Depression is an emotion that all of us experience at some time in our lives, but Depression is also an illness that afflicts many millions of people worldwide. When we become depressed or sad because of something upsetting that has happened to us, this is an expected and normal response to our circumstances. In depression-the-illness we become sad at times there is nothing to be sad about or we become sadder than our situation warrants.
We call depression-the-illness “Major Depression” in psychiatry. It is often accompanied by other symptoms, including difficulty enjoying life, alterations in our sleep and appetites, poor energy and reduced activity, feelings of worthlessness and guilt, and sometimes even thoughts that we would be better off dead. Interestingly, Major Depression requires either that a person be unexpectedly sad or that they be less able to experience pleasure and joy than is usual for them. That “either, or” opens the way for a person to meet criteria for Major Depression and not to feel sad (based on the presence of a reduced experience of pleasure accompanied by the other required symptoms).
Our emotions give salience to our emotions. For example, if we unexpectedly pass a person on the street who was a close friend in high school, we may feel surprise, happiness, excitement, and other positive emotions that distinguish that person to us from the next person whom we have never seen before in our life. Our daily experiences of work, love, friendships, etc., are similarly colored by our emotional responses to them that in turn give these experiences meaning and importance, or lack of importance, to us. When our feelings of sadness become detached from our experiences, that is, when we feel sad when there is little to really feel sad about (“my life is so good – why, then, do I feel so sad”) or when we feel sadder than we should feel in the context of an experience (“it really wasn’t such a big deal, but I just can’t get it out of my mind”), this represents a distortion of our perception. Not only do we feel unnecessary pain and distress during these times of depression, but decisions we make based on these feelings may not accurately reflect our world and therefore may lead to other problems, like loss of relationships, jobs, friendships, etc, that in turn reinforce or maintain these very feelings of depression.
Major Depression, like most psychiatric disorders, reflects subtle malfunctioning of the central nervous system. Studies, for example, of identical and non-identical twins have shown that when one identical twin is depressed, the other is depressed about 55 % of the time. By contrast, when one non-identical twin is depressed the other twin is depressed only 15 % of the time. The rate of Major Depression in the general population is about 5 % at any given time. This is strong evidence that at least some depressions have a genetic basis. Other studies of the brains of depressed people have shown changes in the functioning in the frontal cortex, cingulate gyrus, hippocampus, and other structures of the central nervous system. Alarmingly, when the brains of people who have been depressed are compared to the brains of people who are not depressed, the hippocampus and amygdala actually have less volume, suggesting that when we are depressed parts of the brain we use less become smaller, possibly by losing neurons. This is additional evidence that Major Depression has a biological basis in many cases. This is also consistent with our clinical experience that the more time a person spends depressed the harder the depression is to treat, suggesting that being depressed is bad for the brain. By contrast, recent work has shown that antidepressants (and lithium) increase Brain Derived Growth Factor (BDNF), a protein in the central nervous system known to facilitate the healthy maintenance and even growth of normal nervous tissue in the brain.