How Bilateral Stimulation Works.
What is bilateral stimulation?
Bilateral stimulation (BLS) refers to any form of sensory input that alternates between the left and right sides of the body — left-right-left-right — in a rhythmic, repeating pattern. In EMDR (Eye Movement Desensitization and Reprocessing) therapy, BLS is the mechanism that supports trauma processing while the client holds a distressing memory in mind.
The term "bilateral" simply means two-sided. The alternating quality — the fact that the stimulus moves back and forth rather than stimulating both sides at once — is what matters for the therapeutic effect.
The three modalities
BLS can be delivered in three ways, and research suggests all three are comparably effective for most clients. Therapist and client preference typically guide the choice:
- Visual: a moving light bar or the therapist's fingers moving side to side — the original method developed by Francine Shapiro
- Auditory: alternating tones played through headphones — a tone in the left ear, then the right
- Tactile: alternating taps or vibrations in each hand — the most commonly used modality in modern practice, especially for clients who find visual tracking difficult
TheraJoy delivers all three modalities in one iPhone app — visual (on-screen light bar), auditory (stereo tones), and tactile bilateral stimulation via Joy-Con controllers.
Why alternating? The dual attention hypothesis
The leading explanation for why BLS works is the dual attention hypothesis: holding a traumatic memory in mind while simultaneously tracking an external alternating stimulus taxes working memory in a way that reduces the emotional charge of the memory. The brain cannot hold a vivid, emotionally charged image with full intensity while simultaneously processing an external alternating cue.
The repetitive bilateral movement also resembles the rapid eye movements of REM sleep — the phase when the brain is thought to consolidate and reprocess emotional memories. This parallel has led some researchers to suggest EMDR may work in part by activating similar neurological processes.
Both the dual attention and REM-sleep hypotheses are active areas of research. What's well-established is the clinical outcome: EMDR with bilateral stimulation consistently outperforms EMDR without it in controlled trials, and is recognized by the WHO, APA, and VA/DoD as a first-line treatment for PTSD.
Speed and intensity matter
BLS is typically delivered at 0.5–2 Hz (cycles per second) — about one complete left-right cycle every half to two seconds. The speed is adjusted based on the phase of work:
- Slower (0.25–0.5 Hz): highly activated trauma, early-phase processing, clients who need a gentler pace
- Moderate (0.5–1.5 Hz): standard reprocessing
- Faster (1.5–3 Hz): resource installation, positive memory work, cognitive interweaves
Intensity — how strongly the tactile signal is felt — is adjusted to client preference. Present enough to notice and track; not so strong it becomes distracting or aversive.
Tactile BLS: why haptic hardware matters
For tactile bilateral stimulation, the quality of the vibration directly affects the therapeutic experience. Clients need to clearly feel the alternation — left, then right — throughout a set. If the signal is too diffuse or the two sides are hard to distinguish, the dual-attention state is harder to maintain.
Cheap vibrating devices typically use ERM (eccentric rotating mass) motors — a spinning weight that creates a diffuse, buzzy sensation. Higher-quality devices use LRA (linear resonance actuator) motors — the same technology in iPhone's Taptic Engine — which produce a clean, precise, distinct tap.
TheraJoy uses Joy-Con controllers, which contain LRA haptic motors, for tactile bilateral stimulation. This is the same haptic hardware class as dedicated clinical tappers costing $100–$450, delivered through a bilateral stimulation device most clients already own.
BLS in teletherapy
Remote EMDR sessions require the client to have their own BLS source. Visual BLS can be delivered on-screen. Auditory BLS works through any headphones. Tactile BLS requires the client to have a device — either dedicated hardware ($100–$450) or a pair of Joy-Con controllers ($80, or free if they already own a Nintendo Switch).
TheraJoy's remote session model lets therapists control BLS settings — speed, intensity, modality — while the client uses their own device at home, joining via a shared session code for free. See the EMDR teletherapy guide for a full setup walkthrough.