Image Credit: JCInactive1
As a human, the concept of aggression should be a familiar one. We’ve all expressed aggressive behaviours, and experienced them in others – those who follow soon-to-be-former President Trump’s Tweets should be particularly familiar. But how does the brain produce aggressive behaviour? How is aggression coordinated and controlled?
A recent review by Julieta E. Lischinsky and Dayu Lin at New York University School of Medicine, gathers research from many studies and summarises what we currently know about the biology of aggression.
Aggression Provoking Stimuli
Aggression, as a behaviour, is usually a response to a trigger. What is an ‘aggression provoking stimulus’ and how is it detected? For humans, there are multiple - from physical threats to name-calling. In other animals, the trigger can be something as simple as a smell or sound, such as in songbirds who have evolved specific neural pathways to respond to intruders’ songs with aggressive attacks.
Neural Circuitry of Aggression
How does detection of an aggression-provoking stimulus lead to the production of an aggressive response? In response to the trigger, the body’s detectors send signals to the Core Aggression Circuit (“CAC”) in the brain, comprising four subcortical brain regions.
An interesting study by researchers at New York University School of Medicine, published only last year, investigated a specific subset of neurons within the ventrolateral part of the ventromedial hypothalamus (one of the four regions participating in the Core Aggression Circuit). Dayu Lin and the team performed a series of experiments to investigate this brain region’s role in aggression and defence against aggression in mice. Mice studies are useful, as mice are biologically similar to humans – both being mammals, and sharing most of the same gene (and thus organ) functions.
This study used ‘optogenetics’, a technique that fires light rays at cells which have been genetically modified to express light-sensitive ion channels. Put more simply, this technique allows scientists to use lasers to activate specific neurons. They found that, in the presence of a female intruder, light stimulation caused almost half of the animals to attack the female – with the rest of them responding defensively. Other papers on the other brain regions involved in the CAC have recorded similar results.
Artificially activating populations of neurons within the Circuit has the ability to stimulate aggression, and inhibiting them suppresses aggressive behaviour. Just by changing the function of ONE member of the Circuit can cause such drastic behavioural changes! This shows how important each brain region involved in the Circuit is in mediating an aggressive response.
So, aggression-provoking triggers are detected, and these detectors send signals to the CAC, which determines whether aggression is produced. But what happens to the signal after this? In short, the CAC sends signals to regions of the midbrain to produce an innate aggressive action (hardwired into the organism, such as biting in mice). It also sends signals to another region of the midbrain, which in turn sends signals to the striatum to promote learned aggressive actions.
Control of Aggression
The CAC is under tight control by other pathways of neurons, as aggression is very risky. Think about it, if the brain cannot control aggression, this could result in death for the organism… it’s important to pick your battles.
Whether an animal will respond with aggression ultimately depends on a multitude of internal states such as stress and discomfort, hunger, and even the time of day – as well as past experience, such as a consumption of violent media in the case of humans. There is also some internal cognitive control of aggressive behaviour, especially in humans, through belief systems and the knowledge of certain laws - this is performed by other brain regions which send signals to the CAC.
Ultimately, the CAC receives a bunch of signals from all over the place, each either increasing or decreasing the likelihood of sending more signals to produce an aggressive response. For example, an aggressive action may be produced by the brain when the organism is starving, in order to increase the likelihood of survival.
What is an aggressive action? It’s anything from biting, in the case of rodents, to slaps and insults in the case of humans. As a species, we are now quite far from fist fights, with technological advances giving us access to knives, guns, and even nuclear weapons.
We can also premeditate aggression, showing aggression-seeking behaviour without detecting an aggression-provoking stimulus. Our aggressive acts can also be much more complex and extreme than other animals, with war as an example - what was once an evolutionary advantage now has the ability to cause mass suffering.
In an interview with Medical Xpress, the authors of the review state that future studies in the field aim to focus more on the ‘control’ aspect of the Aggression Circuit, i.e. which factors are taken into account before production of an aggressive act, and which brain regions are involved. This will help researchers understand why and when aggressive behaviours occur.
The review, “Neural mechanisms of aggression across species” published in Nature Neuroscience describes the studies mentioned above in great depth, and is a great read for those with a background in biology who wish to learn more about recent studies on the neuroscience of aggression.