Aggression is used as a term in referring to any attacking behaviour that is directed against oneself, other individuals (mostly) or at objects. These can be expressed as verbal or physical attacks against persons or objects. Aggression in its extreme form is defined as violence. Aggressive behaviour is innate in both animals and humans. It is believed that aggression is or was helpful in the competition for resources such as food, in the defence of territories or in hierarchies, and in the competition for a sexual partner.
Emotions in research
Triggering of aggressive behaviour occurs through specific emotions. These are complex mental or psychosomatic processes through which procreation and physical and mental homeostasis are probably preserved. They occur in both humans and in many animal species. Experts dispute the precise definition. According to neurobiologist David Anderson from the California Institute of Technology, emotions are but “kinds of internal brain states with certain general characteristics that are independent of subjective or conscious experience”. Yet how can those cells responsible for certain emotions be identified?
In their study, the group led by David Anderson observed the behaviour of test animals and ascertained the underlying emotions. For example, the behavioural actions “freezing” or “flight” express the emotional state “fear”. Forms of behaviour which indicate “anger” take in variable forms of aggression. Scientists provide evidence for the neurons responsible for emotions by way of gene-based techniques (eg. optogenetics). In this instance, genetically labelled nerve cells are activated using light in order to initiate specific events.
Male-specific cells increase aggressiveness
Using this technique, the Californian scientists discovered already two years ago [Paywall] gender brain cells in male Drosophila melanogaster which increased the aggressive behaviour of the animals. When these neurons were activated, the aggressiveness increased. If on the other hand they were blocked, the aggressive behaviour diminished. Courtship behaviour was not affected. “The gender specific cells that we have found exert their effect in the form of fighting behaviour by releasing a specific neuropeptide or hormone that in mammals – mice and rats included – produces aggression as a result”, says David Anderson. This result correlates with other studies which were also able to demonstrate an increased concentration of the hormone tachykinin in humans with personality disorders.
“Aggression” and “pairing neurons”
Looking at male mice the group led by David Anderson [Paywall] found some 2,000 neurons which affected the aggressiveness of the animals. These were found in the ventromedial hypothalamus – a region that was actually associated with reproductive behaviour. These “aggression neurons” were mixed in with the nerve cells responsible for mating. About 20 percent of these neurons controlled both of these behaviours, aggression and sexual intercourse. The scientists have shown this [Paywall] by stimulating the neurons in male mice opto-genetically. While at a lower light intensity behaviours such as sniffing or mounting other animals increased, at a higher stimulation intensity the animals were stimulated to go on the attack – regardless in both cases whether the other individual was a male or female. Since the hypothalamus of the human brain is structurally similar to that of mice, the results should be transferable to human models, according to David Anderson.
Looking at male Drosophila melanogaster the David Anderson group found eight to ten so-called P1 neurons per hemisphere. These occur only in male flies and connect pheromonal and visual stimuli, thereby promoting the courtship behaviour of the animals. One small subunit is able not only to affect courtship behaviour, but also aggressiveness. Opto-genetic experiments showed that the animals behaved increasingly aggressively at a lower light intensity. The flies made threat gestures, for example, using their wings, or boxed one another. At a higher intensity, the scientists observed that the animals stretched their wings and thus vibrated – the courtship song of fruit flies. The results were published in the open-access journal eLife.
From the fly to man
“We are studying these problems, since these are fundamental for understanding how the brain works. However, it does not escape our attention that violence is an omnipresent public health problem. In my view, we need to understand the brain circuitry responsible for the emergence of aggression, if we are ever to understand abnormal forms of aggression such as sexual violence”, says David Anderson. This is because, according to the scientist’s beliefs, “incorrectly connected” neurons in humans could potentially trigger violent behaviour.
But why are both neuronal populations ever so close together? Reason for this might be that aggressive behaviour among men secured food, territories and sexual partners. Therefore, those nerve cells responsible for sexual behaviour might have caused the development of “aggression neurons”, which would explain their spatial proximity. In addition, both neuronal populations reinforce each other. After all, animals with sexual experience would likely be more aggressive than those without sexual experience.