Understand the brain through damage

The brain is not well-understood. We are gradually uncovering its mysteries, but there is a long way to go. One interesting way of finding out about the brain is looking at what happens when it goes wrong. A common way of doing this is looking at what happens after a stroke, when a specific brain region is damaged. The resulting behavioural deficits can tell you about the role of that damaged region.

There are some fascinating and debilitating conditions that come about as a result of brain lesions. For example, bilateral damage to a region of the brain called the parietal lobe can result in simultanagnosia, the inability to perceive more than one object at a time: sufferers are unable to recognise more than one object in their visual field. Damage to a different brain structure, the amygdala, can result in reduced ability to feel emotion. Since patients with amygdala damage suffer these social and emotional deficits, neuroscientists have dubbed the amygdala the ‘emotional centre’ of the brain.

But the brain can go wrong in other ways when there is no obvious damage. Psychiatric disorders such as depression must be caused by a disturbance in biological function in the brain, if you don’t believe in dualism (the belief that the mind and body are separate). If the mind solely arises from activity within the brain, then any problem or characteristic of the mind must be due to something happening in the brain. However, the cause is not as simple as a damaged part of the brain; something more subtle is going on.

Depression is a very common disorder, affecting 10-15% of the population. No-one knows the cause, but there are several hypotheses as to the biological basis. The classic theory is that it is caused by a deficit in serotonin, a neurotransmitter that supposedly regulates mood. Evidence for this came from the fact that drugs that increased serotonin levels in the brain (SSRIs – selective serotonin reuptake inhibitors) like Prozac can help treat depression. However, there seems to be more going on. SSRIs don’t always work, and other treatments like cognitive behavioural therapy can help depression without tackling any biological deficits. Clearly there are many environmental and social factors contributing to depression. There might be biological susceptibility (having specific versions of certain genes does make having depression more likely), but as with everything, this can be enhanced or diminished by the environment.

Though the cause may not be clear, you’d still think the definition of depression would be simple: an abnormally low mood. But there are problems with this definition – how do you define when low mood becomes abnormal?  There is no naturally occurring cut-off between someone who’s in a negative mood a lot of the time and someone who is clinically depressed. And what about the other end of the spectrum?  Are there people who have abnormally high mood? Bentall (1992) wrote a paper making a case for happiness being classed as a disorder, since it is uncommon and probably reflects processing abnormalities, but was not currently characterised as a disorder because it was “not negatively valued”. This paper was, of course, a joke (or what passes for a joke in the scientific community), but does raise interesting questions about how we judge mental illness.

This lack of clarity about where abnormality begins makes it difficult to diagnose psychiatric disorders.  A blood test cannot tell you whether you have depression. And an incorrect diagnosis can have implications: on a medical level you may get incorrect medication, and on a social level a diagnosis of mental illness could have affect your self-perception, especially considering the social stigma unfortunately still attached to mental illness. In medicine it is almost always better to have a diagnosis; in psychiatry this isn’t always the case.

Brain damage has told us a lot about the role of various regions of the brain by showing what happens when they go wrong.  But psychiatric disorders remind us that we don’t really know anything.  Working out what each brain area does is all very well, but disruptions in normal processing can occur without any clear damage.  And to understand these disruptions we need to know what normal is, which isn’t always easy.