Joseph LeDoux has worked on emotion, memory, and consciousness in the brain since the mid-1970s. He is a Professor of Neural Science at New York University and an elected member of the National Academy of Sciences and the American Academy of Arts and Sciences. He has received numerous awards for his work and is the author of several books, including The Emotional Brain, Synaptic Self, Anxious, The Deep History of Ourselves, and the Four Realms of Existence. In addition, LeDoux is the lead singer and songwriter in the rock band The Amygdaloids. His music is the subject of a play called Map of Your Mind, and he subject of a documentary called Neuroscience and Emotions: The Life, Work, and Music of Dr. Joseph LeDoux. He and his music are also featured in Werner Herzog’s 2024 film, Theatre of Thought. In this Recently Published Book Spotlight, he discusses what it means to be a human being, his taxonomy of behavior, and how we might better understand behavor.

WHAT IS YOUR LATEST BOOK ABOUT?

In The Four Realms of Existence, I ponder questions about what it means to be a human being. I start with the fact that our species has long thought of bodies and minds as separate spheres of existence. The body is physical—the source of life and its aches and pains. But the mind is mental—it perceives, remembers, believes, feels, and imagines. Although modern science has largely eliminated this mind–body dualism, people still tend to feel that their mind is somehow different from their physical being. Indeed, even in research, the notion of a “self” that is somehow distinct from the rest of the organism persists. Yet, there is little agreement about what “self” refers to or even whether it refers to a real entity, as opposed to just being a shorthand label for a variety of psychologically interesting phenomena. In part, this may be because constructs like “the self” that goes back to ancient times are inadequate as scientific conceptual hooks on which to hang the phenomena that have been discovered in the name of “the self.” Accordingly, the empirical phenomena discovered in the name of “the self” and related notions like “personality” might be better served by a new conceptual home, especially one grounded in empirical findings about the relation of the human mind to the human brain.

In the Four Realms, I constructed such a conceptual framework, proposing that a human being can be characterized as a composite of four fundamental, parallel, entwined realms of existence: biological, neurobiological, cognitive, and conscious. All four are, deep down, biological. But the neurobiological realm transcends the biological, the cognitive transcends the neurobiological, and the conscious transcends the cognitive.

The realms coalesce as an ensemble of being that encompasses all of what and who we are as individual human organisms, including those aspects of us that fall under the rubrics of the self and personality. A case could be made for social and cultural realms, but I believe that society and culture are things done with cognitive and conscious realms.

The realms not only reflect our present ways of being but also our evolutionary past. The biological realm made life possible. Every organism that has ever lived has existed biologically. Animals, and only animals, came to supplement mere biological existence with a nervous system that constitutes the neurobiological realm. Armed with a nervous system, animals could control the biological activities of their bodies with degrees of speed and precision unseen in other life forms. Some animals with nervous systems evolved the ability to create internal representations of the world around them and use these in thinking, feeling, and planning. These cognitive capacities are not necessarily conscious. The evolution of a conscious realm made it possible for cognitive animals to experience thoughts and feelings and be aware of who they are in relation to the world that they share with other organisms.

Biological beings are easily distinguished from non-living matter and neurobiological beings from mere biological ones based on physical properties. But things are considerably more complex when considering the cognitive and conscious realms since there are no physical properties that unequivocally identify which organisms are cognitive and conscious beings. However, my primary goal was not to offer an account of which animals are cognitive and or conscious. It was instead to account for how our four realms interact and, in so doing, make human beings what and who we are.

WHY DID YOU FEEL THE NEED TO WRITE THIS BOOK?

I did my Ph.D. studying consciousness in split-brain patients. But one patient in particular, Paul, or PS, had a tremendous impact on me. Studies of him suggested that our brains evolved the capacity to generate stories, narrations, to prevent unconsciously controlled behaviors from challenging our sense of mental unity and free will, and thereby maintain a sense of volitional control of behavior, even if that sense is illusory. One of the studies of PS was about emotional consciousness and led to the idea that emotional behaviors might just be the kinds of unconsciously controlled responses that challenge our senses of mental unity and agency.

Though emotion was a minor part of my graduate work, it defined much of the rest of my career in which I pursued what emotions are and how they are represented in the brain.  Primarily, I researched rodents using Pavlovian fear conditioning of freezing behavior. I had no illusion about being able to study consciousness in rats. I assumed because emotional behaviors are conserved across mammals, I could understand the brain mechanisms that control unconscious emotional behaviors in humans through studies of rodents. I went on to spend much of my career exploring the role of amygdala brain circuits in controlling fear. But I did not go looking for the amygdala. My research just took me there.  

Over time, though, I became uncomfortable with the use of the term “fear” to describe both behaviors controlled by amygdala circuits and the conscious feeling of fear. I had, since the split-brain days, thought of conscious emotions as cognitively constructed experiences and realized it was confusing, in fact, wrong to use the term “fear” to name both. As a result, I began to vigorously pursue the distinction between fear and ancient survival behaviors controlled by the amygdala.

One implication of this line of thought was that the failure of psychiatry to deliver effective treatments for disorders involving fear was due to the conflation of the amygdala’s control of ancient defensive survival circuits with cognitive circuits that construct fear and other emotions. The two occur in parallel and are not one and the same. A related implication is that drugs that change rat behavior will never effectively relieve the feelings of fear or anxiety that prompt people to seek help in the first place.

These ideas led Nathaniel Daw and I to propose a new taxonomy for defensive behaviors that spanned various hierarchical levels of neural control in the brain:

Table 1: A Taxonomy of Defensive Behavior in Relation to Neural Control

• reflexes
• fixed action patterns
• instrumental habits
• instrumental goal-directed behavior
• consciously controlled instrumental goal-directed behavior

These levels were the foundation for the four realms idea. Specifically, in The Four Relams, I distributed the five behavioral categories Daw and I proposed within three of the four realms that depend on having a nervous system.

Table 2: How Behavior Relates to the Three Realms of Neural Control

• The Neurobiological Realm (reflexes, fixed action patterns, habits)
• The Cognitive Realm (non-conscious goal-directed actions)
• The conscious Realm (conscious goal-directed actions)

WHICH INSIGHTS IN YOUR BOOK DO YOU FIND MOST EXCITING?

I have found that when I write a book, I often discover ideas that were not on my mind, and that’s always exciting. Below are three that stand out from writing The Four Realms.

Insight 1: Turning the “two system” view of cognition into a “three system” framework.  As popularized by Daniel Kahneman, System 1 is fast and implicit (unconscious). It controls innate capacities we’ve inherited from and share with other mammals. But it also learns to form primitive conditioned associations, as occurs in fear conditioning. His System 2, by contrast, is slow and uses the deliberative powers of working memory to explore alternative plans and make decisions about actions. The upside of fast, unconscious System 1 processing is that it is computationally and energetically cheap. The upside of System 2 processing is that it makes possible purposive, conscious deliberation in pursuing goals. But this comes with high energy costs.

Table 3: Behavioral Control in the Two Systems Framework

• System 1: Fast and non-conscious. Includes reflexes, instincts, habits, and non-conscious cognitive control of behavior.

• System 2: Slow and Deliberative. Includes conscious control of behavior.

What I noticed was that System 1 was a grab-bag concept—it consisted of anything that was not conscious. The thing is, a lot of our cognition occurs unconsciously, which meant that non-conscious cognition was packaged with reflexes, instincts and habits. That seemed wrong. As a result, I proposed that the various processes might be fruitfully viewed as being distributed across three systems that reflect different levels of behavioral control by the brain.

Table 4: Behavioral Control in a Three Systems Framework

• System A: Behavioral Control that does not depend on Cognition or Consciousness. Included are reflexes, innate behaviors, and habits.

• System B: Behavioral Control that depends on Cognition but not on Consciousness. Included are non-conscious cognitive processes that control goal-directed behavior.

• System C: Behavioral Control that depends on Cognition and Consciousness. This encompasses conscious, volitional control of goal-directed behavior.

Insight 2: My Multi-State Hierarchal Neuroanatomical Framework for Understanding Cognition and Consciousness. The neural basis of consciousness is often talked about in terms of interactions between circuits in sensory areas of the cortex and cognitive areas of the lateral prefrontal cortex. While not wrong, this way of thinking is overly simplistic, especially given how much more we know about the neuroanatomy of the human cerebral cortex.

The fact is, a more complex anatomical arrangement of connections exists, forming a hierarchy of interactions, each of which creates different kinds of states that contribute to higher-order mental modeling and conscious experience. I call this a multistate, hierarchical framework for understanding consciousness. This is a framework more so than a theory. But, regardless of your preferred theory of consciousness, this framework gives you a richer perspective for thinking about your theory.

Insight 3: Human mentation, including conscious mentation, is a product of a-modal pre-conscious narrative streams. The very last part of the book is what I am most fond of. With the help of the multistate hierarchical framework, I was able to close the circle that I opened in my graduate work on consciousness in split-brain patients. The basic idea is that we use lower-order information processing, especially related to sensation and memory, to construct mental models in working memory when thinking and having experiences. That’s hardly novel. What’s new is the notion that at least two working memory mental models are in play simultaneously, one non-conscious (i.e., pre-conscious), and the other conscious.

The output of the pre-conscious mental model is a narrative stream that feeds three neural distributaries: of action, of verbal expression, and of consciousness. But the narration is not encoded verbally or visually, or in any other specific way. It is a-modal. That is, it is a generic neural code that reflects any and all lower-order inputs processed by the non-conscious mental model.

The key to the story is that the third distributary generates a second mental model in working memory, a conscious mental model, from which three additional consequences result. The first consequence is that the conscious mental model sends an a-modal summary of what you are conscious of back to the non-conscious mental model, closing the loop and allowing the content of your present conscious experiences to non-consciously contribute to what you are about to be conscious of. The other two consequences are that the conscious mental model, like the non-conscious mental model, generates feedforward outputs that allow you to control verbal and non-verbal goal-directed behaviors, but consciously.

You may be wondering what the evidence is for the idea that we act non-consciously. Think about it. Whether speaking or writing, words typically come out as sentences without you choosing each individual word and its grammatical placement—sometimes you do, but that is the exception. The same goes for bodily actions. Most of the cognitive work when talking, writing, or behaving occurs sub-rosa, that is, by non-conscious processes. Because conscious processes use a lot of energy, non-conscious control is the default mode.

Suppose I am correct that pre-conscious and conscious mental models can separately control overt behavior and verbal expression. In that case, the effort to scientifically understand consciousness may be even more complicated than we thought. Why? Because it would mean that we would not know, at a given moment, which mental model is in charge of what one says and/or does. This is indeed a complication, a challenge, for experiments on consciousness. But there may be an upside. Knowing the complications might help us to construct theories in which the complications are known features rather than impediments. Paraphrasing Sun Tzu in The Art of War, you have to know your enemy to win the battle. Indeed, knowing that verbal reports and behavior can be controlled consciously and pre-consciously may help explain why scientific solutions to the nature of consciousness seem so elusive. It might also help explain why it is so difficult to resolve debates about whether the sense that we consciously control our actions is illusory or real. From this perspective, the feeling of conscious agency that comes with responses produced by the pre-conscious mental model would be illusory, but actual conscious agency would be involved when the conscious mental model controls the responses.

I can now suggest what might have been going on in the brain of PS. When we asked him why he did what he did, his momentary self-awareness was colored with tacit dissonance (a feeling of wrongness) since he (his talking left hemisphere) did not generate the response. In a flash, a pre-conscious mental model emerged and spun a mentalese narrative that flowed to his speech-control processes, allowing an explanation to roll off his tongue non-volitionally. Simultaneously, the mentalese narrative spawned a conscious mental model that interpreted his speech as an adequate explanation of why he did what he did, thereby reducing any unsettling feeling of dissonance and making him consciously feel that “he” was in charge of his actions and speech. He was not simply covering up a neurological deficit. He was just doing what we humans do all the time. We construct narratives to make sense of the plethora of unconsciously controlled actions we emit.

WHAT’S NEXT FOR YOU?

On August 31, 2025, I will retire from NYU. Friends and colleagues, and even strangers, have cautiously asked me: “What are you going to do with all that time?” I assure them that have no concerns, as my three favorite activities will keep me quite busy. One is writing. I am working on a memoir (which is almost done), a novel (which is a mere idea at this point), and various articles about consciousness. Another activity is lecturing. For some reason, I have more invitations to speak in the U.S. and abroad for 2025 than I have ever had. The third is music. I never tire of listening to, playing, and writing songs. Now, all I have to worry about is what the hell is happening, and going to happen, to the world.

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