Why Do We Separate Psychiatry and Neurology?

As advances in neurobiology and genetics reveal complex associations between brain structure, function, and the symptoms of mental illness, there have been renewed calls to reposition mental illness as disease of the nervous system. This is highlighted in public statements by prominent figures in American psychiatry, such as Thomas Insel's assertion that mental illness is brain disease and Eric Kandel's proposal to merge psychiatry with neurology.

The psychiatrist and philosopher Karl Jaspers argued it is a logical error to hold that mental disorders are reducible to diseases of the brain.

Source: Public domain

The relationship between psychiatry and neurology has always been a fascinating and contentious one, and these debates surrounding the relationship between mental and neurological disease are nothing new. Almost two hundred years ago, the eminent neurologist and psychiatrist Wilhelm Griesinger (1845) insisted that "all mental illnesses are cerebral illnesses," an argument that is echoed in more recent assertions like those of Insel and Kandel.

In contrast, the psychiatrist and philosopher Karl Jaspers (1913), writing almost a century after Greisinger, argued that "there has been no fulfillment of the hope that clinical observation of psychic phenomena, of the life-history and of the outcome might yield characteristic groupings which would subsequently be confirmed in the cerebral findings" (p. 568).

A recent paper published in the Journal of Neuropsychiatry and Clinical Neurosciences begins, "While most organs have one dedicated medical specialty, the brain has been historically divided into two disciplines, neurology and psychiatry" (Perez, Keshavan, Scharf, Boes, & Price, 2018, p. 271), squarely positioning psychiatry as a specialty that deals with diseases of the brain.

I argue that these proposals to reclassify mental illness as neurological disease are based on a basic category error and that the distinction between psychiatry and neurology is not an arbitrary one.

This is not to deny physicalism, that is, that the mind exists because of the brain, and I submit that it is possible to simultaneously accept that the mind is a function of the brain and that mental disorders are not reducible to brain disorders. To do this, let us first examine the difference between mental and neurological illness and then evaluate the claim that mental disorders can be reduced to the pathologies of the brain.

Neurological diseases are, by definition, diseases of the central and peripheral nervous system, and they can generally be identified on the basis of objective medical testing, such as electroencephalography for epilepsy and magnetic resonance imaging for a brain tumor. Many neurological diseases can be localized, meaning found to exist as a lesion in a particular area of the brain or nervous system. While some neurological diseases can cause mental symptoms, such as changes in mood or perception, neurological illness is not chiefly associated with these psychological abnormalities, and they exist secondary to the disease's deleterious effects on the nervous system.

In contrast, mental or psychiatric illness is characterized by a clinically significant disturbance in an individual's thoughts, feelings, or behaviors. The Diagnostic and Statistical Manual of Mental Disorders is theoretically neutral on the cause of mental disorders, and, despite claims to the contrary by antipsychiatrists, organized American psychiatry has never officially defined mental illness as "chemical imbalance" or brain disease (see Pies, 2019).

While many advances have been made in the realms of neuroscience and genetics that aid our understanding of mental illness, there remains not a single identifiable biomarker for any mental disorder. Historically, mental disorders have been considered functional diseases, due to their impairment of functioning, rather than structural diseases, which are associated with known biological abnormalities. The American Psychiatric Association (2013) defines mental disorders this way:

A mental disorder is a syndrome characterized by clinically significant disturbance in an individual's cognition, emotion regulation, or behavior that reflects a dysfunction in the psychological, biological, or developmental processes underlying mental functioning. Mental disorders are usually associated with significant distress in social, occupational, or other important activities (p. 20).

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Thus, while neurological diseases can generally be identified via objective biological means and may present with psychological sequelae, psychiatric diseases or mental disorders cannot be identified biologically and are defined by their disturbances in thought, emotion, and behavior.

Another distinguishing characteristic of mental disorder is its relationship to environmental factors. Whereas a variety of life circumstances and experiences are associated with the development and course of mental disease, such as childhood trauma, no significant relationship exists between environmental-psychological factors and the onset of neurological disease. While psychotherapy is a treatment for mental illness, it makes no sense to speak of psychotherapy as a treatment for brain cancer or stroke.

The now-abandoned Norwich State Hospital in Norwich and Preston, Connecticut.

Source: Norwich Hospital District, used with permission.

Perhaps the best example of the difference between mental and neurological disease is conversion disorder, now also referred to as functional neurological symptom disorder. Conversion disorder, previously subsumed under the broad category of hysteria, was fundamental in Freud’s development of psychoanalysis. It's characterized by the presence of neurological symptoms, such as weakness, paralysis, or blindness, without organic (neurological) cause. Conversion symptoms are often precipitated by psychological trauma and are interpreted psychoanalytically as a hidden or symbolic means of communication. Unable to express themselves via conventional means, patients with conversion disorder resort to a symbolic protolanguage to communicate their psychological conflicts.

Nevertheless, there have been recent attempts to reduce conversion disorder to dysfunction of neurobiology, and research has shown dorsal anterior cingulate cortex structural alterations in conversion disorder patients compared to controls (Perez et al., 2018). But the question we must ask ourselves is, is this where we think the locus of conversion disorder is? Or is this merely a neurological correlate of a psychological phenomenon?

I argue that the locus of mental disorder is better conceived as the person rather than the brain. Whatever the etiology of mental disorder—whether it is genetic, the product of gene-environment interactions, psychological, or social—we identify it on the basis of its disruption to the person's mental life or relationships. This is not so with neurological or other physical diseases, which can be identified on the basis of structural abnormality and often presents without any disruption at all to the person or the self.

Banner (2013) summarizes it this way:

There is much talk in the empirical literature about brain 'abnormalities', 'deficits' and 'alterations' associated with mental disorder symptoms. But conceptually, what marks these differences as indicative of disorder or dysfunction? It is the association they bear with deviations from epistemic, evaluative, emotional, moral and social norms of functioning, in other words … person‐level [emphasis added] constructs… (p. 512).

Conceiving of mental disorders as disorders of the person, who has a brain as well as a complex psychosocial environment—rather than as diseases of the brain—does not discount the many significant advances made through biological and genetic investigation. Nor does such a conceptualization rely on an erroneous mind-body dualism or negate the effectiveness of biological therapy. To the contrary, conceiving of mental disorders in this way is both philosophically sound and places us in the best position to treat them, from a unified biopsychosocial perspective.

As psychiatrists are first and foremost medical doctors, there is an understandable hesitation to construing mental illness in this manner. If mental illness is not a brain disease, then what organ is affected? Is mental illness merely a metaphor, or fake disease as the psychiatrist Szasz argued?

A thorough analysis of the meaning of disease (dis-ease) in the history of medicine reveals that the classification of a particular entity as a disease requires no known, or putative, biological cause, and the classification of all diseases, not just mental ones, relies on a subjective determination regarding abnormality. What is of consequence in disease classification is the degree of suffering and incapacity rather than the presence of a biological lesion. Thus, to accept that mental disease is a disease of the person rather than a disease of the brain is not to say that mental diseases are not diseases at all.

Rather than attempting to reorient itself as a specialty of brain science alongside neurology and neurosurgery, psychiatry would be better served by orienting the rest of medicine towards itself, as a specialty that operates uniquely as a multi-level science. Regardless of future advances in neurobiology, psychiatry will remain the specialty that deals with the complex problems of the individual person (mental disorders), problems that are qualitatively different from those treated by neurologists—and it should embrace its role in this regard.