Empirical evidence has shown that EMDR is very effective in helping people eradicate negative symptoms that result from traumatic events. The evidence shows that EMDR does work for most people. However, they do not know exactly why it works.
Studies are currently being done that hope to prove the mechanisms for "why" / "how" EMDR works. Below is my best understanding for "why / how" it works.
Pharmaceutical companies, in the process of studying the side effects of various drugs for sleeping aids have done a lot of sleep studies. In these studies, they found by waking people up several times a night that people often have the same dream that is repeated 8-15 times a night. Each dream sequence is slightly different from the previous one. And the first dream presents a problem or "trauma" that the dreamer is wrestling with during the day (my boss looked at me wrong, my dog almost got hit by a car, the guy in traffic in front of me was a jerk, etc). By the end of the last dream, the "problem" or "trauma" is creatively dealt with. The theory is that during the REM (Rapid Eye Movement) portion of sleep, the left half of the brain (the rational part) is able to communicate with and self-soothe the right half of the brain (the emotional/religious/love/fear part of the brain). Actually both parts of the brain are interactively engaged to come up with a crazy and bizarre dream solution to our daily problem(s). And with each dream sequence there is an improvement in the theme or story of the dream that allows the unsoothed right hemisphere of the brain where the stress of the day is stored to be soothed. This may explain why we can have a fight with our spouse or have a lousy day at work and wake up in the morning and feel fine. Basically, our mind is created in a way that is generally effective at self-soothing.
When a traumatic event happens, the primitive part of the brain that protects us from danger (attacking lions, dinosaurs, crazy spouses, etc), gets triggered. And we are in "fight or flight" mode. This means that we are in a constant vigilance. We have less energy focused to the pre-frontal cortex (the most developed front part of the brain). This is our body's way of making sure that we get the burst of energy necessary to survive the threatening event.
The theory is that for some people when there is a traumatic event, that for some reason the right hemisphere in their brain biologically locks up the traumatic event into an inaccessible portion of the brain. It does this because the brain determines that traumatic event was so threatening that the brain biologically needed to compartmentalize it in an area separate from the rest of the brain in order to survive the traumatic event.
PTSD is basically when that part of the brain gets stuck on and does not go away when the threat is gone.
Many people can experience a very stressful event (rape, robbery, a car accident, etc) and after several weeks of talking about it and having dreams about the event, their brain is able to self-soothe and tell them that the threat that was there is no longer there. And the "fight or flight" portion of the brain gets the message that "we are safe now". When this happens, the person is able to go back to "normal". They may have very disturbing memories of what happened, but are not experiencing their every day life as if the event is continuing to happen.
For some people, according to this theory, the traumatic event gets stuck in the right part of the brain in an non-accessible space. This makes it so that when they talk about the event with their friends and when they dream about it that their left brain is working as hard as it can to self-soothe the situation, but it is unable to calm down the part of the right brain that gets stuck. It also means that for these people that REM when we sleep does not work. Instead of dreams self-soothing the individual, there is no resolution and the person instead has night-terrors. Also, regular talk-therapy is rather ineffective to the extent that the "left brain" where rational conversations are stored is stimulated without adequate access to the "right brain" where emotions and the traumatic event are stored. The person talks and talks and talks about the event, but never gets the relief they need.
As a reminder, what happens in REM, the normal process of accessing the person's self-soothing techniques is "bi-lateral stimulation". This means, both the right brain and the left brain are engaged.
With EMDR, we replicate REM by stimulating "bi-lateral stimulation". This can be done by having the client look at my hand as I wave it left and right with them focusing their eyes on my fingers. It can be done by me tapping on their right knee and their left knee. And it can be done by listening to sounds in expensive fancy headphones that make sounds that go back and forth between the right and left ears. I personally use my hands and not the other techniques. I think it has an advantage because the proximity and interpersonal connection between the therapist in the client can create an additional therapeutic benefit that I personally believe that the other techniques are not as effective at.
So, through replicating REM while talking about the trauma in a systematic way, the therapist insures that there is a safe context to insure that the trauma gets talked about while the previously inaccessible part of the client's brain is stimulated. This means that the client may even say the same things to the EMDR therapist that they have already (ONLY WITH ONLY THEIR LEFT GRAIN ENGAGED) journaled about, talked with their friends about, or talked with another therapist about with talk-therapy for months or years. However, with the whole brain being stimulated while processing the trauma, the part of the brain that holds the traumatic memory can be accessible so it can actually be soothed. Once it is soothed, then hyper-vigilant responses are no longer necessary.
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