The Better Angels of Our Nature: Why Vio - By Steven Pinker Page 0,303

large cavities in the brain. The periaqueductal gray contains circuits that control the sensorimotor components of rage. They get inputs from parts of the brain that register pain, balance, hunger, blood pressure, heart rate, temperature, and hearing (particularly the shrieks of a fellow rat), all of which can make the animal irritated, frustrated, or enraged. Their outputs feed the motor programs that make the rat lunge, kick, and bite.49 One of the oldest discoveries in the biology of violence is the link between pain or frustration and aggression. When an animal is shocked, or access to food is taken away, it will attack the nearest fellow animal, or bite an inanimate object if no living target is available.50

The periaqueductal gray is partly under the control of the hypothalamus, a cluster of nuclei that regulate the animal’s emotional, motivational, and physiological state, including hunger, thirst, and lust. The hypothalamus monitors the temperature, pressure, and chemistry of the bloodstream and sits on top of the pituitary gland, which pumps hormones into the bloodstream that regulate, among other things, the release of adrenaline from the adrenal glands and the release of testosterone and estrogen from the gonads. Two of its nuclei, the medial and ventrolateral, are parts of the Rage circuit. “Ventral” refers to the belly side of the animal, as opposed to its “dorsal” or back side. The terms were grandfathered over to the human brain as it evolved its perpendicular perch atop a vertical body, so in the human brain “ventral” points to our feet and “dorsal” to the top of our scalp.

Modulating the hypothalamus is the amygdala, Latin for “almond,” the shape it takes in the human brain. The amygdala is a small, multipart organ connected to brain systems for memory and motivation. It applies the emotional coloring to our thoughts and memories, particularly fear. When an animal has been trained to expect a shock after a tone, the amygdala helps to store the connections that give the tone its aura of anxiety and dread. The amygdala also lights up at the sight of a dangerous predator or of a threatening display from a member of the same species. In the case of humans, for example, the amygdala responds to an angry face.

And sitting on top of the entire Rage circuit is the cerebral cortex—the thin layer of gray matter on the outer surface of the cerebral hemispheres where the computations behind perception, thinking, planning, and decision-making are carried out. Each cerebral hemisphere is divided into lobes, and the one at the front, the frontal lobe, computes decisions relevant to how to behave. One of the major patches of the frontal lobes sits on top of the eye sockets in the skull, also known as orbits, so it is called the orbitofrontal cortex, orbital cortex for short.51 The orbital cortex is densely connected to the amygdala and other emotional circuits, and it helps integrate emotions and memories into decisions about what to do next. When the animal modulates its readiness to attack in response to the circumstances, including its emotional state and any lessons it has learned in the past, it is this part of the brain, behind the eyeballs, that is responsible. By the way, though I have described the control of rage as a topdown chain of command—orbital cortex to amygdala to hypothalamus to periaqueductal gray to motor programs—the connections are all two-way: there is considerable feedback and cross talk among these components and with other parts of the brain.

As I mentioned, predation and rage play out very differently in the behavioral repertoire of a carnivorous mammal and are triggered by electrical stimulation of different parts of the brain. Predation involves a circuit that is part of what Panksepp calls the Seeking system.52A major part of the Seeking system runs from a part of the midbrain (not shown in figure 8–1) via a bundle of fibers in the middle of the brain (the medial forebrain bundle) to the lateral hypothalamus, and from there up to the ventral striatum, a major part of the so-called reptilian brain. The striatum is composed of many parallel tracts (giving it a striated appearance), and it is buried deep in the cerebral hemispheres and densely connected to the frontal lobes.

The Seeking system was discovered when the psychologists James Olds and Peter Milner implanted an electrode into the middle of a rat brain, hooked it up to a lever in a Skinner box, and found that the rat would press the lever to stimulate