Feeling down or depressed from time to time happens to most people, but usually passes, and the person can improve his or her mood naturally. However, some people cannot break out of a depressed state over an extended period of time. In those cases, a person is considered to have clinical depression, a psychological diagnosis
In reality, there is much research demonstrating that depression is neurological, not psychological. Certain brain patterns are linked to depression.
There are a lot of different brain patterns for depression. One of the most common is called alpha asymmetry, which is too much alpha (especially 10-12 Hz) in the left hemisphere compared to the right. Another common one is excessive alpha in the front of the head. Others are very low alpha levels in the back of the head, or failure to block alpha with eyes open or at task, or any of a variety of tone strategies.