These are norepinephrine, dopamine and seratonin. Depressive is caused by a deficiency of one or more of these neurotransmitters. An interesting research tool has been developed for studying various illnesses including depression. This apparatus can actually evaluate the intensity of brain activity. It has been helpful in showing how depression effects the brain.
Depression reduces the intensity of brain activity.
It is like the brain is in a state of hibernation. The intensity of brain activity can be measured by the colors. The most activity is the red, followed by yellow, green, and finally blue. The depressed brain is in a very low activity state almost like hibernation. You can see why mental concentration is not very good in someone who is depressed.