What Is Bilateral Stimulation? And Why It Works (According to Neuroscience)
As a trauma therapist, I get this question a lot:
“What is the eye movement even doing?”
That back-and-forth movement? It’s called bilateral stimulation (BLS), and it’s a cornerstone of EMDR (Eye Movement Desensitization and Reprocessing) therapy. While it may seem simple, there’s a growing body of neuroscience that shows why this rhythmic pattern helps the brain process trauma more effectively.
Let’s explore what bilateral stimulation is, what it does in the brain, and how BLS helps stuck memories get unstuck.
What Is Bilateral Stimulation?
Bilateral stimulation (BLS) refers to alternating sensory input across the left and right sides of the body—often visual, auditory, or tactile. In EMDR, this could look like:
Watching a therapist’s hand or light bar move side-to-side
Listening to alternating tones in each ear
Feeling rhythmic taps on opposite sides of the body (e.g., knees, hands, or via buzzers)
Using the “Butterfly Hug” to self-administer BLS
What all of these have in common is that they activate both hemispheres of the brain in a rhythmic, alternating fashion. But why does that matter?
The Science of Why Bilateral Stimulation Works
Researchers are still unpacking the full story, but here are several key mechanisms—backed by neuroscience—that explain why BLS supports trauma healing.
1. Working Memory Taxation
One well-supported theory is that BLS reduces the vividness and emotional charge of traumatic memories by taxing your brain’s working memory.
Think of working memory as the brain’s mental “desktop”—used for holding and manipulating information.
When you recall a distressing memory while doing a task (like following a moving light or tapping), your brain’s attention is split. This “dual attention” leads to less vivid, less distressing recollections.
Research insight:
A 2013 meta-analysis by Lee & Cuijpers confirmed that BLS during memory recall significantly reduces emotional intensity, compared to recall without BLS. This supports the idea that taxing working memory disrupts emotional overactivation.
2. REM Sleep Mimicry
Another compelling theory is that BLS mimics REM sleep, which is the phase of sleep when the brain consolidates memories and processes emotions.
During REM sleep, our eyes naturally move from side to side. Francine Shapiro, the founder of EMDR, believed that replicating this lateral movement while awake could activate similar brain processes.
Neurobiological perspective:
REM sleep has been shown to decrease activity in the amygdala (the brain’s fear center) while strengthening communication between the hippocampus (memory integration) and prefrontal cortex (rational thinking). BLS may trigger a similar neuro-synchrony, allowing the brain to reprocess trauma without becoming overwhelmed.
3. Hemispheric Integration
Trauma tends to activate the right hemisphere of the brain—the emotional, sensory, nonverbal side—while suppressing the left hemisphere, which handles logic, language, and narrative.
In a trauma state, people often “feel” the memory but can’t put it into words.
BLS appears to stimulate both hemispheres simultaneously, promoting cross-communication and integration. This helps clients turn fragmented, implicit memories into coherent, verbalized experiences—something the brain can file away as “in the past.”
Supporting evidence:
Neuroimaging studies show that EMDR with BLS increases connectivity between the amygdala, hippocampus, and anterior cingulate cortex—regions associated with emotion regulation, memory, and sense-making.
4. Safety and Parasympathetic Regulation
The rhythmic, predictable nature of BLS can also signal safety to the nervous system, especially through tactile forms like gentle tapping or bilateral audio tones.
Think of how rocking or swaying soothes a distressed infant. It’s the same principle.
BLS appears to activate the parasympathetic nervous system (the “rest and digest” branch), reducing fight-or-flight responses and helping clients stay grounded while engaging traumatic material.
Clinical observations:
Therapists and clients often report a sense of calm, stability, or “safe distance” from traumatic content during BLS—a sign that the body isn’t being flooded with cortisol or adrenaline in the moment.
So, What Happens When You Use BLS in EMDR?
Here’s the short version:
A distressing memory is activated (with images, sensations, and emotions).
Bilateral stimulation is introduced—often via eye movements, tapping, or tones.
The brain stays engaged in “dual attention”: one foot in the memory, one foot in the present.
Over time, the emotional charge decreases. New insights emerge. The memory shifts from reactive to resolved.
This process, called adaptive information processing (AIP), is the core model behind EMDR. BLS seems to accelerate and deepen this reprocessing, helping the brain move the trauma from “hot and now” to “filed and finished.”
Can You Use BLS Outside of EMDR Therapy?
Yes—and in many cases, it’s encouraged.
Bilateral stimulation can be used outside of trauma reprocessing to support:
Emotion regulation (e.g., calming anxiety or panic)
Resourcing (e.g., reinforcing internal strengths or positive memories)
Grounding (e.g., staying anchored during a triggering moment)
Try this: Cross your arms over your chest in a Butterfly Hug, and gently tap back and forth while focusing on your breath or a calming image. This simple practice can lower arousal and help you reconnect with the present.
Final Thoughts
While bilateral stimulation may look simple from the outside, inside the brain it’s doing complex, integrative work—stimulating memory networks, regulating arousal, and fostering healing across hemispheres.
It’s not a magic trick. It’s not hypnosis. It’s a science-backed way to help your brain do what it was always designed to do: process, adapt, and heal.
If you’re in EMDR therapy—or thinking about starting—know that BLS is more than a technique. It’s a bridge between the pain of the past and the freedom of the present.
