We cannot talk about traumatic memory, or how EMDR works for that matter, without talking about the right and left hemispheres of the brain. Under normal circumstances, these two sides of the brain communicate and work together effectively to process everyday events and store them adaptively in our memory networks. However, when one side of the brain is shut down, as it is during a traumatic event, the effects can be debilitating.
The right side of the brain is intuitive, emotional, visual, spatial, and tactile. It remembers sights, sounds, body sensations, smells, and emotions related to events. The left side of the brain is linguistic, sequential, and analytical. It remembers facts, statistics, and the words that go with an event.
When something happens that causes intense emotion, the limbic system (particularly the amygdala) and the visual cortex- both in the right side of the brain- show increased activation.
At the same time, the speech center (Broca’s area) in the left frontal lobe of the brain shows decreased activation. Interestingly, Broca’s area is also affected in stroke patients. When the left side of the brain is deactivated, it is difficult to organize experiences into logical sequences, or translate feelings into words. Executive functioning is impaired.
Knowing all of this, it is not difficult to imagine what happens when traumatic material is triggered or recalled: the images, sounds, body sensations, smells and emotions related to it are intensely experienced, with no words or facts to explain them. It can feel as though the trauma is happening in the present. This is how a flashback is experienced. This also helps explain why although the sights, sounds, smells and sensations of a memory may be vividly remembered, the sequential details of the story may be forgotten or difficult to put together.
By applying bilateral stimulation (back and forth eye movements, alternating tapping on knees, or alternating tones in ears) during EMDR reprocessing, we are activating both sides of the brain while revisiting the upsetting event, enabling the memory to be processed and stored more adaptively. This results in more insight and understanding of the traumatic event, along with less intensity of emotions and sensations related to it.
*Summarized from The Body Keeps the Score by Bessel van der Kolk, MD