It was first discovered that EMDR does work - and has been proved to be effective for PTSD, acute distress disorder, phobias and many other conditions - it was then the task to find out how and why it works. The short answer is that no-one is absolutely certain, however various ideas have been put forward:

REM Mechanism

It has been known for quite a while now that memory is reorganised during the REM - rapid eye movement - phase of sleep. During this period of sleep, the memories and experiences from the day, and before, are pruned of some of their vividness and power so that we are just left with the gist of what went on. This is necessary to organise our memories and sense of ourselves, otherwise we would be flooded by experience.

It is thought that a similar mechanism goes on in EMDR and this is likened to the mechanism that occurs when we are startled. For example, if a deer hears a noise in the bushes they will orient themselves to that sound and there will be a spike of arousal. Similarly in EMDR, the person first orients themselves to the distressing experience and there is a similar spike of arousal.

Meanwhile the amygdala kicks into gear - the brain’s ‘smoke alarm’ - to appraise the potential danger, comparing this scenario with previous experience to know whether to prepare for fight or flight. If this automatic mechanism gives the all clear, the level of arousal goes down.

For someone who has been traumatised the amygdala remains in a hyper-aroused (or sometimes a hypo-aroused) state. The theory goes that, by focusing on the original trauma while watching a boring, repetitive hand movement, the amygdala’s level of arousal goes down. This is sometimes called the ‘compelled relaxation response’.

Disruption of Working Memory

There is a large amount of research which shows that eye movement and other kinds of ‘bi-lateral stimulation’ (the use of the tappers or fluctuating music), disrupts working memory where the traumatic memory is held. The example is given of someone finding it impossible to try to remember a long number whilst eye movement is going on.

Thalamus response

One of the effects of trauma is to disrupt the thalamus, that part of the brain which integrates the different parts of our experience, putting together the somatic, visual, olfactory, sensory, emotional and cognitive elements to make a coherent whole. Due to this disruption, our experience of traumatic events becomes fragmented, so that we might get auditory, visual or emotional flashbacks, which intrude and seem to come out of nowhere. It has been shown that EMDR enhances the 40 Hz signal given out by the thalamus that serves to bind together the different parts of the brain, thus repairing the fragmentation due to trauma and dissociation.


EMDR has been described as ‘mindfulness on steroids’ as, like mindfulness, it is good at bringing the prefrontal orbital cortex back on line. This prefrontal system allows regulation of the amygdala - the emotional activation centre of the brain - allowing down-regulation of stress, and facilitating an observing watchfulness similar to that achieved through mindfulness or meditation.

Cerebellum Theory

The cerebellum is one of the primitive parts of the brain that helps regulate our emotional life. Those people who have experienced, for example, significant early neglect, and will often rock when agitated in order to comfort themselves, have been found to have the two halves of their cerebellum not fully connected. Through the bi-lateral stimulation, EMDR is thought to bring the two halves of the cerebellum into closer relationship.