Mirror neurons—nerve cells that fire both when an individual performs an action and when they observe the same action in others—have mainly been described in the neocortical structures of primates and rodents. Their function nevertheless remains debated. A classic hypothesis holds that their emergence in the neocortex is merely a by-product of associative learning, which obscures their evolutionary significance. This article comments on a study that radically shifts the perspective on these cells: it identifies mirror neurons dedicated to aggressive behavior in a phylogenetically very ancient brain structure, the ventromedial hypothalamus, and more specifically its ventrolateral part.
The study under discussion first recorded the activity of labeled neurons using fiber photometry during a classic resident-intruder test used to assess aggression in males. Because this technique cannot determine whether the same cells discharge in the aggressor and in the observer, a finer imaging method using a miniscope was developed. It revealed that these neurons increase their activity during the attack and threat phases of a territorial conflict, but also when a male simply observes two other males fighting. To rule out the hypothesis of a mimicry effect, the observer's rare movements or tail flicks were controlled for and proved to be uncorrelated with neuronal activity. Moreover, in mice lacking the Trpc2 gene, which are anosmic and non-aggressive, these neurons still became active at the sight of a fight, confirming that the triggering stimulus is indeed the visual information of another's aggressive behavior.
A final series of experiments established the causal role of these cells. Their chemogenetic inhibition, using a DREADDi actuator, reduced attack behaviors without affecting other social behaviors; conversely, their forced activation by a DREADDq actuator increased aggression directed toward males, females, and even the animal's own reflection in a mirror. These hypothalamic mirror neurons are therefore both necessary and sufficient for the expression of territorial aggression.
This finding invites a reconsideration of the origin of mirror neurons. The hypothalamic nuclei, highly conserved throughout vertebrate evolution, regulate functions essential to survival—reproduction, territorial aggression, feeding, and sleep rhythms. The mirror neurons they harbor appear to operate according to a principle close to the ethologists' "sign stimuli," activating "hard-wired" networks that underlie stereotyped, species-specific behaviors. By contrast, neocortical mirror mechanisms, which appeared later, would be more plastic and shaped by experience. Long associated with empathy, mirror neurons here reveal a darker side: depending on the context, they could underpin a regulatory function that activates aggressive rather than empathic responses, raising concerns about the impact of exposure to violence on mental well-being, particularly during sensitive developmental periods.