What bilateral stimulation is, why it works, and the three types used in clinical EMDR practice.
BLS stands for bilateral stimulation — the alternating left-right sensory input that is the defining mechanism of EMDR therapy. During a standard EMDR session, a client holds a target memory in mind while receiving BLS in sets of 15–30 seconds. Between sets, they briefly report what came up. The process continues until the memory's emotional charge (measured on the SUDS scale) drops toward zero.
The "bilateral" part refers to the alternating side-to-side pattern — not both sides at once, but left, then right, then left again. This rhythmic alternation appears to be what distinguishes EMDR from other exposure-based approaches, though the exact mechanism remains an active area of research.
A moving stimulus — therapist's fingers, a light bar, or a screen animation — that the client tracks with their eyes. The original EMDR modality.
Alternating tones in left and right channels of headphones. Often used in combination with tactile BLS, or for clients who prefer to close their eyes.
Alternating vibration or physical taps in each hand — via hardware tappers, buzzers, or Joy-Con controllers. Standard for remote sessions and teletherapy.
Research has not established a clear superiority of one modality over others, and clinicians often let client preference guide the choice. Many therapists use tactile BLS for remote EMDR sessions because it doesn't require the client to track a moving visual element on a shared screen.
The honest answer is that the mechanism isn't fully resolved, but the most empirically supported hypothesis is the working memory theory: bilateral stimulation occupies the same attentional and visuospatial processing resources required to hold a traumatic memory in vivid, emotionally intense form. By taxing those resources, BLS reduces the memory's distress while it remains sufficiently active for reprocessing.
The working memory account explains several EMDR findings: why the bilateral element seems necessary (compared to simple eye movements or attention tasks), why BLS should be fast enough to tax attention without being distracting, and why sets should be time-limited. It also explains why EMDR doesn't cause forgetting — the memory becomes less distressing, not less accessible.
A second hypothesis draws parallels to REM sleep, during which rapid eye movements occur naturally during emotional memory consolidation. Some researchers propose that waking bilateral stimulation may activate similar neural processes. This theory has intuitive appeal but less direct empirical support than the working memory account.
Standard EMDR protocol typically uses a speed of 1–2 Hz (one complete left-right cycle per second to two per second), though therapists adjust based on client response. Slower speeds are often used for resource installation and stabilization work; faster speeds for active trauma processing.
The number of BLS sets, the number of taps per set, and the inter-set interval are all clinician-adjustable. In TheraJoy, speed is adjustable from 0.25 Hz to 3 Hz, and the therapist can modify settings mid-session without interrupting the client's focus.
Tactile bilateral stimulation has historically required dedicated hardware — EMDR tappers or buzzers costing $100–$450 per set. These devices connect to a control unit that the therapist operates, alternating a vibration motor in each handset. They're effective but expensive, bulky, and impractical for remote sessions.
Nintendo Joy-Con controllers contain the same class of haptic motor (linear resonance actuators) used in dedicated tappers, with substantially more precision than older vibration motors. TheraJoy turns Joy-Cons into BLS tools via Bluetooth — the therapist and client each use their own phone, and the therapist controls BLS settings in real time. See the guide to EMDR tappers for a comparison of hardware and app options.
Tactile BLS is particularly well-suited to remote EMDR because it doesn't require the client to track a stimulus on a shared video call. The client holds Joy-Con controllers or another BLS device, the therapist controls the session from their own device, and the video call serves as the communication channel between them. For more on setting up remote EMDR sessions, see the EMDR teletherapy guide.
BLS is the mechanism — EMDR is the full protocol. EMDR includes assessment, history-taking, preparation (stabilization, resource installation), active processing using BLS, and closure phases. BLS is what happens during the processing phase.
Self-directed BLS for resource installation (grounding, safe-place exercises, positive cognition strengthening) is considered appropriate and is sometimes assigned by therapists between sessions. Active trauma processing using BLS should be done with a trained therapist. TheraJoy includes slow-speed resourcing presets for between-session use.
It varies widely by client and target. A typical processing session might include 10–30 sets, each lasting 20–30 seconds. A full EMDR protocol across all phases spans multiple sessions, with the number depending on the complexity of the target material.
With tactile BLS, clients typically describe a gentle alternating vibration or tap in each hand — calming and rhythmic rather than intrusive. Most clients habituate quickly and find it easy to maintain dual awareness (holding the memory while receiving BLS). Auditory BLS is described as a soft alternating click or tone. Visual BLS requires more active participation (tracking) but is the most widely used in face-to-face sessions.
TheraJoy delivers tactile, visual, and auditory bilateral stimulation via iPhone. Remote sessions included in Pro. Free 7-day trial.
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