Imagine a person, physically and mentally healthy, totally satisfied with life. Though an ideal picture, this image, if real, would be of a person who has their hormonal and neurotransmitter systems in a balanced state. The neuroendocrine balance is reflected by an appropriate feedback between sympathetic and parasympathetic systems in this person.

Physiologically, the balance of various systems within the body can be understood on the basis of electromagnetism (e.g. ECG, EEG), or levels of elements, hormones, enzymatic secretions or metabolic products. Whenever their levels become abnormal, the body tends to become imbalanced in certain predictable ways causing a state of illness. Physical conditions can be measured in above terms. But measurements of mental conditions were till recently unacceptable in the above terms.

Depression is a mental condition (also having physical effects) in which a person is unhappy, with loss of concentration, energy and ability to enjoy life. There is an accompanying obvious physiological imbalance in the body systems. Autonomic disturbances and increased levels of cortisol are characteristic features of depression. Depression is a state of stress in which the body (and the brain) is uncomfortable. This discomfort is due to a "perceived" threat to the body or a part of the body. In the course of Nature, the body and mind would like to "undo" the effects of "perceived threat". The sympathetic part of autonomic nervous system plays a more active role in "undoing perceived threat."

With sympathetic arousal, the senses of perception