Normal and Abnormal Mood States
Mood disturbances in depression lead us to feel persistently and pervasively low or unable to experience joy or pleasure. When depressed, we may be pessimistic and tend to view the “glass half empty” rather than half-full. We may be so devoid of hope that we see no reason to go on living, our future (and past) colored in a negative light as far as the eye can see. Our sense of physical vitality and health fades, and seems nowhere to be found. In addition, there are changes in our emotions: we may be frequently sad or tearful, anxious, irritable, or incapable of laughter and the positive feelings associated with it. Reduced appetite, weight, and sexual interest, as well as impaired sleep, represent disturbed drives that are often experienced in depression. We might represent an episode of depression as a line remaining below a baseline, normal mood state, until an eventual return to normal in this example:
Each point on this line represents total depression severity at a given point in time, by adding up a score of individual symptoms and rating their severity. People with major depression have severe enough symptoms that their score exceeds a certain severity threshold (dotted line). There are numerous rating scales for depression that could be used in this way. For example, here is the Zung self-rated scale. Here is a modified Young Mania rating scale, which would allow a similar score for the presence and severity of manic symptoms.