Dissociation is a psychological state best described as a loss of awareness. It is a reaction to some form of perceived psychological stress, whether real or remembered. The defense mechanism is to disconnect the sense which causes the pain from the consciousness. It's very similar to the cringing flinch response of the eyelid when the eye detects an projectile or other threat. It occurs automatically rather than electively. It should always be kept in mind that dissociation is an extremely human behavior. To try and escape from pain and its sources at any cost, through any amount of abandonment necessary is basic human nature, at least as the pain becomes extreme enough. When it's extreme enough, we're even willing to sacrifice feeling if it is the only means of escape possible.
The specific cause of dissociation can be quite difficult to identify because it can occur as an automatic reaction to something subconsciously perceived as threatening, which depends on personal history. It's possible to discern the types of things which will trigger it by developing an awareness of when one is dissociating and trying to analyze the environmental conditions of the time. Regardless, it is known that severe psychological traumas, such as those known to cause Post Traumatic Stress Disorder also cause Dissociation. Events where there is a perceived threat to human life, physical integrity, or emotional attachments including rape, murder, death, and natural disasters are good examples. Beginning from an actual danger, dissociation as a mental state grows to become a self defense response which is triggered when one's environment somehow begins to resemble the conditions on which a trauma was predicated.
As a very simplistic example, a man enters his home and notices a quiet hissing sound and smells a strange fume, his dog is barking wildly from within it's cage. He releases the dog and as he is taking it outside his home is rocked suddenly by a blaring explosion. They escape with moderate burn injuries and the house goes up in flames. This event would have been perceived as severely life threatening and terrifying, definitely qualifying for psychological trauma. What happens a month later however, when the man walks into a hardware store and smells the same smell? Like Pavlov's famous dinner bell, it could induce an automatic reaction, a dissociative one. The reason is that dissociation occurs in reaction to a perceived threat, the memory itself and the possibility of recurrence is the threat. Dissociation is defending him from the pain of reliving the memory by trying to ignore the sense which triggers it. It should come as no surprise most victims of severe traumas, especially repeat victims are unable to remember the experiences. Quite probably, they either never fully perceived the experience, or they slowly detached themselves from the memories by repeatedly automatically avoiding them.
All of this talk about losing something, about losing awareness, about losing senses hasn't yet covered what is actually being lost. First of all, there are certain mental states which are universally recognized as being normal though they resemble aspects of dissociation. Dissociation is really about the depth and breadth of one's awareness. Here is another example for the purposes of illustrating gradations of awareness.
I give you a pen and tell you to draw a short vertical line and two half as short horizontal lines centered over both ends of it. You have just written the capital I character of the alphabet. Each one of these 3 actions involved was specific and deliberate, and you were aware of them as you did them. Now I tell you to draw a letter I, you do it again but this time you do it differently. The stroke order and direction is different, and although you wrote the same character, you weren't aware this time of how you were doing it. Once more I ask you to sign your name now, you whip the signature off very quickly, but each character is blending into a single movement of your hand. If I ask you to write an e within the middle of your name can you do it in a relatively similar amount of time? The point we often act based on an emotion or desire at a high level, and carry it out in the form of rote procedure. Procedure which requires very little attention or awareness to the specific actions.
Dissociation takes it to another level, a level where you can work at a simple job without ever really feeling like you're doing something, without ever really doing more than feeling or knowing what you have to do. It turns reality into a sort of script, a movie where you are playing the lead role and the material is very boring. Everything is happening as it normally would, but a sense of doing has been lost and you become disconnected. Is it really very strange? Have you ever noticed the way your attention to driving or other tasks can slip when something else such as a cell phone takes center stage? Have you ever been an athlete and struggled to keep track of the movements and rhythms of your body while under stress? Have you ever been somewhere, or been listening to someone and slowly drifted away into your own thoughts? Have you ever been so caught up in what you're thinking that you started "staring off into space"? Have you ever day dreamed?
Disinvolvement from processes and tasks is one manifestation of dissociation, but it has other more powerful instances of separation. In order to understand why and how we react to pain, it's crucial to understand what the origin of psychological suffering is in the first place. To put it simply, it's caring. Example number three.
Suppose you're my friend and we're walking out of a movie theater, as we walk out someone runs past us and punches someone in the face. It might upset you, moderately, but suppose the scenario is replayed. Suppose the man runs up to us and punches me in the face, and we've been friends for five years. It's going to be even more painful for you. Taking it to an exaggerated level, suppose you're walking out of the movie theater with your mother, and someone runs up and punches her in the face. I would expect it to be horrifying. The difference between each victim is the amount that you know and care for the particular person. You wouldn't feel any pain or fear if you werent attached to the well-being of any of us in some way, but it's basic instinct to have such attachments. I choose to call those life or death attachments survival ethics. Our most basic survival ethic concerns the status of our bodies, whether we are in pain or not. It's easy to see how you would feel threatened if someone were to swing a butcher knife toward your immobilized fingers for example. But a human being's body can become a source of pain, and when that pain is inescapable, one can begin to separate from the body itself. Dissociation strongly affects the relationship of body and mind. For an instance of their separation see example number 4.
At any time if I ask you to take a deep breath and let it out you can, but even if neither one of us has any desire for you to keep breathing, you will. Contrarily even if you wanted to quit, it would be almost impossible. The point here is that although you keep breathing, and although your heart keeps beating, you are by no means aware of it, though you can be.
In the same way it's possible for people to lose feeling in their bodies, and to even lose the physical integrity survival ethic. Because pain comes from the body, one loses bodily feeling psychologically and to a lesser extent, physically. A person in that state of mind wouldn't be afraid of the butcher knife coming down, but would still feel a muted pain. There are a lot of reasons why people engage in behaviors of self-harm, and one of them is certainly to regain some kind of feeling, even if it is a painful one. Another reason is that losing feeling in your body, and at the same time losing fear of physical harm is a provocative, like an attack. It's as though your body has become your enemy now and it is casually stealing your sense of feeling. Who wouldn't be tempted to hit back with such impunity?
Think if you've ever been in a severe car accident, as you watched it happening around you how did you feel? Do you even remember what happened with any degree of clarity? Did your memory of the experience fade and become vague over time?
I haven't taken one class of psychology, and I've barely researched the topic. You're getting theories, thoughts, and perceptions entirely based on my own empiricism. Empiricism. Experiences.
I have had plenty of them. 2 near fatal car accidents, the bloody death of my pet, sexual, physical, and emotional abuse, at home and at school for periods of years. Domestic violence, bipolar generational legacies, and Post Traumatic Stress Disorder are my sources. I have the deepest sense of compassion for people who have serious mental illnesses, and the deepest respect for those who refuse to make excuses out of them, or excuses for them.
Experiences. We're both human beings, and if you had the same experiences as I have, you would have the same damage.
This is my straight-through unrevised, somewhat poorly structured first draft. I wrote it because it's the best I can do for now. I appreciate your feedback.