What Is Bilateral Stimulation in EMDR Therapy?

Written by Murat Yön, clinical psychologist, EMDR Europe accredited therapist

Updated 2026-10-14 · For EMDR-trained clinicians

Bilateral stimulation (BLS) is the rhythmic left–right sensory input used during EMDR therapy while a client holds a distressing memory in mind. This guide covers the three delivery modalities, how clinicians choose between them, and what the research does and does not establish. To hear auditory alternation for yourself, the free online bilateral stimulation tool runs in a browser.

The basic definition

Bilateral stimulation means alternating sensory input between the left and right sides of the body at a steady rhythm. In EMDR it is applied during the reprocessing phases while the client attends to a target memory, the associated body sensation, and the negative belief attached to it.

The original protocol developed by Francine Shapiro used guided horizontal eye movements. Auditory and tactile forms were added later and are now standard clinical options, particularly in remote work (see EMDR via Zoom and telehealth: bilateral stimulation setup).

The three modalities

Visual (eye movements)

The client tracks a moving stimulus — the therapist's hand, a light bar, or a dot moving across a screen — while holding the target in mind. This is the modality used in most of the original research base. It requires the client to be able to sustain visual tracking, which makes it harder in low-vision, migraine-prone, or heavily dissociative presentations.

Auditory (alternating tones)

A tone alternates between the left and right ear through stereo headphones. It frees the client's gaze entirely, which many clients find less effortful, and it lets them close their eyes — often useful when eye contact itself is activating. It is also the modality that travels most easily: it needs only headphones and a phone. Clinicians weighing it against a light bar can read EMDR light bar alternatives.

Tactile (alternating taps or buzzes)

Alternating physical taps, delivered by handheld pulsers, by the therapist tapping the client's knees or hands, or by the client crossing their arms and tapping their own shoulders (the butterfly hug). Useful when auditory and visual channels are both unavailable or overwhelming.

Choosing a modality in practice

There is no clinical consensus that one modality outperforms the others across the board, so the choice is usually driven by the client and the setting:

• Client preference and tolerance. A client who finds tracking tiring or who reports headaches after eye-movement sets will usually process more comfortably with tones.
• Remote sessions. Visual BLS delivered over a video call is at the mercy of frame rate and connection quality; a dropped frame breaks the rhythm at exactly the wrong moment. Auditory BLS played locally on the client's own device is unaffected by connection quality.
• Dissociation. Some clinicians prefer tactile or auditory input when grounding is a concern, since it keeps a body-based anchor available.
• Environment. In a shared or noisy clinic, headphones give a boundary that hand movements do not.

Many clinicians switch modality mid-course rather than committing to one — for example, using eye movements in the consulting room and tones for remote sessions with the same client.

Speed, sets and tone

Beyond modality, three parameters shape how a set feels:

• Speed. Faster alternation is generally used during desensitisation; slower rhythms are typical for resource installation, calm-place work and closure.
• Set length. Set length is adjusted continuously in response to what the client reports between sets, rather than fixed in advance.
• Tone or pitch. With auditory BLS, a lower tone is often experienced as less intrusive; a client who winces at a bright tone will process differently once it is softened.

The clinically important point is that all three are adjusted during processing, not set once at the start. That is why therapist-side control of the stimulation matters in remote sessions: if the client is the one holding the controls, each adjustment interrupts the very state you are trying to maintain.

What the research says

EMDR as a whole is recommended for post-traumatic stress disorder in major clinical guidelines, including those of the World Health Organization and the UK's National Institute for Health and Care Excellence. That recommendation concerns the full eight-phase protocol delivered by a trained clinician; it does not isolate bilateral stimulation. The question of what BLS itself contributes is a separate one, and the literature below reads as describing an open question, not a settled answer. All four papers are indexed on PubMed and none carries a retraction notice.

Is the stimulation an active component?

Jeffries and Davis (2013) reviewed the evidence on whether eye-movement bilateral stimulation is an essential component of EMDR. They reported that clinical research evidence is contradictory on how essential eye movements are, that analogue studies provide more positive support, and that some evidence suggests bilateral stimulation may increase access to episodic memories and act on components of working memory. Behavioural and Cognitive Psychotherapy, 41, 290–300. doi:10.1017/S1352465812000793 (PMID 23102050).

Laboratory work on memory vividness

Leer, Engelhard and van den Hout (2014) randomised 73 undergraduates to recall negative autobiographical memories with and without eye movements. Recall with eye movements, but not recall alone, was followed by an immediate reduction in memory vividness, and the group with the longer intervention also showed reductions in vividness and emotionality at a 24-hour follow-up. The authors noted that only self-report measures were used, and the participants were not a clinical sample. Journal of Behavior Therapy and Experimental Psychiatry, 45, 396–401. doi:10.1016/j.jbtep.2014.04.004 (PMID 24814304).

Proposed mechanisms

Landin-Romero, Moreno-Alcazar, Pagani and Amann (2018) systematically reviewed 87 studies of suggested mechanisms of action. They reported reasonable empirical support for the working memory hypothesis and for physiological changes associated with successful therapy, and concluded that research into mechanisms was still in its infancy, with larger samples and tighter methodological control needed before firm conclusions. Frontiers in Psychology, 9, 1395. doi:10.3389/fpsyg.2018.01395 (PMID 30166975).

Calancie, Khalid-Khan, Booij and Munoz (2018) reviewed neurobiological theories involving working memory, interhemispheric communication, de-arousal and memory reconsolidation, and put forward a hypothesis about the timing of sensory stimulation and baseline saccade metrics. The authors present it as a hypothesis; it is not an established finding. Annals of the New York Academy of Sciences, advance online 21 June 2018. doi:10.1111/nyas.13882 (PMID 29931688).

What follows for practice

Across these sources, the working-memory account has received the most attention, the clinical evidence on any single modality is described as mixed or limited, and whether bilateral stimulation is necessary for the effects of EMDR remains debated. Studies comparing modalities have generally not found large differences between visual, auditory and tactile delivery, although this is a thin evidence base and should not be over-read. A clinician can therefore reasonably choose the modality that suits the client and the setting while delivering the rest of the protocol as trained. If you are comparing delivery options in the consulting room, EMDR light bar alternatives sets out the practical differences, and the eight-phase reference shows where stimulation sits in the protocol.