Emotional Intelligence

APPENDIX C

The Neural Circuitry of Fear

The amygdala is central to fear. When a rare brain disease destroyed the amygdala (but no other brain structures) in the patient neurologists call "S.M.," fear disappeared from her mental repertoire. She became unable to identify looks of fear on other people's faces, nor to make such an expression herself. As her neurologist put it, "If someone put a gun to S.M.'s head, she would know intellectually to be afraid but she would not feel afraid as you or I would."

Neuroscientists have mapped the circuitry for fear in perhaps finest detail, though at the present state of this art the full circuitry for none of the emotions is completely surveyed. Fear is an apt case in point for understanding the neural dynamics of emotion. Fear, in evolution, has a special prominence: perhaps more than any other emotion it is crucial for survival. Of course in modern times misplaced fears are the bane of daily life, leaving us suffering from frets, angst, and garden variety worries—or at pathological extreme, from panic attacks, phobias, or obsessive-compulsive disorder.

Say you're alone one night at home, reading a book, when suddenly you hear a crash in another room. What happens in your brain over the next moments offers a window into the neural circuitry of fear, and the role of the amygdala as an alarm system. The first brain circuit involved simply takes in that sound as raw physical waves and transforms them into the language of the brain to startle you into alertness. This circuit goes from the ear to the brainstem and then to the thalamus. From there two branches separate: a smaller bundle of projections leads to the amygdala and the nearby hippocampus; the other, larger pathway leads to the auditory cortex in the temporal lobe, where sounds are sorted out and comprehended.

The hippocampus, a key storage site for memory, quickly sorts that "crash" against other similar sounds you've heard, to see if it is familiar—is this "crash" one that you immediately recognize? Meanwhile the auditory cortex is doing a more sophisticated analysis of the sound to try to understand its source—is it the cat? A shutter banging in the wind? A prowler? The auditory cortex comes up with its hypothesis—it might be the cat knocking a lamp off the table, say, but it might also be a prowler—and sends that message to the amygdala and hippocampus, which quickly compare it to similar memories.

If the conclusion is reassuring (it's only the shutter that bangs whenever it gets too windy) then the general alert does not escalate to the next level. But if you are still unsure, another coil of circuitry reverberating between amygdala, hippocampus, and the prefrontal cortex further heightens your uncertainty and fixates your attention, making you even more concerned about identifying the source of the sound. If no satisfying answer comes from this further keen analysis, the amygdala triggers an alarm, its central area activating the hypothalamus, the brainstem, and the autonomic nervous system.

The superb architecture of the amygdala as a central alarm system for the brain becomes evident in this moment of apprehension and subliminal anxiety. The several bundles of neurons in the amygdala each have a distinct set of projections with receptors primed for different neurotransmitters, something like those home alarm companies where operators stand at the ready to send out calls to the local fire department, police, and a neighbor whenever a home security system signals trouble.

Different parts of the amygdala receive differing information. To the amygdala's lateral nucleus come projections from the thalamus and auditory and visual cortices. Smells, via the olfactory bulb, come to the corticomedial area of the amygdala, and tastes and messages from the viscera go to the central area. These incoming signals make the amygdala a continual sentinel, scrutinizing every sensory experience.

From the amygdala projections extend out to every major part of the brain. From the central and medial areas a branch goes to the areas of the hypothalamus that secrete the body's emergency-response substance, corticotropm-releasing hormone (CRH), which mobilizes the fight-or-flight reaction via a cascade of other hormones. The amygdala's basal area sends out branches to the corpus striatum, linking into the brain's system for movement. And, via the nearby central nucleus, the amygdala sends signals to the autonomic nervous system via the medulla, activating a wide range of far-flung responses in the cardiovascular system, the muscles, and the gut.

From the amygdala's basolateral area, arms go to the cingulate cortex and to the fibers known as the "central gray," cells that regulate the large muscles of the skeleton. It is these cells that make a dog snarl or that arch the back of a cat threatening an interloper on its territory. In humans these same circuits tighten the muscles of the vocal cords, creating the high-pitched voice of fright.

Still another pathway from the amygdala leads to the locus ceruleus in the brainstem, which in turn manufactures norepinephrine (also called "nor-adrenaline") and disperses it throughout the brain. The net effect of norepinephrine is to heighten the overall reactivity of the brain areas that receive it, making the sensory circuits more sensitive. Norepinephrine suffuses the cortex, the brainstem, and the limbic system itself, in essence setting the brain on edge. Now even the ordinary creaking of the house can send a tremor of fear coursing through you. Most of these changes go on outside awareness, so that you are not yet aware you feel fear.

But as you begin to actually feel fear—that is, as the anxiety that had been unconscious pierces awareness—the amygdala seamlessly commands a wide-ranging response. It signals cells in the brainstem to put a fearful expression on your face, make you edgy and easily startled, freeze unrelated movements your muscles had underway, speed your heart rate and raise your blood pressure, and slow your breathing (you may notice yourself suddenly holding your breath when you first feel fearful, all the better to hear more clearly what it is you are fearful of). That is only one part of a wide, carefully coordinated array of changes the amygdala and connected areas orchestrate as they commandeer the brain in a crisis.

Meanwhile the amygdala, along with the interconnected hippocampus, directs the cells that send key neurotransmitters, for example, to trigger releases of dopamine that lead to the riveting of attention on the source of your fear—the strange sounds—and put your muscles at readiness to react accordingly. At the same time the amygdala signals sensory areas for vision and attention, making sure that the eyes seek out whatever is most relevant to the emergency at hand. Simultaneously cortical memory systems are reshuffled so that knowledge and memories most relevant to the particular emotional urgency will be most readily recalled, taking precedence over other less relevant strands of thought.

Once these signals have been sent, you are pitched into full-fledged fear: you become aware of the characteristic tightness in your gut, your speeding heart, the tightening of the muscles around your neck and shoulders or the trembling of your limbs; your body freezes in place as you strain your attention to hear any further sounds, and your mind races with possible lurking dangers and ways to respond. This entire sequence—from surprise to uncertainty to apprehension to fear—can be telescoped within a second or so. (For more information, see Jerome Kagan, Galen's Prophecy. New York: Basic Books, 1994.)