What Happens in the Brain During Bilateral Stimulation?
If you search online for an explanation of bilateral stimulation, you will quickly encounter some very confident neuroscience.
You may read that it:
“balances the left and right hemispheres.”
“switches off the amygdala.”
“moves trauma from the emotional brain into the logical brain.”
“rewires traumatic neural pathways.”
“recreates REM sleep.”
Those explanations are memorable.
They are also considerably cleaner than the science.
The real answer is more interesting:
Bilateral stimulation appears to interact with attention, working memory, emotional arousal, and memory processing—but researchers have not identified one single brain mechanism that explains what happens during EMDR or ART.
And even the phrase bilateral stimulation may place the emphasis in the wrong place.
First: what is bilateral stimulation?
Bilateral stimulation refers to stimulation that alternates from one side to the other.
In therapies such as EMDR, this may involve:
horizontal eye movements,
alternating auditory tones,
or alternating tactile stimulation.
Accelerated Resolution Therapy, or ART, primarily uses visually guided horizontal eye movements.
The obvious assumption is that alternating left-right stimulation must work by alternately activating the brain's two hemispheres.
But that is not where the strongest evidence currently points.
The brain does not operate as a simple left-brain/right-brain system
The popular idea of a “logical left brain” and an “emotional right brain” is far too simplistic for understanding trauma.
Memory, attention, emotion, threat detection, bodily awareness, self-reflection, and regulation depend on networks distributed throughout the brain.
Those networks communicate constantly across both hemispheres.
So while bilateral eye movements certainly involve neural activity related to vision and eye movement, we do not have good evidence that trauma treatment works because the therapist is simply alternating activation between the left and right sides of the brain.
That is a much bigger claim than the science supports.
A better starting point may be working memory
One of the strongest current explanations for the eye-movement effect involves working memory.
Working memory is the brain's limited-capacity system for temporarily holding and manipulating information.
When you bring a vivid emotional memory to mind, working memory becomes involved.
You may be holding:
an image,
sounds,
body sensations,
emotional meaning,
and details of what happened.
At the same time, tracking a moving visual target also requires attention and working-memory resources.
The two tasks compete.
That competition may alter the way the memory is experienced.
What does that competition do?
Research on eye movements and emotional memories has repeatedly found that recalling a distressing memory while simultaneously performing a demanding eye-movement task can reduce the memory's subsequent vividness and emotional intensity.
The most consistent effect appears to be on vividness.
In other words, the memory may become:
less sharp,
less immersive,
less visually intense,
or less immediate.
The emotional response may also diminish.
This working-memory account has substantial experimental support and is one of the strongest current explanations for why eye movements can alter emotional memories.
But where is working memory in the brain?
There is no single “working-memory center.”
Working memory depends on distributed networks that include areas of the prefrontal and parietal cortex, among others.
These systems help us:
maintain information,
direct attention,
update what we are holding in mind,
and coordinate competing demands.
So when someone recalls a traumatic memory while simultaneously tracking a moving target, several processes are occurring at once.
The brain is retrieving autobiographical material.
Processing visual movement.
Directing attention.
Monitoring internal emotion.
Tracking bodily sensations.
And updating information moment by moment.
That is much more complicated than “stimulating both sides of the brain.”
Trauma also involves networks—not one damaged brain region
PTSD research increasingly describes trauma-related symptoms in terms of altered functioning across large-scale brain networks, rather than one malfunctioning structure.
A 2021 systematic review and meta-analysis of resting-state functional connectivity studies found PTSD-related differences involving interactions among networks related to affect, default-mode processing, and somatomotor function.
More recent PET research likewise describes PTSD-related alterations across large-scale systems involved in emotion regulation, self-referential processing, executive control, stress, and memory rather than a single “trauma center.”
That is an important shift in how we think about the brain.
What is the amygdala doing?
The amygdala is often described as the brain's “fear center.”
That description is useful for teaching.
It is also incomplete.
The amygdala helps detect emotionally important information and contributes to learning about threat.
It interacts with many other structures and networks involved in memory, attention, perception, and physiological responses.
PTSD research often finds altered amygdala activity or connectivity.
But bilateral stimulation should not be described as simply “turning off the amygdala.”
There is no switch.
What about the hippocampus?
The hippocampus plays an important role in episodic and contextual memory.
Context matters enormously in trauma.
A major distinction in recovery is:
“This is happening now”
versus
“This happened then.”
The hippocampus helps organize memory within context—where and when something happened and how one experience relates to another.
PTSD research has repeatedly implicated hippocampal function alongside broader networks involved in emotional memory.
That makes the hippocampus relevant to understanding trauma processing.
But again, it would be too strong to say that bilateral stimulation directly “repairs the hippocampus.”
What about the prefrontal cortex?
The prefrontal cortex participates in:
attention,
working memory,
decision-making,
cognitive control,
and regulation of emotional responses.
Trauma-related disorders have often been associated with altered communication between prefrontal regions and limbic systems involved in threat and emotion.
Successful trauma treatment may be associated with changes in these systems.
A 2023 systematic review and meta-analysis examining functional MRI studies of PTSD treatment—including EMDR, CPT, and Prolonged Exposure—found that psychotherapy can be associated with changes in neural functioning.
But this is crucial:
Those findings do not show that bilateral stimulation uniquely causes the brain changes.
Different effective trauma therapies can alter brain functioning.
That distinction matters
If someone's brain looks different after EMDR, we cannot automatically conclude:
“The eye movements caused this particular neural change.”
The person also:
recalled the trauma,
changed beliefs,
experienced new emotional learning,
worked with a therapist,
repeatedly encountered previously avoided material,
and developed different associations.
Any or all of those processes may contribute.
Neuroimaging can show us that something changed.
It is much harder to determine exactly which treatment component caused it.
What happens to attention during eye movements?
Attention is probably central.
During eye-movement processing, attention is divided between:
the internal memory,
and an external visual task.
You are attending inward.
And outward.
At the same time.
This is often referred to as dual attention.
That divided-attention state may help prevent complete immersion in the traumatic memory.
Part of the mind is contacting what happened.
Part remains oriented to what is happening now.
Why might that help?
Traumatic memories can feel unusually present.
Instead of:
“I remember being in danger,”
the experience may become:
“I feel as though danger is happening again.”
The image becomes vivid.
The body reacts.
Emotion arrives immediately.
Dual attention may help maintain contact with both:
the remembered experience,
and the current safe context.
That could help the brain update the meaning of the memory.
The memory says:
“Danger.”
The present says:
“Not now.”
Is that memory reconsolidation?
Possibly.
Memory reconsolidation refers to the finding that when an established memory is reactivated, it may temporarily become modifiable before being stored again.
That idea is highly relevant to trauma therapy.
A memory is activated.
Something different happens while it is active.
The emotional intensity changes.
New information becomes available.
The memory may later be retrieved differently.
It is tempting to describe this entire process as reconsolidation.
But we need to be precise.
Reconsolidation is a well-established phenomenon in memory science.
Exactly how much it explains EMDR, ART, or other trauma therapies remains under investigation.
Eye movements may change what is being re-stored
Here is one plausible possibility.
You activate a distressing memory.
While it is active, eye movements tax working memory.
The image becomes less vivid.
The emotional response changes.
You simultaneously remain aware that you are safe in the present.
If that altered version of the memory is then stored again, its later retrieval may feel different.
That is a coherent model involving both working memory and memory updating.
But a coherent model is not the same thing as definitive proof.
What about the orienting response?
The brain is naturally drawn toward novel or changing stimuli.
Movement attracts attention.
A sound attracts attention.
Something entering your visual field causes you to orient.
Some researchers have proposed that bilateral stimulation repeatedly activates this orienting response.
Initially:
“Something changed. Pay attention.”
Then, when nothing dangerous happens:
“This is not a threat.”
Physiological arousal may begin to decrease.
This has led to theories that eye movements may support a shift from heightened alertness toward reduced arousal.
It may contribute.
But evidence has not established the orienting response as the complete mechanism.
What about the autonomic nervous system?
Trauma is not purely cognitive.
It affects:
heart rate,
breathing,
muscle tension,
startle responses,
sweating,
digestive sensations,
and other physiological processes.
During successful trauma processing, clients may experience changes in those body responses.
A memory that once caused immediate activation may become easier to recall without the same physiological surge.
That may reflect changes in how threat is predicted and regulated across multiple brain-body systems.
But again, it is too simplistic to say:
“Bilateral stimulation activates the parasympathetic nervous system and clears trauma.”
The physiology is more complex.
Do eye movements recreate REM sleep?
This hypothesis has been around for years.
During REM sleep, the eyes move rapidly.
REM sleep also plays roles in memory and emotional processing.
That makes the comparison attractive.
But there is a major problem:
Two things involving eye movements does not mean they use the same mechanism.
The eye movements during therapy occur while a person is awake, attending deliberately, interacting with a therapist, and recalling targeted material.
REM sleep is a very different neurophysiological state.
There may be interesting overlaps in memory processing.
We cannot simply say therapy recreates REM sleep.
What about the default mode network?
The default mode network, or DMN, is involved in functions including autobiographical memory, self-referential thought, and internally directed cognition.
Because trauma therapy involves recalling autobiographical experiences and updating their meaning, researchers have examined whether this network is involved in EMDR's effects.
A neurobiological review of EMDR proposed possible roles for the default mode network and cerebellum in memory retrieval, associative learning, and relaxation during predictive eye movements. But the authors presented this as a hypothesis requiring further testing—not a settled mechanism.
That distinction is exactly how neuroscience should be communicated.
What about the salience network?
The salience network helps identify what deserves attention.
Threat is highly salient.
So are bodily sensations, emotionally significant cues, and unexpected events.
People with PTSD may show altered functioning in networks involved in salience detection and emotion regulation.
If trauma therapy changes how the brain evaluates a previously threatening cue, salience processing may also change.
But we cannot currently point to bilateral stimulation and say:
“This directly resets the salience network.”
The evidence is not that specific.
What about the default mode, salience, and executive networks together?
This may actually be a better way to conceptualize trauma than focusing on isolated structures.
PTSD can involve disruptions in how several large systems coordinate:
networks that detect emotionally important information,
networks involved in autobiographical memory and self-related thought,
networks involved in executive control and regulation,
and networks related to bodily experience and action.
A recent systematic review of PET studies similarly identified abnormalities across large-scale networks rather than one isolated region.
Trauma treatment may involve changes in how these systems communicate.
That is much more plausible than the idea that one eye movement is simply “switching off the fear center.”
What happens during ART?
Accelerated Resolution Therapy, or ART, also uses visually guided eye movements.
But ART combines them with a different clinical structure.
ART includes:
memory activation,
attention to body sensations,
eye-movement sets,
and deliberate imagery procedures.
That means whatever cognitive and neural effects the eye movements have are occurring alongside imagery rescripting and structured changes in how the internal experience is represented.
We do not yet have enough ART-specific neuroscience research to determine precisely how those components interact.
Could ART and EMDR produce some of the same brain effects?
Possibly.
Both activate emotional memory while the client performs visually guided eye movements.
So mechanisms involving:
working memory,
attention,
arousal,
and memory updating
could plausibly contribute to both.
But ART and EMDR do not use those eye movements in identical ways.
So it would be inappropriate to assume the total brain mechanism is identical.
Why ART may be especially interesting from a memory-science perspective
ART deliberately works with imagery.
That raises a fascinating possibility.
Eye movements may reduce the vividness of an emotionally charged image.
Then the client may intentionally change aspects of the image.
The brain is not simply recalling.
It is retrieving, modifying, and re-experiencing the representation.
That could create conditions for meaningful memory updating.
But we still need ART-specific mechanistic research before making stronger claims.
Does bilateral stimulation “rewire the brain”?
Technically, virtually all learning changes the brain.
Reading this sentence changes neural activity.
Learning to play piano changes the brain.
Psychotherapy changes the brain.
Relationships change the brain.
So saying therapy “rewires the brain” is both broadly true and not very informative.
The important question is:
What changes, where, through what mechanism, and with what clinical effect?
That is where the science is still developing.
Can neuroimaging prove that a treatment worked?
Not by itself.
Brain scans are fascinating.
But they are easy to overinterpret.
A change in blood flow or functional connectivity does not automatically tell us:
why the client improved,
which treatment component mattered,
or whether that neural finding is the cause or consequence of symptom change.
Clinical outcome research and mechanistic neuroscience answer different questions.
Both matter.
Neither should be overstated.
What does the research actually support?
At this point, a cautious synthesis looks something like this:
Eye movements performed while emotional memories are active can affect memory vividness and emotional intensity.
Working-memory competition has substantial experimental support.
Dual attention may help the person remain connected simultaneously to memory and present context.
Changes in arousal and orienting may contribute.
Memory updating or reconsolidation may contribute.
Successful trauma therapy is associated with changes in brain systems involved in emotion, memory, threat, attention, and regulation.
But no single neural mechanism has been established as the explanation for bilateral stimulation.
What research does not justify saying
Current evidence does not justify confidently telling clients that bilateral stimulation:
“balances the hemispheres,”
“switches off the amygdala,”
“moves trauma into the logical brain,”
“reprograms the nervous system,”
“clears trauma from the body,”
or
“recreates REM sleep.”
Those are attractive simplifications.
They are not established neuroscience.
Why uncertainty is not a problem
People sometimes assume that if researchers do not completely understand the biological mechanism of a therapy, the therapy itself must be questionable.
That is not how science works.
Effectiveness and mechanism are separate questions.
We can have meaningful evidence that a treatment helps before we fully understand every pathway producing that change.
EMDR has been studied extensively as a PTSD treatment, including randomized clinical trials and meta-analyses.
The mechanism question remains more open.
That makes it worth studying—not dismissing.
Why I care about this distinction
I am trained in both ART and EMDR.
One of the things I find most interesting about both therapies is that the clinical change can sometimes feel much more dramatic than the current neuroscience explanation would lead you to expect.
That makes me want better explanations.
Not bigger claims.
What happens to visual memory?
What happens to working memory?
How does the brain update emotional predictions?
What role does the body play?
What happens when imagery changes?
Why can one memory suddenly feel distant when it felt immediate moments earlier?
What mechanisms do ART and EMDR share?
And where do they diverge?
Those are exciting questions precisely because the answers are not finished.
Working with Laura Geftman, LCSW
I use Accelerated Resolution Therapy and EMDR as part of a broader trauma-focused practice.
My work may also incorporate IFS, Cognitive Processing Therapy, Prolonged Exposure, CBT, psychodynamic, interpersonal, attachment, and schema therapies depending on the client and clinical question.
I offer focused trauma therapy and therapy intensives for adults in Pennsylvania, New Jersey, New York, and Florida.
In-person therapy intensives are available in Ardmore, PA, on the Main Line near Philadelphia.
Virtual therapy may also be available when clinically appropriate.
Interested in ART, EMDR, or focused trauma therapy?
If a particular memory, image, body response, or trigger continues to feel present even though you intellectually understand that the event is over, ART, EMDR, or another focused trauma approach may be worth exploring.
The goal is not to “hack” the brain.
It is to help an experience that still feels current become integrated more fully as something that happened in the past.
Suggested Reading
If you are interested in bilateral stimulation, EMDR, ART, trauma memory, or how eye movements may work, these related articles may be helpful:
What Does Research Actually Say About Eye Movements in EMDR?
How Do Eye Movements Work in Accelerated Resolution Therapy?
ART vs. EMDR: How Is Bilateral Stimulation Used Differently?
FAQ
What happens in the brain during bilateral stimulation?
There is no single established mechanism. Research suggests that eye movements can compete with emotional memories for limited working-memory resources while attention is divided between the memory and the present. Changes in arousal, attention, and memory updating may also contribute.
Does bilateral stimulation activate both sides of the brain?
Eye movements involve activity across multiple brain systems, but there is not strong evidence that the therapeutic effect comes simply from alternating activation of the left and right hemispheres.
Does bilateral stimulation calm the amygdala?
That is too simplistic. The amygdala participates in emotional salience and threat learning, but trauma involves multiple interacting brain regions and networks. We do not have evidence that bilateral stimulation simply switches the amygdala off.
What does working memory have to do with bilateral stimulation?
Recalling an emotional memory and tracking a moving visual target both require cognitive resources. Because working memory has limited capacity, doing both at once may make the memory less vivid or emotionally intense.
Does bilateral stimulation rewire the brain?
Effective learning and psychotherapy can change brain function, but “rewiring” is too vague to explain what bilateral stimulation specifically does. Researchers are still identifying the neural mechanisms associated with EMDR and other eye-movement therapies.
Is memory reconsolidation involved?
Possibly. When memories are reactivated, they may become temporarily modifiable. Trauma therapies may create conditions in which new information or a changed emotional experience becomes associated with the memory, but reconsolidation has not been established as the sole mechanism of EMDR or ART.
Does bilateral stimulation recreate REM sleep?
That is an unproven hypothesis. REM sleep and eye-movement therapies both involve eye movements, but they occur in very different brain states.
Does EMDR change the brain?
Neuroimaging research suggests that successful trauma therapies, including EMDR, can be associated with changes in brain activity and connectivity. Similar changes can occur with other effective trauma treatments, so neuroimaging does not show that eye movements alone caused those changes.
Do ART and EMDR affect the brain in the same way?
They may share mechanisms involving attention, working memory, and emotional memory because both use eye movements while distressing material is active. But ART and EMDR have different protocols, and their complete mechanisms should not be assumed to be identical.
Is the neuroscience of bilateral stimulation settled?
No. Working-memory research provides a strong explanation for some eye-movement effects, but the relationships among attention, arousal, memory updating, neural networks, and clinical recovery are still being studied.