Dr. David Anderson

It's one of the big differences between defensive aggression and offensive aggression.

And maybe these two regions are close to each other to facilitate inhibition

of aggression by the fear neurons.

We know for a fact that if we deliberately stimulate those fear neurons at the top of the pair, when two animals are involved in a fight, it just stops the fight dead in its tracks and they go off into the corner and freeze.

So at least hierarchically, it seems like fear is the dominant behavior over offensive aggression.

I think that's the way I tend to think about why these neurons are all mixed up together.

And it's not just fight and flight.

There are also metabolic neurons.

that are mixed together in VMH as well.

One way that is helpful, at least for me, to break this question apart and think about it is to distinguish homeostatic behaviors, that is need-based behaviors, where the pressure is built up because of a need like,

I'm hungry, I need to eat, I'm thirsty, I need to drink, I'm hot, I need to get to a cold place.

It's basically the thermostat model of your brain.

You have a set point and then if the temperature gets too hot, you turn on the AC and if the temperature gets too cold, you turn on the heater and you put yourself back to the set point.

You can think of this accumulated hydraulic pressure either being based on something that you were deprived of, creating an accumulating need, or something that you want to do, building up a drive or a pressure to do that.

And the natural way to think about that, at least for me, is as gradual increases in neural activity in a particular region of the brain.

So for example, in the area of the hypothalamus that controls feeding, Scott Sternson and others have shown that the hungrier you get, the higher the level of activity in that region in the brain.

And then when you eat, boom,

the activity goes right back down again.

And I think in the case of aggression, our data and others show that the more strongly you drive this region of the brain optogenetically, the more of just a hair trigger you need to set the animal off to get it to fight.

VMH projects to about 30 different regions in the brain.