Is depression a brain disease ?

A kuro5hin.org article on 'Demystifying depression' gives an excellent account of the experience of depression, but uncritically repeats some common assumptions about the condition - namely that it is a 'physical illness' caused by 'low serotonin'.

Despite the familiarity of these claims, both are problematic.

* * *

The article by an author entitled Name of Feather takes a comprehensive look at clinical depression, and vividly describes the experience at the heart of the author's malady. It is also abound with good advice, such as seeking the help of a competent well-informed professional early in an episode.

It also attempts to describe what causes depression but makes several points that are often repeated as facts, but have surprisingly little support, or are highly controversial in the scientific literature.

Depression as a physical disease

The author asks us to "forget purely psychological explanations of the illness", "clinical depression is a physical illness" and claims that dualism, the idea that mind and brain are separate entities, is responsible for this false view of mental illness.

On a pragmatic level however, clinical depression is defined as mental phenomena. The criteria used by psychiatrists for diagnosing a Major Depressive Episode lists 'depressed mood' or 'loss of interest or pleasure' as the core feature and the majority of the additional features are purely psychological in nature.

If we want to believe that depression is a purely 'physical disease', then we could in fact feel pushed into dualism. Perhaps thinking that depression affects the brain and somehow the separate mind reacts to this impairment of thinking or emotion to produce the conscious experience of depression.

More likely, the view that depression is purely a physical illness reflects a school of thought known as epiphenomenalism, which argues that the mind has no causal effect at all, and is just the subjective experience of our brain at work.

However, both of these theories are roundly rejected by the majority of contemporary neuroscientists, psychologists and philosophers.

The most common view is that mind and brain are exactly the same sort of thing, but described at different levels of explanation - a school of thought known as property dualism. In other words, the mind is changes in the physical structure of the brain, and changes in the physical structure of the brain are the mind.

To make an analogy, no-one would deny that the economic system exists in the physical world, but to try and explain unemployment in terms of atomic physics would be folly, as would trying to solve economic problems by using a particle accelerator. In a similar way, we can accept that the mind and brain are both based in the physical world, but explaining the mind, or mental illness, purely in physical terms, may not always be appropriate or useful.

In a recent article for the American Journal of Psychiatry psychiatrist Kenneth Kendler cautions against exactly these sort of simple 'physical' explanations for mental illness and argues that comprehensive explanations and treatment will have to involve both psychological and biological theories.

If the logic of this argument is not convincing enough, recent studies have shown that psychotherapy has a measurable influence on brain function, with the neuroscience of psychotherapy now becoming an exciting complement to the vast amount of research on the psychological effects of physical treatments.

Depression as an illness of 'low serotonin'

In Name of Feather's article, he or she suggests that depression is caused by exhausting levels of serotonin in the brain. Unfortunately, there is little support for this simple theory.

If depression is nothing more than low serotonin, drugs that specifically lower serotonin levels in the brain should lead to depression or at least low mood. Studies which have tried this in both healthy participants and depressed patients show remarkably little effect on mood, with a mild dysphoria being the only occasional effect.

Furthermore, drugs which increase serotonin levels in the brain typically do not start having an effect on mood for several weeks, despite affecting serotonin levels immediately.

It is likely that serotonin plays some role in mood, but in a recent article for Nature Reviews Neuroscience, neuroscientist Eero Castrén criticises the oversimplified view of depression, stating:

Over the last few decades, the view that depression is produced by a chemical imbalance in the brain has become widely accepted among scientists, clinicians and the public.

However, during the past decade, several observations indicated that there might be an alternative hypothesis to the chemical view of depression. This network hypothesis proposes that mood disorders reflect problems in information processing within particular neural networks in the brain and that antidepressant drugs and other treatments that alleviate depression function by gradually improving information processing within these networks

It is notable that Name of Feather does mention an information processing approach to understanding depression, although it is important to note that this theory is a more complex and nuanced explanation than a simple 'low serotonin' theory can support.

Should we be cautious of purely biological theories of mental illness?

One motivation sometimes given for stating that mental illness is a purely 'physical disease' is to draw parallels with physical ailments, to try and make mental illness less stigmatised. Nevertheless, some research has suggested that purely biological explanations might have the opposite effect.

One study asked groups of participants to give their views on a person describing their experiences of mental illness. In one group, participants were subsequently given a biological and genetic explanation of mental illness, in another, they were given a social and psychological explanation. The group given the biological explanation were much more likely to rate the person as dangerous and unpredictable. Other research has suggested that clinicians with a purely biological perspective are likely to rate patients as more disturbed than other clinicians.

So why do simplified theories - like the 'low serotonin' theory of depression, persist - despite overwhelming evidence to the contrary ?

One view is from noted psychiatrist and psychopharmacologist David Healy who has criticised drug companies for promoting simplified biological theories of mental illness that seem to imply the primacy of drug treatments while ignoring social and developmental factors, which are known to be important influences in the development of mental illness.

Focusing specifically on depression and the development of antidepressant medication in his book The Antidepressant Era, he argues that drug companies have spent as much time marketing diseases as treatments, and laments the influence of pharmaceutical companies on scientific understanding.

Healy's views are not without controversy and need more unpacking than is space for here, although perhaps we can forgive overworked clinicians for seeing the attraction of simple 'one sentence' explanations for mental distress, despite the obvious complexity of the issue.

Conclusion
It is clear from the scientific literature that a purely biological theory of mental illness is not sufficient to explain and treat the experience of mental distress. Furthermore, simplified theories, that argue, for example, that depression is 'caused by low serotonin' are lacking in support and best avoided.

Psychological factors are equally important as biological factors in both the treatment and understanding of mental distress. Denying one or the other will undoubtedly slow scientific progress and lead to further misunderstanding of ourselves and each other.