Among the inhabitants of my household is a five-year-old pug, Alfred. After a somewhat rocky start, my relationship with Alfred has blossomed: When I return home—much less feed him or take him for a walk—he scampers around appreciatively, snarling and grunting in the manner I’ve come to associate with the expression of canine excitement. But any fondness that Alfred holds for me is far eclipsed by his relationship with my wife, with whom he has lived his entire life. When she leaves home, Alfred wanders the apartment listlessly before finally taking up a spot that affords him an unobstructed view of the front door. And there he sits, staring intently, alert to the faintest hint of his rightful owner’s return. Reflecting on his behavior, my wife and I often find ourselves speaking of how worried poor Alfie seems in her absence; occasionally, in a fit of diagnostic excess, we declare that he is suffering from separation anxiety.

As a historian of philosophy and mental health, such talk leaves me uneasy. What does it mean to say that an animal, even one as keenly attuned to its surroundings as Alfred, is apprehensive or concerned? Surely the experience of such states requires a capacity for self-reflection lacking in animals. Isn’t my speculation about Alfred’s psychological states just the crudest anthropomorphic sentimentality? My wife has no such qualms. Alfred is anxious; what more is there to say?

Quite a lot, it would seem, at least according to Joseph LeDoux’s Anxious. Currently the Henry and Lucy Moses Professor of Science at New York University, LeDoux has been grappling for the better part of the last four decades with questions of how emotions are processed by human and animal brains. As a graduate student at the State University of New York at Stony Brook in the mid-1970s, LeDoux and his supervisor, Michael Gazzaniga, conducted pioneering research on patients with split-brain syndrome, a condition in which the flow of information between the two hemispheres of the brain is disrupted, resulting in profound shifts in behavior. Based on their experiments, LeDoux and Gazzaniga concluded that much of the emotional processing that takes place within the brain occurs nonconsciously—that is to say, prior to the organism becoming actively aware of the stimuli producing the response.

LeDoux’s work on nonconscious emotions led him to take up the question of how the brain processes fear. After carrying out a series of experiments on the brains of rats, his attention was drawn to the amygdalae, two clusters of neurons located deep inside the medial temporal lobe of complex vertebrates such as rodents and humans. LeDoux has been researching and writing about the amygdala ever since; indeed, such is his dedication to all things amygdalar that he even named his rock band, in which he plays guitar and sings, the Amygdaloids (the band is a recurring feature of Anxious).

Though discovered in the early 19th century, the amygdala wasn’t first associated with fear (and, even more, aggression) until the late 1930s, when the neuroscientists Heinrich Klüver and Paul Bucy began experimenting on rhesus monkeys whose temporal lobes—and hence amygdalae—had been removed. Following their lobectomies, the monkeys exhibited less aggression and fear; where before they’d been afraid of humans, now the monkeys approached the doctors without hesitation, even allowing themselves to be touched. Other changes in behavior also followed, including the development of an oral fixation and a marked increase in sexual activity (at times these traits overlapped, with Klüver and Bucy noting that the monkeys would occasionally fall asleep with their erect penises in their mouths). More studies related to the amygdala appeared in the 1950s, but for several decades it remained obscure even to scientists.

All that changed with LeDoux’s 1996 book The Emotional Brain, which helped to bring the amygdala to the attention of the public at large. It was here that LeDoux identified the amygdala as the brain’s “fear system,” responsible for both the nonconscious detection of threats (as well as the coordination of the organism’s defensive response) and the emergence of the conscious feeling of fear. The amygdala, in this sense, is like an alarm that silently calls the police before informing the homeowner that this is indeed the time to panic. In the wake of LeDoux’s book, the amygdala became a cultural phenomenon, popularized in countless articles with headlines like “Fearless Woman Lacks Key Part of Brain” and “Crayfish, Like Humans, Are Anxious Worrywarts.”

Though doubtless a boon to human-crayfish relations (“Crustaceans Can Get Anxiety, So Maybe We Shouldn’t Boil Them Alive,” declared an article published in 2014, before asserting that invertebrates possess the capacity to feel “true anxiety,” whatever that might mean), such headlines—­and, more signifi­cantly, the science that underpins the accompanying stories—­fundamentally misconstrue the amygdala’s place in the emotional lives of humans, LeDoux has now come to believe. Given his role in popularizing the idea that the amygdala is the brain’s “fear center,” his new book can be read as something of a mea culpa—though not entirely, because he is keen to point out that the misunderstanding lies less with him than with his readers.

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Two related claims lie at the heart of Anxious. The first is that a conceptual muddle has, in recent years, plagued our understanding of states like fear and anxiety. When I tell a friend that I’m anxious about an upcoming exam or interview, I am using that term in its every­day, received sense: I’m declaring that I consciously feel worried or concerned about an impending event, and that this feeling is accompanied by various symptoms ranging from mild preoccupation to sleeplessness, loss of appetite, and panic attacks. Implied in such statements is the idea that I recognize myself as the entity experiencing these feelings and have, in some sense, reflected upon them.

But, as LeDoux points out, when scientists discuss states like fear and anxiety, they are usually not referring to this common-sense understanding; rather, they are referring to nonconscious systems of threat detection and reaction. As noted above, the brain is capable of detecting a threat, processing it, and initiating a response without our ever becoming consciously aware of the grizzly bear lurking just beyond the trees. There are various threat responses at the disposal of the organism, including the classic trio of freeze, flight, and fight; the regulation of energy; a rise in heart rate, etc. This defensive system is essential to the basic survival of all complex organisms, and the amygdala lies at the heart of it.

The confusion, according to LeDoux, comes when this brain system is straight­forwardly equated with the feeling of fear or anxiety. Although brain systems like the amygdala create the basis for the conscious feeling of fear and other emotions, the two are distinct; indeed, one may say that they constitute entirely different ontologies. LeDoux spends a significant portion of Anxious outlining the many ingredients that must be added to nonconscious brain systems before emotions like fear and anxiety can be experienced. Emotions are what neuroscientists term “cognitively assembled conscious feelings”—that is to say, the human mind actively constructs emotions out of a variety of components in a process LeDoux likens (not entirely convincingly) to the notion of “bricolage” advanced by the French anthropologist Claude Lévi-Strauss. Just as the bricoleur arranges her tools in a variety of different configurations in order to meet the ever-shifting demands of her environment, the brain uses the material available to it—the flow of sensory input from the outside world, but also various forms of memory, body-response feedback, and the ability to organize information into categories via the use of natural language—to produce emotions.

The feeling of fear thus occurs when a threat detected by our nonconscious brain is interpreted by our cognitive systems and rises to the level of consciousness. In LeDoux’s words, “The amygdala circuitry…does not make fearful feelings; it detects threats and orchestrates defensive responses to help keep the organism alive and well.” Fear, in short, is the emotional response, not the process by which a given stimulus is identified as a threat. Prior to our being consciously aware of a threat, LeDoux contends, we cannot meaningfully describe what is taking place as fear. Scientists are aware of the distinction between nonconscious brain systems and conscious feelings, but often refer to the latter when they are in fact discussing the former. Because the public and many journalists lack the expertise to distinguish between the two levels of feeling, the result has been a widespread misunderstanding as to how the brain functions and what role it plays in producing emotions. LeDoux therefore concludes that we ought to restrict the use of words like “fear” and “anxiety” to the conscious experience of those feelings, and that scientists working on brain systems should adopt a different vocabulary.

The second major claim advanced in Anxious is that it’s a mistake to base conclusions about the human experience of feelings like fear and anxiety on evidence gathered from animals. When scientists carry out experiments designed to assess fear levels in laboratory rats, for example, they are measuring fluctuations within the brain system rather than the conscious experience of fear. On the surface, this seems quite obvious: After all, whatever their cognitive capacities—and LeDoux devotes significant space to the current debates regarding the existence and meaning of consciousness, especially self-consciousness, in animals—they lack the ability to self-report their emotions (assuming they can be said to possess them in the first place—more on this below). Impressive though his capacity to communicate a desire for Iams Woof Delights may be, Alfred has yet to engage me in a meaningful exchange as to his feelings of existential dread. But humans can and in fact do report our feelings on a daily basis, and we consciously recognize our fear and anxiety to be problematic in ways that animals do not.

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From an evolutionary perspective, then, LeDoux is arguing that what we’ve inherited from our animal forebears is not fear, per se, but rather a circuit that nonconsciously detects and responds to threatening stimuli. This makes a great deal of intuitive sense: Fear, as an emotion, is considerably less useful to an organism’s survival than the system that identifies and responds to threats in the first place. As many soldiers have reported, the hesitation and indecision that can result from the conscious awareness of, and reflection upon, a dangerous situation often serves as an impediment to survival. A survival circuit is exactly that: a system that aims to keep an organism alive in the face of a variety of threats. How the organism feels about those threats (or anything else) is not only irrelevant to this aim; it is a fundamentally different kind of question.

For LeDoux, the problem once again comes when scientists fail to recognize the distinction between nonconscious brain systems and conscious feelings and draw conclusions about the latter from animals that, in all likelihood, have no conception of what fear and anxiety feel like. Complex organisms are capable of producing a physiological reaction to a stimulus without ever being conscious of the threat itself, so we cannot infer from Alfred’s whimpers that he actually experiences fear. It’s possible that he does, but the evidence, according to LeDoux, generally suggests otherwise. In the absence of certainty, LeDoux holds that the only responsible course of action is to proceed as though the capacity to consciously experience emotions is unique to humans. This line of argument is not quite the conversation stopper that LeDoux suggests. The vexed nature of equating human and animal physiology was acknowledged as far back as the 17th century, and it seems unlikely that many contemporary scientists have forgotten the lesson. Either way, it’s hard to see how awareness of the difficulty would impact the daily practice of neuroscientists and psychologists; LeDoux’s call for linguistic clarity holds valid whether the research is being conducted on humans or animals.

LeDoux’s skepticism regarding the emotional capacity of animals has consequences beyond settling a dispute in the present author’s household. When scientists design pharmacological solutions to anxiety disorders, they primarily target the non­conscious brain system, leaving the subjective feeling of anxiety largely untouched. Such, LeDoux claims, is the case with anxiolytic drugs such as Valium, Xanax, and Klonopin. Given that almost 20 percent of adult Americans (roughly 40 million people) are affected by an anxiety disorder, of whom many are prescribed medication, this is no small matter. Even if these medications do work—and the jury is still very much out when it comes to the efficacy of benzodiazepines and SSRIs—the mechanism by which they affect our conscious emotions remains obscure. (Their effect on our collective wallet is not: Americans spend almost $10 billion a year on antidepressants alone.) Although he is clear that research based on animal physiology is indispensable to expanding our understanding of how human brains function, LeDoux concludes that the uniquely human dimensions of fear and anxiety must be recognized for us to fully understand and treat them.

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Anxious has been interpreted in some quarters as a call for a return to a more philosophical, specifically existential understanding of anxiety. If that is LeDoux’s intention—and, in truth, it’s difficult to say—his discussion is woefully underdeveloped. Few would doubt that LeDoux is a highly accomplished neuro­scientist with a formidable understanding of the evolutionary development and functioning of the brain. He is also a gifted writer with a rare talent (especially among scientists) for rendering complex ideas in a manner both intelligible and engaging for the lay reader. But he is neither a philosopher nor a historian, and he merely gestures at the potential contribution of a genuinely humanistic perspective to the contemporary understanding and treatment of anxiety.

Early in the book, LeDoux notes: “Scientists and mental health professionals today are greatly influenced in their views of fear and anxiety by both Freud and Kierkegaard.” This is patently untrue, at least if LeDoux means a conscious recognition of (much less reflection upon) the ideas of those thinkers. Contemporary scientists generally understand fear as the recognition of an identifiable and present threat, and anxiety as the anticipation of an undefined future threat: definitions that bear only the slightest resemblance to Kierkegaard’s and Freud’s conceptions of those states, which themselves differ considerably. Although it was not always this way, the aims, methods, and language of humanistic endeavors like psychoanalysis and philosophy have been almost entirely expelled from the discourse of modern science and medicine. To the extent that they reflect upon it at all, most scientists greet this situation with relief, if not outright joy, at being rid of the unsystematic and unquantifiable speculations of their former associates.

Were LeDoux and his colleagues as familiar with the contributions of thinkers like Kierkegaard as he suggests, however, they might have been exposed to alternative ways of understanding states like anxiety. For just over a century—beginning with the 1844 publication of Kierkegaard’s The Concept of Anxiety, to the rise of psychopharmacology in the 1950s and ’60s—a group of existential philosophers, theologians, psychiatrists, and psychotherapists developed a distinctive conception of anxiety. Rather than viewing it in purely negative terms as a malady of the soul, as ancient and medieval audiences did, or a pathology to be conquered, as contemporary medicine does, these thinkers saw in anxiety the potential for something constructive.

Although it could be debilitating, even terrifying, anxiety was regarded by the likes of Kierkegaard, Martin Heidegger, Kurt Goldstein, Paul Tillich, and Reinhold Niebuhr as the price we pay for some of the values we hold most dear, including freedom, imagination, and creativity. There is, according to the existentialists, a positive correlation between anxiety and human potentiality; as Kierkegaard put it, “the more profoundly he is in anxiety, the greater is the man.” The perceived association between creativity and anxiety was especially strong in postwar America. Rollo May, a psychologist and the author of 1950’s The Meaning of Anxiety, declared: “One has anxiety because it is possible to create.” Patients, he continued, should be encouraged to recognize that the “presence of anxiety means a conflict is going on, and so long as this is true, a constructive solution is possible.”

Any notion of a positive understanding of anxiety has almost entirely collapsed over the last three decades. To be sure, there are legitimate reasons to be skeptical of the kind of treatment advocated by May and other existentially oriented therapists. Merely asserting a link between anxiety and artistic fulfillment, for example, does little to help a patient in the grip of a panic attack. But the proposition that anxiety can be channeled into constructive behavior or activity is surely an idea worth considering—one with the potential to provide comfort and hope to many thousands of patients, especially those whose disorders are comparatively mild. That LeDoux is almost entirely silent on the matter is especially puzzling given his call for an understanding of anxiety that encapsulates the everyday experiences of human beings. The omission is even more glaring considering that his suggestions for treating anxiety are limited to medication and meditation. Likewise, LeDoux does not consider the idea, advanced most persuasively by the political theorist Corey Robin, that fear is a political idea as much as a conscious state.

LeDoux’s inability to recognize alternate ways of conceiving of fear and anxiety is perhaps related to his very serious failure to adequately distinguish between the two concepts themselves. In the book’s preface, he notes that “fear and anxiety are complexly entwined, and must be understood both separately and together.” While he does differentiate between them early on, explaining that each is governed by a different mechanism of the brain, LeDoux essentially treats the two states as synonymous. In fact, although the book is titled Anxious, its overwhelming focus is fear. On the rare occasions when anxiety takes center stage, LeDoux regards it as virtually the same as other emotions like worry and concern, even going so far as to point out that there are “more than three dozen English words that are either synonyms, variants, or aspects of ‘fear’ and ‘anxiety.’”

But anxiety is not the same as fear or worry or panic. It conveys something distinctive and important: a kind of ineffable and elusive, yet instantly recognizable, dread that strikes at the core of what it means to be human. The faculty of language offers us the ability to make fine distinctions between ostensibly interchangeable terms. In the case of anxiety, we would be well advised to distinguish it from the concepts that masquerade in its place. LeDoux’s tendency to conflate anxiety with fear is especially unfortunate in view of his call for scientists to more accurately differentiate between the conscious, everyday feelings of fear and anxiety and their nonconscious equivalents. It also perhaps suggests the limitations of even the most avowedly humanistic scientific accounts of human emotions.