The Safe and Sound Protocol:
From Brain Stem to Hypotheses on Gene Expression.

Historically, ‘safety’ was treated by psychology and medicine as a cognitive construct - an absence of perceived threat. Polyvagal Theory, developed by Dr. Stephen Porges over more than four decades, showed that safety is primarily a measurable physiological state, regulated by the autonomic nervous system and operating largely outside the conscious mind.

The three phylogenetic stages

The autonomic nervous system did not emerge fully formed. It evolved in three stages, each of which built on what came before - and each of which remains active in the modern human nervous system:

The hierarchy is not merely descriptive - it is prescriptive for therapy. A nervous system stuck in sympathetic activation or dorsal shutdown has reduced access to the prefrontal cortex and can process language less meaningfully. Therapies that start ’top-down’ - with insight, language or cognitive re-approach - may reach a system that has gone partially offline less effectively.

The SSP works ‘bottom-up’: it targets the brainstem and the auditory system, aiming to strengthen the physiological basis that can make everything else more accessible.

The Social Engagement System: a symphony of brain nerves

The ventral vagal complex does not work in isolation. It coordinates an ensemble of cranial nerves that make up the Social Engagement System (SES) - the biological basis of human connection:

When the neural tonus of the cranial nerves feeding these structures is impaired by trauma, chronic stress or neurodevelopmental differences, the Social Engagement System becomes diminished available. The SSP aims to support this neural tonus - through sound, non-invasively.

The middle ear muscles and the biology of hyperacusis

In mammals, the middle ear muscles - the musculus stapedius and musculus tensor tympani - evolved to perform a selective function: actively muffling low-frequency background noise. Very low frequencies (below ~500 Hz) are instinctively associated with potential danger: the rumble of a predator, the thump of a threat. By modulating these frequencies, the middle ear muscles tune hearing to the range of 500-4,000 Hz - the natural bandwidth of the human voice.

This mechanism provides an explanation for hyperacusis - the phenomenon in which ordinary environmental sounds feel unbearably loud or threatening. From a polyvagal perspective, this is not only a defect in the cochlea, but also possibly the result of impaired middle ear muscle function, which causes the brainstem to be constantly exposed to frequencies that are interpreted as danger.

The technology of acoustic filtering

The SSP uses computer-modified vocal music - mostly contemporary folk or pop songs sung by female vocalists. The music is processed through a proprietary algorithm that dynamically modulates low and very high frequencies, limiting the acoustic envelope to the safety band of 500-4,000 Hz.

Crucially, it is not just the frequency selection that matters - it is the dynamic modulation itself. The filter provides the middle ear muscles with consistent, pulsating acoustic challenges, teaching them to actively tune in. As the nervous system constantly scans the environment for safety or threat (neuroception), the filtered music delivers repeated signals in the safety range directly to the brainstem.

Headphone instruction: For the SSP, a over-ear stereo headphones required - headphones where the ear cups are completely covered. In-ear earbuds and earbuds are not appropriate. Headphones with active noise cancellation (ANC) may be used, provided that noise cancellation and all other sound adjustments are completely turned off during the session.

The three pathways

The SSP is structured into three sequential programs, each with its own function:

• SSP Connect - A gentle introduction with unfiltered music. Prepares the autonomic system for the active intervention. (~1 hour)
• SSP Core - The active neural phase. Progressively filtered music challenges the middle ear muscles over their full retraining range. (~3-5 hours)
• SSP Balance - The integration phase. Lighter filtering supports Core's results over time. (Continuous)

Early research on cellular effects

Transcriptomic studies suggest that activation of the ventral vagal complex is associated with increased gene expression of genes such as Mbp, Myrf and Snap25 in neurons of the nucleus ambiguus - genes relevant to neurosignaling and myelin synthesis. This is a promising insight that opens the possibility that vagal neuromodulation may be not only functionally but also structurally relevant.

Physiological biomarkers: HRV and the Middle Ear Reflex

The scientific validity of the SSP is enhanced by the use of quantifiable biomarkers. Two are particularly relevant: heart rate variability (HRV) and the middle ear muscle reflex (MEMR).

Heart Rate Variability (HRV). is the variation in time between successive heartbeats and is recognized worldwide as an indicator of autonomic flexibility. Higher HRV indicates stronger parasympathetic influence and greater regulatory capacity.

Middle Ear Muscle Reflex (MEMR). - the middle ear muscle reflex - provides a second objective method of measurement, closer to the primary mechanism of action of the SSP. The MEMR can be measured by wideband tympanometry, which determines the sound intensity at which the stapedius muscle contracts. Recent research (medRxiv, 2026) has shown that age, hearing loss and co-activation all affect the MEMR and the medial olivocochlear reflex - scientifically supporting the development of more subtle MEMR measurements as a biomarker for changes in neural middle ear tone after SSP interventions. An ongoing clinical trial (NCT07309354) is specifically investigating the relationship between acoustic reflexes and muscle relaxation.

Within advanced clinical practice, one of the most valuable functions of the SSP is not what it does directly, but what it enables. When the nervous system is stuck in sympathetic activation or dorsal vagal shutdown, higher cortical centers are functionally less available. The SSP can expand the neural window making other interventions more accessible.

Somatic Experiencing and the SEGAN model

The synthesis of SSP with Somatic Experiencing (SE) - a body-centered trauma approach - is clinically promising. Within SE protocols, this combination has been formalized through the SEGAN model (Seeking Awareness by Embracing the Awakening of a Vision), developed by Ana do Valle and Laura Piche. The approach teaches clients to notice and embody shifts in their physiological arousal during SSP listening sessions - hypothesizing that the physiological safety provided by the SSP creates a favorable context for processing somatic memories.

EMDR: expanding the tolerance window

EMDR is one of the most evidence-based treatments for trauma. The main clinical challenge is to keep the client within the ’tolerance window’ - the autonomic zone in which memories can be processed without re-traumatization. The SSP is increasingly used as an autonomic preparation for EMDR because it can stabilize vagal tonus before bilateral stimulation begins - which can make the processing process more accessible and less destabilizing, especially for clients with complex trauma.

Neurofeedback: two bottom-up techniques

The combination of SSP and neurofeedback (NFB) is a clinically interesting pairing. NFB is designed to calm overactive brainwave patterns, but can reduce anxiety without necessarily restoring the experience of social safety. Clinically, it is suggested that NFB and SSP are complementary: NFB reduces overactivation, while SSP supports vagal capacity for social engagement.

Somatic applications: voice, throat and Functional Neurological Disorder

The vagus nerve innervates virtually all vital organs above the diaphragm - heart, lungs, larynx, pharynx. This explains why the SSP can be effective in seemingly non-psychiatric complaints.

Voice and throat complaints - published research (Grooten-Bresser et al., 2024)
A study published in Music and Medicine examined 33 individuals with unexplained voice, throat and respiratory symptoms. After five days of SSP, participants reported significant decreases in anxiety, depression and autonomic reactivity (measured with HADS), and specifically improvements in functions controlled by the vagus nerve above the diaphragm. The mechanism is anatomically coherent: the nerves controlling the larynx and pharynx (CN IX and X) lie in the same brainstem regions as the nerves for the ear and heart. When the autonomic state normalizes via auditory input, it has a direct effect on the tension of the throat muscles and the quality of the voice.

Functional Neurological Disorder - Harvard Review of Psychiatry (Rajabalee, Kozlowska, Porges et al., 2022)
A case study published in the Harvard Review of Psychiatry, co-authored by Dr. Stephen Porges, described a 10-year-old child with Functional Neurological Disorder (FND) - paralysis and tremors that did not respond to standard treatments including high doses of sertraline and CBT. Application of the SSP, embedded in a polyvagal-informed treatment plan, led to significant reductions in physical symptoms. The authors stated that through auditory stimulation of the brainstem, neural networks responsible for motor control and physiological state were supported, enabling recovery. A systematic review (Vincent et al., 2025, Occupational Therapy International) identified this as one of two published SSP studies in children, in addition to the Okayama study. Because this is an individual case study, no overall efficacy in FND can be inferred from it.

Psychoneuroimmunology (PNI) has identified precise neurological pathways through which psychological states regulate immune function. From this framework, the possible physical effects of the SSP are a relevant research question - although direct evidence for the SSP specifically is still limited.

The cholinergic anti-inflammatory pathway

Strong vagal tonus - measurable via heart rate variability (HRV) and respiratory sinus arrhythmia (RSA) - is associated with lower concentrations of pro-inflammatory cytokines, including TNF-alpha. The mechanism is relatively well documented: vagal activation stimulates the release of acetylcholine, which binds to nicotinic receptors on macrophages and can suppress the production of cytokines via the cholinergic anti-inflammatory pathway.

Chronic stress, trauma and social isolation reduce the accessibility of the ventral vagus nerve. The result can be persistent sympathetic dominance and systemic chronic inflammation - a mechanism associated with anxiety disorders, depression, cardiovascular disease, autoimmune disease and pulmonary COVID.

Lung COVID, ME/CFS and vagal dysautonomia

Research suggests that postviral conditions, including Long COVID, may be associated with a form of vagal dysautonomia. Studies suggest that vagal neuromodulation can potentially reduce excessive cytokine responses and support autonomic balance. Some of our clients with Long COVID and ME/CFS report physical improvements in addition to psychological. This is consistent with the PNI hypothesis, but is based on practical observations - not verified evidence.

Sociostasis, oxytocin and co-regulation

The process of ‘sociostasis’ - the co-regulation of physiological state via social connection - links the psychological with the immunological. Positive social buffering via prosodic cues (vocal warmth, gentle touch) is associated with oxytocin release, which is directly linked to the nucleus ambiguus and NTS - the brainstem centers for heart and vagus nerve. Central OT release can directly inhibit the HPA axis and calm the sympathetic nervous system. Because the SSP acoustically mimics the prosodic signature of safe social contact, it is plausible that it creates similar physiological conditions - although direct evidence for this specific mechanism in the SSP is still limited.

Epigenetics: hypotheses about molecular effects

Epigenetic research shows that early adversity and attachment trauma can alter gene expression via DNA methylation. Crucial finding: some epigenetic changes appear transmissible through the sex line - the dysregulation of the nervous system of traumatized parents can be found in the physiology of their children.

From a polyvagal perspective, sensory sensitivities in ASD are partly autonomic in nature - the nervous system filters human speech frequencies less effectively. Evidence ranges from two randomized controlled trials in children to a pilot study in adults.

The Listening Project Protocol studies - two RCTs (n=146)

Before the SSP became commercially available, it was investigated as a “Listening Project Protocol” in two consecutive randomized controlled trials involving a total of 146 children with ASD:

Trial II is particularly valuable scientifically: by comparing filtered music to unfiltered music, the study showed that the effects are specifically attributable to the filtering algorithm - not to listening to music per se. Children who showed improvement in hearing sensitivity also showed significant progress in social sharing behavior and interaction.

In both studies, the Respiratory Sinus Arrhythmia (RSA) measured as an objective physiological outcome measure. Participants in the intervention group showed a significant increase in baseline RSA post-intervention - objective validation that the intervention measurably affected autonomic state. Post-intervention, children also showed more stable RSA under cognitive load.

Okayama University Hospital - Exploratory pilot study in adults (n=6)

An exploratory pilot study at Okayama University Hospital examined the SSP in six adults with ASD (ages 21-44). Results showed statistically significant improvement on the ‘Social Awareness’ subscale of the SRS-2, correlated with improvements in physical health (WHOQOL-BREF) and decreases in anxiety (STAI) and depression (CES-D). A systematic review (Vincent et al., 2025) confirmed this as one of two published SSP studies in this population.

ADHD: regulation over attention

Although ADHD is primarily categorized as an attention disorder, researchers suggest that the underlying cause often lies in poor regulation of the nervous system. Many individuals with ADHD are in a state of physiological overdrive, manifesting as hyperactivity and impulsivity. Auditory processing problems are frequent: the inability to filter the teacher's voice from background noise creates a tremendous cognitive load.

From a polyvagal perspective, middle ear function support could contribute to an improved ‘signal-to-noise ratio’ in some clients - the ability to distinguish relevant sounds (the teacher's voice) from background noise. Clinical reports show that after the SSP, the frequency of emotional outbursts (meltdowns) may decrease because the nervous system is less likely to reach critical stress levels.

Foster care and child welfare - breaking the cycle

The prevalence of mental health problems is disproportionately high within the U.S. foster care system - estimates run as high as 4 in 5 foster children, primarily due to complex early childhood trauma. Organizations such as Alternative Family Services (AFS) have integrated the SSP to contribute to regulation on a physiological level - complementing existing therapeutic care.

“Blocked care” among foster and adoptive parents

The SSP is also employed in “blocked care” - the physiological exhaustion that foster and adoptive parents may experience when chronically overwhelmed by the care of severely traumatized children. By applying the SSP to both the child and the parent, it attempts to break the mutually disruptive neuroceptive loop - a theoretically coherent approach that is found to be clinically promising.

Education: Safe and Sound Schools

Within the American educational system, programs such as Safe and Sound Schools align with Multi-tiered Systems of Support (MTSS) frameworks. Here, the SSP is used as a physiological intervention targeting the underlying dysregulation manifested in behavioral problems, poor concentration or social withdrawal.

First responders and stress from critical incidents

Police officers, firefighters and paramedics carry an increased risk of allostatic overload and complex PTSD due to repeated exposure to existential threat. The SSP is increasingly being integrated into first responders' therapy programs and Critical Incident Stress Management protocols, where professionals can learn to make the physiological transition from chronic combat fatigue to real presence.

PTSD - Ongoing studies and institutional validation.

Department of Defense - $3.8 million for a randomized double-blind trial
In 2024, the U.S. Department of Defense (DoD) awarded nearly $3.8 million for a large-scale study of the SSP in PTSD - funded through the Peer Reviewed Medical Research Program (PRMRP). Led by Dr. Jacek Kolacz of The Ohio State University, the study is testing whether filtered music from the SSP Core, combined with cognitive processing therapy (CPT), reduces hyperarousal symptoms in PTSD better than CPT alone. The design is randomized and double-blind: SSP Core versus ‘sham’ music (unfiltered, as placebo). The study targets both military and civilian use and specifically measures anxiety, irritability and sleep problems. Data collection was expected to begin in late 2024; results not yet published as of May 2026.

Spencer Psychology pilot study (NCT04999852)
An observational pilot study examines the effects of the SSP on PTSD symptoms and anxiety in adults, using both self-report (PCL-5, GAD-7) and physiological measurements (HRV via earlobe PPG sensor). It is hypothesized that integration of the SSP into standard psychotherapeutic treatment leads to greater reduction in autonomic disruption than therapy alone. Results are expected following the DoD study.

The applications of Polyvagal Theory extend beyond clinical pathology. At the top of performance culture, the same autonomic principles that describe trauma dysregulation also explain certain limitations of peak performance.

The concept of autonomous flexibility

Autonomous flexibility is the ability to switch smoothly between internal physiological states under pressure - to recognize, adjust and return to a state of presence in real time. This is not a simple relaxation technique; it is a measurable, trainable physiological skill.

Many successful athletes and executives have built their results on dysregulated sympathetic activation: perfectionism, chronic vigilance, the adrenaline rush. Their achievements are real. So are the costs - to health, relationships, creativity. The SSP is used as a tool to support the physiological balance that enables sustained performance.

Applications in sports

Top sports programs in Australia and the U.S., among others, have integrated the SSP as part of broader athlete wellness programs. The hypothesis is that a better-regulated autonomic system recovers more quickly from intense activation - thereby easing the transition from competitive stress to recovery. Formal controlled research in sports contexts is still limited; field experiences are promising. For performance contexts, evidence for the SSP specifically is still based primarily on practical experience and theoretical extrapolation from autonomic regulation, not large-scale controlled studies.

Organizational well-being

In organizational contexts, the SSP is increasingly discussed as support for burnout prevention and leadership development. The assumption is that leaders with a better regulated nervous system have more room for empathy, creativity and nuanced decision-making - and are less reactive under pressure. Systematic research in organizational contexts is still scarce, but the theoretical basis is coherent.

One of the most surprising insights of Polyvagal Theory is that co-regulation - the biological process by which one nervous system calms the other - is not limited to the human species. Mammals share the evolutionary hardware for safety detection and social engagement. This has led to early explorations of the SSP in veterinary and animal-assisted contexts.

The SSP in rescue dogs

Carol J.S. Nickerson has explored the SSP as support for traumatic dogs - rescue dogs desensitized after repeated exposure to extreme conditions, or traumatized former shelter animals. The hypothesis is that filtered music in the frequency range of a reassuring human voice can affect neuroceptive processes in dogs as well. Systematic research with control groups has not yet been published; experiences are positive but preliminary.

Horses and the Polyvagal Equine Institute

The Polyvagal Equine Institute (PVEI) has developed Connection Focused Therapy (CFT) - an approach that applies polyvagal principles in the context of horse-human interaction. Horses are exceptionally sensitive to the autonomic states of those around them and in animal-assisted therapy serve as living biofeedback for the human client. Integrating SSP principles into this work is an active area of exploration.

SSP and the Tomatis method.

The Tomatis method, developed by French ENT physician Alfred Tomatis in the 1950s, was one of the early acoustic approaches that used frequency filtering for auditory training. Similarities: both use filtered music, both focus on the middle ear and auditory processing, both aim to improve listening skills and self-regulation. Differences: the Tomatis method has a broader focus on language development, vocal quality and learning; the SSP specifically targets the autonomic nervous system through Polyvagal Theory. The scientific rationale for the SSP through Polyvagal Theory is more recent. This comparison is not a ranking of effectiveness - each method varies in scope, research tradition and quality of available studies.

SSP and neurofeedback

Neurofeedback (NFB) focuses on direct training of brainwave patterns via real-time feedback on EEG activity. Similarities: both are non-invasive, bottom-up approaches aimed at regulation of the nervous system without medication. Differences: NFB works through the cortex and conscious feedback loops; the SSP works through the brainstem and autonomic nervous system. NFB can reduce anxiety without necessarily restoring the experience of social safety - the SSP specifically targets that social safety dimension through the middle ear. Clinically, both approaches are considered complementary.

SSP and HRV biofeedback

HRV biofeedback - the conscious regulation of breathing to increase heart rate variability - has solid empirical support for immune modulating and stress reducing effects. The SSP and HRV biofeedback target overlapping autonomic mechanisms, but through different pathways. HRV biofeedback requires active participation and conscious control of breathing - making it less accessible to clients who are too dysregulated for active exercise. The SSP is passive: the client listens. This is a clinically relevant distinction, not a hierarchical claim about which approach is superior.

Critical comments - what we know and what we don't know

A balanced scientific analysis also requires an honest discussion of limitations and criticisms. The current evidence for the SSP has three relevant limitations.

1. Need for large-scale RCTs in adults.
The strongest data (the LPP RCTs) are from pediatric populations. For adults with diagnoses such as generalized anxiety disorder, depression or chronic PTSD, larger, independent randomized controlled trials are needed to universally validate clinical effectiveness. Pilot studies and practice data are valuable, but not sufficient for broad clinical recommendations.

2. Variability in results
Not every client responds the same way to the SSP. An independent study of adults with self-reported auditory hypersensitivity found no consistent improvement. Factors such as duration of symptoms, degree of neuroplasticity, quality of co-regulation during the intervention and dosage presumably play an important role in outcome.

3. Academic discussion of Polyvagal Theory.
There is academic debate about the PVT itself. Some critics argue that the theory oversimplifies the complexity of the autonomic nervous system, particularly around its phylogenetic claims. Porges has responded to these criticisms in several scientific publications, and a recent publication (PMC, 2026) offered a direct scientific refutation of the most commonly cited objections. For clinical practice, the PVT remains a useful and coherent framework, even if some of the details are explored further.

Clinical perspective

Reconciliation: a new explanation of a persistent myth

One of the most influential recent contributions of Polyvagal Theory is the work of Porges, Bailey and Dugard (2023) on what they call “Atonement” (Appeasement) - replacing the term “Stockholm Syndrome.” The classic Stockholm syndrome implies a pathological emotional response to abductees. The polyvagal explanation is fundamentally different: under extreme, sustained threat - when fight-or-flight is not possible - the nervous system chooses its most sophisticated survival strategy: social connection to the aggressor as a means of physical survival.

This is not weakness. It is biology - the most adaptive behavior the autonomic nervous system can generate under such circumstances. This redefinition has far-reaching implications for how we understand trauma in survivors of abuse, hostage-taking and imprisonment. It shifts the perspective from pathology to physiological intelligence.

Porges, S.W., Bailey, R., & Dugard, J. (2023). Appeasement: replacing Stockholm Syndrome. European Journal of Psychotraumatology, 14(1).