How Does Auditory Bilateral Stimulation Reorganize the Brain During Fear Processing? An fMRI Study

Rousseau PF, Boukezzi S, Garcia R, Chaminade T, Khalfa S. Cracking the EMDR Code: Recruitment of Sensory, Memory and Emotional Networks During Bilateral Alternating Auditory Stimulation. Australian & New Zealand Journal of Psychiatry, 2020. Read the paper.

Researchers used the same fear-conditioning paradigm as in Boukezzi et al. (2017), but combined it with functional MRI (fMRI) to examine what happens in the brain when bilateral auditory stimulation (BLS) is applied during fear extinction learning.

Participants: 37 healthy volunteers (mean age 32.6 years; 25 men).

• Participants first underwent fear conditioning by repeatedly pairing the presentation of a geometric shape with a mild electric shock.
• This was followed by the extinction phase, during which the same stimulus was presented without the shock.
• Each participant completed two experimental conditions in a randomized order: one with auditory BLS (alternating 1 Hz auditory clicks, 60 BPM) during extinction learning and one without BLS.
• On the following day, researchers assessed extinction recall to determine whether the reduction in fear had been retained.
• Brain activity (BOLD signal) and functional connectivity between brain regions were measured throughout the experiment.

1. BLS activates more than the auditory cortex. It recruits networks involved in fear processing and memory.

During the early phase of extinction learning, when fear levels were still high, BLS increased activity in the right precuneus, a region involved in context-dependent episodic memory, multisensory integration, and self-referential processing, as well as the left medial frontal gyrus, which contributes to decision-making, sensory integration, and evaluating the significance of incoming information.

2. BLS produces lasting changes in brain networks.

The same brain regions remained more active during the extinction recall session on the following day, despite the absence of bilateral stimulation.

“Remarkably, on the day following extinction, we found again an increased activation in the medial frontal gyrus and the precuneus, together with numerous changes in functional connectivity despite the absence of BLS. The BLS effects therefore persisted from one day to the next.”

3. BLS strengthens communication between major brain networks.

Functional connectivity analysis showed that BLS enhanced communication between regions involved in sensory integration, executive control, emotional processing, salience detection, and memory.

“The connectivity analysis found a significant increase in connectivity during bilateral alternating stimulation versus without bilateral alternating stimulation in the early extinction and recall between the two superior temporal gyri, the precuneus, the middle frontal gyrus and a set of structures involved in multisensory integration, executive control, emotional processing, salience and memory.”

4. BLS may help the brain reinterpret bodily signals of fear.

The insula emerged as a key hub in the BLS network. Its strengthened functional connectivity with the auditory cortex persisted during both fear extinction and extinction recall on the following day. This pattern suggests that BLS may facilitate the reassignment of bodily sensations associated with fear to a newly learned safe context. As the brain repeatedly pairs the previously threatening stimulus with neutral bilateral sounds and the absence of danger, physiological responses to the trigger become progressively updated.

5. The effects of BLS depend on emotional intensity.

BLS had its strongest impact during the early stages of extinction learning, when fear responses were greatest. Once fear had substantially diminished later in extinction training, BLS no longer produced measurable changes in brain activation.

“BLS had a maximal effect on brain activation at the beginning of extinction learning, when fear was high. However, at the end of extinction learning, when fear had markedly decreased, BLS no longer influenced brain activation. Thus, the BLS effect may depend on the negative emotional load of the memory, as if the memory network were more labile when emotions are experienced.”

When a person recalls a frightening experience while receiving bilateral auditory stimulation, the brain gradually updates the way that memory is represented. The memory becomes less strongly associated with threat and acquires a safer emotional meaning. This process appears to involve coordinated activity across multiple brain regions, including the prefrontal cortex, which supports cognitive control and evaluation, the precuneus, which integrates memory and self-related processing, the insula, which represents bodily sensations and emotional awareness, and sensory cortices. Together, these networks contribute to integrating the original memory into a new, non-threatening context.

Three things are worth carrying away from this, and one thing is worth leaving behind.

• The signal is doing something measurable. A plain alternating click is not merely pleasant background — it changes activity and connectivity in networks that handle memory and emotion, which is the mechanism described in how and why bilateral stimulation works.
• The change outlasts the session. The differences were still there a day later with no stimulation running, which is an argument for practising regularly rather than only in the moment.
• It matters most when something is actually charged. The effect was largest while fear was high and disappeared once it had faded. Applied to ordinary use: a session is likelier to do something on an evening when your head is genuinely busy than on one when it is not.

What to leave behind: this is 37 healthy volunteers and a shape paired with a shock — not patients, not treatment, not a clinical outcome. It says the mechanism is real and describes how it behaves. It does not say that listening to alternating tones treats anything, and the practice itself should be judged on the modest, everyday terms it is offered on.