In a remarkable series of experiments, researchers have located a trigger for aggression in mice—providing us with fresh insights into the workings of our human consciousness.
Our story starts in the hypothalamus, an ancient region of the brain, conserved throughout mammalian evolution. In humans, it is about the size of an almond, housing a motley collection of neurons. These cells regulate distinct bodily functions such as temperature, circadian rhythms, sleep, hunger, thirst, sex, anger, aggression and response to stress. Earlier work showed that electrical stimulation of some of these sites provokes cats and rats to sudden bouts of rage and that the ventromedial hypothalamus (VMH) has some involvement in sexual behaviors. Yet the precise location of attack-promoting neurons, their mode of action, and the interplay between aggression and mating—normally two opposing forms of social interactions—had remained deeply mysterious.
Enter a team from the California Institute of Technology, under the leadership of neurobiologist David J. Anderson. In four steps, the seven scientists, spearheaded by postdoctoral fellow Dayu Lin (now at New York University), nailed down the critical role of aggression neurons in the VMH. The setting was the home cage of an individually housed, sexually experienced male mouse. When another mouse, either a male or a sexually receptive female, entered the cage, the resident male mouse usually attacked the former but mated with the latter. The scientists video recorded the behavior so that the detailed time course of interaction of every pair of animals—the cautious sniffing and retreating, the pushing, shoving and biting, the mounting and consummatory activities—in hundreds of encounters could be statistically analyzed and time-aligned using software developed by machine vision engineers Piotr Dollar and Pietro Perona.