What is Bilateral Stimulation (BLS), and why is it important in EMDR?
If you've been researching EMDR therapy, you've probably come across the term bilateral stimulation. You may be wondering what it actually means, and why it matters. It sounds clinical and a little mysterious, but the concept is simpler than it sounds. And once you understand what it is and why we use it, you’ll have a clearer understanding about how this type of healing actually happens.
Let's Start with the Basics
Bilateral stimulation, or BLS, refers to any form of sensory input that alternates from one side of the body to the other. Left, right, left, right.
In an EMDR session, this typically looks like one of three things:
Eye movements. You follow a moving dot, a light bar, or my finger back and forth across your screen or visual field. This is the most commonly known form of BLS, and the one that gave EMDR its name.
Tapping. You tap alternately on your knees, your shoulders, your thighs, or even on an object such as the desk in front of you. You can do the tapping yourself, or with the help of handheld tappers that vibrate gently from side to side.
Audio tones. You listen through headphones to a tone that alternates from ear to ear.
All three accomplish the same thing: alternating stimulation from the left side of the body to the right. The method we use is largely a matter of personal preference and what feels most comfortable for you.
Why Does It Work?
This is the question most people really want answered, and it's a fair one.
The honest answer is that researchers are still working to fully understand the mechanisms behind BLS, but there are several compelling theories, and a growing body of research supporting its effectiveness.
The working memory hypothesis suggests that engaging in bilateral stimulation while holding a distressing memory in mind taxes your working memory—the mental system that holds information in the short term. When your brain is occupied with tracking the left-right movement, it has fewer resources available to process the memory with the same emotional intensity. The memory becomes less vivid, less charged, and easier to work with. In short, the BLS is providing just the right level of distraction to keep “one foot in the present, one foot in the past.” You’re focused enough on the past memory to activate it and move through the trauma, but distracted enough that you don’t become emotionally overwhelmed, or flooded, by slipping too deeply into the memory.
The orienting response theory proposes that the rhythmic back-and-forth movement mimics the natural eye movements we make when scanning our environment for safety. This activates a calming response in the nervous system, similar to what happens when we feel safe enough to look around and take in our surroundings.
The connection to REM sleep is perhaps the most intriguing theory. The eye movements used in EMDR closely resemble the rapid eye movements that occur during REM sleep, the stage of sleep associated with memory consolidation, and emotional processing. This is also, not surprisingly, the stage of sleep in which dreams happen. Since dreams are widely understood to be a mechanism for the brain to process and make sense of your experiences and thoughts, that makes a lot of sense. Some researchers believe BLS may activate a similar process, helping the brain integrate and make meaning of difficult experiences in a way it does naturally during sleep.
So, can you do EMDR without BLS? Yes and no. To do EMDR without BLS is essentially to provide talk therapy. And what we know from decades of research is that EMDR with bilateral stimulation is more effective than EMDR without it, and that, when administered properly, EMDR produces measurable changes in how the brain processes traumatic memories.
What Does BLS Actually Feel Like?
Most people find bilateral stimulation surprisingly calming once they get used to it. The rhythmic quality often has a grounding effect, giving your body something predictable to follow while your mind does the deeper work.
Some people have a strong preference for one type of BLS over another. I've worked with clients who find eye movements too distracting and prefer tapping. Others love the headphone tones because it feels more contained. There's no right answer, and we'll figure out together what works best for you.
A Note on Virtual EMDR
If you've wondered whether EMDR works online, the answer is yes. And bilateral stimulation translates well to virtual sessions. I use a browser-based BLS tool that displays a moving dot on your screen, while I control the speed of the dot and the duration of each set of BLS from my computer. I also have the option of adding in audio tones that can be delivered through your headphones. Alternately, tapping can be self-directed with simple guidance. Since I opened my private practice in 2019, I’ve offered exclusively virtual sessions, and the healing my clients report after completing EMDR treatment is real.
The Bottom Line
Bilateral stimulation is a core component of EMDR, and provides an integral piece of the puzzle for trauma resolution through this modality. EMDR’s specific blend of focus all aspects of each memory—the image, the emotion, the body sensation, and the belief— combined with BLS, allows your brain to process stuck memories in a way that talk therapy alone can't access.
If you're curious about what this might feel like, and if EMDR is the right path for you, I offer a free consultation call where we can talk through any questions you have before you commit to anything.
Susan Candiloro, LCSW-R, is an EMDRIA Certified Therapist and EMDRIA Approved Consultant in private practice in New York and Florida. She specializes in working with high-achieving women navigating anxiety, perfectionism, burnout, and people-pleasing.