would be reduced as well. 

The participants were put in fMRIs and their brain activity was measured while they were seeing images of happy, angry or neutral faces. The researchers could see the activation of different areas of the brain as the different faces were shown.

The results showed that there was reduced activity in the frontal lobe during the low serotonin conditions when the participant was viewing the angry face. Moreover, communication between the amygdala and the frontal lobe was weaker in this condition.

By applying what we know about the frontal lobe and its role in regulating impulsive behaviour, as well as the amygdala in emotion and the stress response, this provides plausible clues as to why low serotonin might lead to acts of aggression or violence. If an individual is threatened and they have low levels of serotonin, they may not be able to perform top-down control; that is to say, the lack of activity in the PFC may affect their ability to regulate the stress response as triggered by the amygala’s reactivity towards the threat. This might increase their emotional level and increase chances of a highly emotion reaction to the threat.

The reduced activity in their PFC as a result of the low serotonin may also affect their ability to inhibit impulsive reactions and think through their actions, so if someone has a tendency towards violence they may not be able to reduce an impulsive reaction towards an individual who is threatening them.

This study might not make heaps of sense to you unless you have first learned about the role of the PFC in impulse control and the amygdala in emotion, social threat and the stress response. In the themantic unit on Criminology, these concepts are covered first before going into the role of serotonin on aggression and violence.

Guiding Question

  • How might levels of serotonin influence behaviour?

Dr Molly Crockett was a co-author of this study and you can watch her TED Talk below. In this talk she addresses the requirement for people to beware of the media’s tendency to oversimplify scientific findings.

References

Passamonti, Luca, Molly J. Crockett, Annemieke M. Apergis-Schoute, et al. “Effects of Acute Tryptophan Depletion on Prefrontal-Amygdala Connectivity While Viewing Facial Signals of Aggression.” Biological Psychiatry 71.1 (2012): 36-43.

Passamonti et al, 2012 (Full Study Link)