I affirm that there are brain diseases, but like I said, the mind is not the brain. There are no diseases of the mind. Things do go wrong with the brain, but depression is not a brain disease. It is not a disease at all. Anxiety can make you sick, and it can show up on scans (as you pointed out) with variances in brain activity. In fact, all kinds of emotions can change brain activity, but they are not diseases. That is what the medical profession has been told, and that is what they tell their patients, but the evidence does not support it. There is no biological test to determine that someone has depression.
I don't really want to make several long posts that no one will read, but allow me to give some basics from a biblical perspective. I will start another thread for open discussion Here
Also, I can provide you with several books and articles from some of the leading medical professionals in this country that have debunked the pseudo science of psychiatry. If there is a legitimate problem with the brain, then treat the brain, but that is not what is being done regarding these so-called "mental illnesses". In fact, you only have to go to the websites of anyone who supports the ideas that these "illnesses" exist. They will tell you that counseling is very effective in treating them. Ask your doctor how effective "counseling" is on a real disease. Ask them how effective counseling is on curing diabetes or cancer.
I have spent many years studying this, and I can assure you that the idea of mental illness is one of the biggest hoaxes ever perpetrated on man.
You are mistaken about how "disease" is identified. "Disease" is identified by symptoms and signs, not phyisical deformaty. Symptoms are things the patient complains of and signs are things the practitioner observes, like temperature and pulse rate. The symptoms of diabetes are excessive urination, excessive thirst and excessive eating. As you can see, these are behaviors that the patient performs. A practioner observes the behaviors and makes the diagnosis.
A depressed person has: changed sleep pattern, usually insomnia, decreased appetite frequently with weight loss, feelings of helplessness, hopelessness, lack of energy, enjoyable activities are no longer fun, concentration and decision making are increasingly difficult, suicidal ideation, frequently with attempts to end their own life, agitation, and sometimes physically slowed movements. A person with 5 or more of these symptoms is diagnosable as depressed. A practioner observes the behaviors and makes the diagnosis. Just like a physical illness, because it is a physical illness.