Hot-blooded badness, warmhearted goodness, and the unnerving incongruity of the cold-blooded versions raise a key point, encapsulated in a quote from Elie Wiesel, the Nobel Peace Prize winner and concentration camp survivor: “The opposite of love is not hate; its opposite is indifference.” The biologies of strong love and strong hate are similar in many ways, as we’ll see.
Which reminds us that we don’t hate aggression; we hate the wrong kind of aggression but love it in the right context. And conversely, in the wrong context our most laudable behaviors are anything but. The motoric features of our behaviors are less important and challenging to understand than the meaning behind our muscles’ actions.
This is shown in a subtle study.7 Subjects in a brain scanner entered a virtual room where they encountered either an injured person in need of help or a menacing extraterrestrial; subjects could either bandage or shoot the individual. Pulling a trigger and applying a bandage are different behaviors. But they are similar, insofar as bandaging the injured person and shooting the alien are both the “right” things. And contemplating those two different versions of doing the right thing activated the same circuitry in the most context-savvy part of the brain, the prefrontal cortex.
And thus those key terms that anchor this book are most difficult to define because of their profound context dependency. I will therefore group them in a way that reflects this. I won’t frame the behaviors to come as either pro- or antisocial—too cold-blooded for my expository tastes. Nor will they be labeled as “good” and “evil”—too hot-blooded and frothy. Instead, as our convenient shorthand for concepts that truly defy brevity, this book is about the biology of our best and worst behaviors.
Two
One Second Before
Various muscles have moved, and a behavior has happened. Perhaps it is a good act: you’ve empathically touched the arm of a suffering person. Perhaps it is a foul act: you’ve pulled a trigger, targeting an innocent person. Perhaps it is a good act: you’ve pulled a trigger, drawing fire to save others. Perhaps it is a foul act: you’ve touched the arm of someone, starting a chain of libidinal events that betray a loved one. Acts that, as emphasized, are definable only by context.
Thus, to ask the question that will begin this and the next eight chapters, why did that behavior occur?
As this book’s starting point, we know that different disciplines produce different answers—because of some hormone; because of evolution; because of childhood experiences or genes or culture—and as the book’s central premise, these are utterly intertwined answers, none standing alone. But on the most proximal level, in this chapter we ask: What happened one second before the behavior that caused it to occur? This puts us in the realm of neurobiology, of understanding the brain that commanded those muscles.
This chapter is one of the book’s anchors. The brain is the final common pathway, the conduit that mediates the influences of all the distal factors to be covered in the chapters to come. What happened an hour, a decade, a million years earlier? What happened were factors that impacted the brain and the behavior it produced.
This chapter has two major challenges. The first is its god-awful length. Apologies; I’ve tried to be succinct and nontechnical, but this is foundational material that needs to be covered. Second, regardless of how nontechnical I’ve tried to be, the material can overwhelm someone with no background in neuroscience. To help with that, please wade through appendix 1 around now.
—
Now we ask: What crucial things happened in the second before that pro- or antisocial behavior occurred? Or, translated into neurobiology: What was going on with action potentials, neurotransmitters, and neural circuits in particular brain regions during that second?
THREE METAPHORICAL (BUT NOT LITERAL) LAYERS
We start by considering the brain’s macroorganization, using a model proposed in the 1960s by the neuroscientist Paul MacLean.1 His “triune brain” model conceptualizes the brain as having three functional domains:
Layer 1: An ancient part of the brain, at its base, found in species from humans to geckos. This layer mediates automatic, regulatory functions. If body temperature drops, this brain region senses it and commands muscles to shiver. If blood glucose levels plummet, that’s sensed here, generating hunger. If an injury occurs, a different loop initiates a stress response.
Layer 2: A more recently evolved region that has expanded in mammals. MacLean conceptualized this layer as being about emotions, somewhat of a mammalian invention. If you see something gruesome and terrifying, this layer sends commands down to ancient layer 1, making you shiver with emotion. If you’re feeling sadly unloved, regions here prompt layer 1 to generate a craving for comfort food. If you’re a rodent and smell a cat, neurons here cause layer 1 to initiate a stress response.
Layer 3: The recently evolved layer of neocortex sitting on the upper surface of the brain. Proportionately, primates devote more of their brain to this layer than do other species. Cognition, memory storage, sensory processing, abstractions, philosophy, navel contemplation. Read a scary passage of a book, and layer 3 signals layer 2 to make you feel frightened, prompting layer 1 to initiate shivering. See an ad for Oreos and feel a craving—layer 3 influences layers 2 and 1. Contemplate the fact that loved ones won’t live forever, or kids in refugee camps, or how the Na’vis’ home tree was destroyed by those jerk humans in Avatar (despite the fact that, wait, Na’vi aren’t real!), and layer 3 pulls layers 2 and 1 into the picture, and you feel sad and have the same sort of stress response that you’d have if you were fleeing a lion.
Thus we’ve got the brain divided into three functional buckets, with the usual advantages and disadvantages of categorizing a continuum. The biggest disadvantage is how simplistic this is. For example:
Anatomically there is considerable overlap among the three layers (for example, one part of the cortex can best be thought of as part of layer 2; stay tuned).
The flow of information and commands is not just top down, from layer 3 to 2 to 1. A weird, great example explored in chapter 15: if someone is holding a cold drink (temperature is processed in layer 1), they’re more likely to judge someone they meet as having a cold personality (layer 3).
Automatic aspects of behavior (simplistically, the purview of layer 1), emotion (layer 2), and thought (layer 3) are not separable.
The triune model leads one, erroneously, to think that evolution in effect slapped on each new layer without any changes occurring in the one(s) already there.
Despite these drawbacks, which MacLean himself emphasized, this model will be a good organizing metaphor for us.
THE LIMBIC SYSTEM
To make sense of our best and worst behaviors, automaticity, emotion, and cognition must all be considered; I arbitrarily start with layer 2 and its emphasis on emotion.
Early-twentieth-century neuroscientists thought it obvious what layer 2 did. Take your standard-issue lab animal, a rat, and examine its brain. Right at the front would be these two gigantic lobes, the “olfactory bulbs” (one for each nostril), the primary receptive area for odors.
Neuroscientists at the time asked what parts of the brain these gigantic rodent olfactory bulbs talked to (i.e., where they sent their axonal projections). Which brain regions were only a single synapse away from receiving olfactory information, which were two synapses, three, and so on?
And it was layer 2 structures that received the first communiqués. Ah, everyone concluded, this part of the brain must process odors, and so it was termed the rhinencephalon—the nose brain.
Meanwhile, in the thirties and forties, neuroscientists such as the young MacLean, James Papez, Paul Bucy, and Heinrich Klüver were starting to figure out what the layer 2 structures did. For example, if you lesion (i.e., destroy) layer 2 structures, this produces “Klüver-Bucy syndrome,” featuring abnormalities in sociality, especially in sexual and aggressive behaviors. They concluded that th
ese structures, soon termed the “limbic system” (for obscure reasons), were about emotion.
Rhinencephalon or limbic system? Olfaction or emotion? Pitched street battles ensued until someone pointed out the obvious—for a rat, emotion and olfaction are nearly synonymous, since nearly all the environmental stimuli that elicit emotions in a rodent are olfactory. Peace in our time. In a rodent, olfactory inputs are what the limbic system most depends on for emotional news of the world. In contrast, the primate limbic system is more informed by visual inputs.
Limbic function is now recognized as central to the emotions that fuel our best and worst behaviors, and extensive research has uncovered the functions of its structures (e.g., the amygdala, hippocampus, septum, habenula, and mammillary bodies).
There really aren’t “centers” in the brain “for” particular behaviors. This is particularly the case with the limbic system and emotion. There is indeed a sub-subregion of the motor cortex that approximates being the “center” for making your left pinkie bend; other regions have “center”-ish roles in regulating breathing or body temperature. But there sure aren’t centers for feeling pissy or horny, for feeling bittersweet nostalgia or warm protectiveness tinged with contempt, or for that what-is-that-thing-called-love feeling. No surprise, then, that the circuitry connecting various limbic structures is immensely complex.
The Autonomic Nervous System and the Ancient Core Regions of the Brain
The limbic system’s regions form complex circuits of excitation and inhibition. It’s easier to understand this by appreciating the deeply held desire of every limbic structure—to influence what the hypothalamus does.
Why? Because of its importance. The hypothalamus, a limbic structure, is the interface between layers 1 and 2, between core regulatory and emotional parts of the brain.
Consistent with that, the hypothalamus gets massive inputs from limbic layer 2 structures but disproportionately sends projections to layer 1 regions. These are the evolutionarily ancient midbrain and brain stem, which regulate automatic reactions throughout the body.
For a reptile such automatic regulation is straightforward. If muscles are working hard, this is sensed by neurons throughout the body that send signals up the spine to layer 1 regions, resulting in signals back down the spine that increase heart rate and blood pressure; the result is more oxygen and glucose for the muscles. Gorge on food, and stomach walls distend; neurons embedded there sense this and pass on the news, and soon blood vessels in the gut dilate, increasing blood flow and facilitating digestion. Too warm? Blood is sent to the body’s surface to dissipate heat.
—
All of this is automatic, or “autonomic.” And thus the midbrain and brain-stem regions, along with their projections down the spine and out to the body, are collectively termed the “autonomic nervous system.”*
And where does the hypothalamus come in? It’s the means by which the limbic system influences autonomic function, how layer 2 talks to layer 1. Have a full bladder with its muscle walls distended, and midbrain/brain-stem circuitry votes for urinating. Be exposed to something sufficiently terrifying, and limbic structures, via the hypothalamus, persuade the midbrain and brain stem to do the same. This is how emotions change bodily functions, why limbic roads eventually lead to the hypothalamus.*
The autonomic nervous system has two parts—the sympathetic and parasympathetic nervous systems, with fairly opposite functions.
The sympathetic nervous system (SNS) mediates the body’s response to arousing circumstances, for example, producing the famed “fight or flight” stress response. To use the feeble joke told to first-year medical students, the SNS mediates the “four Fs—fear, fight, flight, and sex.” Particular midbrain/brain-stem nuclei send long SNS projections down the spine and on to outposts throughout the body, where the axon terminals release the neurotransmitter norepinephrine. There’s one exception that makes the SNS more familiar. In the adrenal gland, instead of norepinephrine (aka noradrenaline) being released, it’s epinephrine (aka the famous adrenaline).*
Meanwhile, the parasympathetic nervous system (PNS) arises from different midbrain/brain-stem nuclei that project down the spine to the body. In contrast to the SNS and the four Fs, the PNS is about calm, vegetative states. The SNS speeds up the heart; the PNS slows it down. The PNS promotes digestion; the SNS inhibits it (which makes sense—if you’re running for your life, avoiding being someone’s lunch, don’t waste energy digesting breakfast).* And as we will see chapter 14, if seeing someone in pain activates your SNS, you’re likely to be preoccupied with your own distress instead of helping; turn on the PNS, and it’s the opposite. Given that the SNS and PNS do opposite things, the PNS is obviously going to be releasing a different neurotransmitter from its axon terminals—acetylcholine.*
There is a second, equally important way in which emotion influences the body. Specifically, the hypothalamus also regulates the release of many hormones; this is covered in chapter 4.
—
So the limbic system indirectly regulates autonomic function and hormone release. What does this have to do with behavior? Plenty—because the autonomic and hormonal states of the body feed back to the brain, influencing behavior (typically unconsciously).* Stay tuned for more in chapters 3 and 4.
The Interface Between the Limbic System and the Cortex
Time to add the cortex. As noted, this is the brain’s upper surface (its name comes from the Latin cortic, meaning “tree bark”) and is the newest part of the brain.
The cortex is the gleaming, logical, analytical crown jewel of layer 3. Most sensory information flows there to be decoded. It’s where muscles are commanded to move, where language is comprehended and produced, where memories are stored, where spatial and mathematical skills reside, where executive decisions are made. It floats above the limbic system, supporting philosophers since at least Descartes who have emphasized the dichotomy between thought and emotion.
Of course, that’s all wrong, as shown by the temperature of a cup—something processed in the hypothalamus—altering assessment of the coldness of someone’s personality. Emotions filter the nature and accuracy of what is remembered. Stroke damage to certain cortical regions blocks the ability to speak; some sufferers reroute the cerebral world of speech through emotive, limbic detours—they can sing what they want to say. The cortex and limbic system are not separate, as scads of axonal projections course between the two. Crucially, those projections are bidirectional—the limbic system talks to the cortex, rather than merely being reined in by it. The false dichotomy between thought and feeling is presented in the classic Descartes’ Error, by the neurologist Antonio Damasio of the University of Southern California; his work is discussed later.2
While the hypothalamus dwells at the interface of layers 1 and 2, it is the incredibly interesting frontal cortex that is the interface between layers 2 and 3.
Key insight into the frontal cortex was provided in the 1960s by a giant of neuroscience, Walle Nauta of MIT.*3 Nauta studied what brain regions sent axons to the frontal cortex and what regions got axons from it. And the frontal cortex was bidirectionally enmeshed with the limbic system, leading him to propose that the frontal cortex is a quasi member of the limbic system. Naturally, everyone thought him daft. The frontal cortex was the most recently evolved part of the very highbrow cortex—the only reason why the frontal cortex would ever go slumming into the limbic system would be to preach honest labor and Christian temperance to the urchins there.
Naturally, Nauta was right. In different circumstances the frontal cortex and limbic system stimulate or inhibit each other, collaborate and coordinate, or bicker and work at cross-purposes. It really is an honorary member of the limbic system. And the interactions between the frontal cortex and (other) limbic structures are at the core of much of this book.
Two more details. First, the cortex is not a smooth surface but instead is folded into convolutions.
The convolutions form a superstructure of four separate lobes: the temporal, parietal, occipital, and frontal, each with different functions.
Brain Lateralization
Analytical thought
Detail-oriented perception
Ordered sequencing
Rational thought
Verbal
Cautious
Planning
Math/science
Logic
Right-field vision
Right-side motor skills
Intuitive thought
Holistic perception
Random sequencing
Emotional thought
Nonverbal
Adventurous
Impulse
Creative writing/art
Imagination
Left-field vision
Left-side motor skills
Second, brains obviously have left and right sides, or “hemispheres,” that roughly mirror each other.
Thus, except for the relatively few midline structures, brain regions come in pairs (a left and right amygdala, hippocampus, temporal lobe, and so on). Functions are often lateralized, such that the left and right hippocampi, for example, have different but related functions. The greatest lateralization occurs in the cortex; the left hemisphere is analytical, the right more involved in intuition and creativity. These contrasts have caught the public fancy, with cortical lateralization exaggerated by many to an absurd extent, where “left brain”–edness has the connotation of anal-retentive bean counting and “right brain”–edness is about making mandalas or singing with whales. In fact the functional differences between the hemispheres are generally subtle, and I’m mostly ignoring lateralization.
Behave: The Biology of Humans at Our Best and Worst Page 3