Bilateral stimulation (BLS) is the core technique used during EMDR therapy in which a therapist guides your attention back and forth between the left and right sides of your body. This alternating stimulation happens while you simultaneously recall a distressing memory, and the combination is what makes EMDR distinct from other trauma therapies. BLS can take several forms: following a moving light or the therapist’s fingers with your eyes, holding small vibrating devices that pulse in each hand, or listening to tones that alternate between your left and right ears.
How It Works in a Session
During the processing phases of EMDR, your therapist asks you to hold a traumatic memory in mind, including the image, the emotions it brings up, and where you feel it in your body. While you focus on this memory, the therapist initiates bilateral stimulation. Each set typically lasts 20 to 30 seconds, after which the therapist pauses and asks what you’re noticing: new thoughts, feelings, images, or body sensations. Then the cycle repeats. This back-and-forth pattern continues until the memory loses its emotional charge.
The same bilateral stimulation is also used during a phase called “resource installation,” where positive beliefs and calming memories are paired with the left-right stimulation to strengthen them. So BLS isn’t only used to process painful material. It’s also used to reinforce the healthier perspective that replaces it.
The Three Types of Bilateral Stimulation
The original and most studied form is eye movements. Your therapist moves their fingers side to side in front of your face, or uses a light bar with a dot that sweeps back and forth, and you track it with your eyes. This is the “EM” in EMDR and remains the most commonly used modality.
Tactile stimulation uses handheld devices called pulsators or “tappers” that vibrate alternately in each hand. Some therapists use manual tapping instead, lightly tapping the tops of your hands or your knees in an alternating pattern. Auditory stimulation delivers tones or clicks through headphones, alternating between the left and right ears. Modern wireless EMDR kits allow therapists to control all three modalities through a smartphone app, adjusting speed and intensity for each client.
Tactile taps or auditory tones are commonly used for clients who have difficulty with visual tracking. A meta-analysis of 26 randomized controlled trials found that eye movements make a significant contribution to processing emotional memories, and some researchers still consider them the most effective form of BLS. That said, all three modalities are used successfully in clinical practice, and therapists often adjust the type based on what feels most comfortable and productive for the individual client.
Why Alternating Stimulation Changes How Memories Feel
The leading scientific explanation is called the working memory hypothesis. Your brain has a limited amount of working memory, the mental workspace you use to hold and manipulate information in the moment. When you recall a traumatic memory while simultaneously tracking a moving light or feeling alternating vibrations, both tasks compete for that limited capacity. Your brain can’t fully reconstruct the memory in all its original vividness while also processing the sensory input from the stimulation.
The result is that the memory gets retrieved in a slightly blurred, incomplete way. The disturbing images become less vivid, and the emotions attached to them weaken. When your brain then stores the memory again, it gets filed with that reduced emotional intensity. Over repeated sets, the memory gradually loses its power to trigger the same distress. Research has confirmed this mechanism: recalling a traumatic memory while performing a secondary task shifts attention away from the retrieval process, producing measurable reductions in both vividness and emotionality.
The Calming Effect on Your Nervous System
Beyond the working memory explanation, bilateral stimulation also appears to trigger something called an orienting response. This is your brain’s natural reaction to new or changing stimuli, a brief spike in attention lasting fewer than 10 seconds. Normally, this response involves a quick activation of both your stress system and your calming system simultaneously, followed by a settling down as your brain registers that the stimulus isn’t a threat.
When BLS repeats this orienting response over and over during a session, the cumulative effect is a measurable decrease in physiological arousal. Studies monitoring heart rate and nervous system activity during EMDR sessions with PTSD patients have found that heart rate drops, and the parasympathetic nervous system (the branch responsible for rest and recovery) becomes more active throughout the session. Researchers noted that the speed of these calming effects was too rapid to be explained by simple habituation alone, suggesting that the bilateral stimulation itself actively drives the body toward a more relaxed state. This may be part of why clients can process highly distressing material without becoming overwhelmed.
Where BLS Fits in the Larger EMDR Framework
EMDR is built on the idea that traumatic experiences can get “stuck” in the brain’s information-processing system. Under normal circumstances, your brain processes daily experiences and files them into long-term memory in an organized way. But during overwhelming events, this processing system can get blocked. The memory gets stored in its raw, unprocessed form, complete with the original emotions, physical sensations, and distorted beliefs (like “I’m not safe” or “It was my fault”). These frozen memories are what drive symptoms like flashbacks, nightmares, and hypervigilance.
Bilateral stimulation is the mechanism EMDR uses to restart that stalled processing. By engaging the brain’s attention with rhythmic left-right input while the stuck memory is activated, BLS appears to help the brain do what it couldn’t do at the time of the trauma: process the experience, connect it with more adaptive information, and store it as a resolved past event rather than an ongoing threat. The memory doesn’t disappear. You still remember what happened. But it no longer carries the same emotional weight or triggers the same physical reactions.
Safety Considerations
Bilateral stimulation is generally well tolerated, but there are situations where therapists modify their approach. People with epilepsy or seizure disorders may be at risk from certain types of visual stimulation, so therapists often switch to tactile or auditory BLS instead. For clients with severe dissociative disorders or fragmented identity structures, therapists typically do extensive stabilization work before introducing BLS, since activating traumatic material without adequate preparation can be destabilizing. Trained EMDR therapists assess for these factors before beginning the processing phases and adjust the type, speed, and duration of bilateral stimulation accordingly.