Vestibular Perception Gates Autonomic Responses to Sinusoidal Galvanic Vestibular Stimulation

Detailed Description

Scientific Background and Rationale Occupational stress is recognized as an important contributor to impaired physical and mental health. Continuous exposure to cognitive workload, emotional demands, and psychosocial stressors may alter autonomic nervous system regulation, resulting in reduced physiological adaptability and increased cardiovascular risk. Heart rate variability (HRV) is one of the most widely accepted non-invasive indicators of autonomic regulation, reflecting the dynamic interaction between sympathetic and parasympathetic branches. Reduced HRV has consistently been associated with impaired stress resilience, diminished vagal regulation, increased cardiovascular morbidity, and adverse mental health outcomes.

Galvanic vestibular stimulation (GVS) applies weak electrical currents through electrodes positioned over the mastoid processes behind the ears. The electrical current activates vestibular afferents, influencing vestibular pathways that project not only to cortical vestibular regions but also to autonomic nuclei within the brainstem. Experimental and neuroanatomical studies have demonstrated direct vestibulo-autonomic connections involving the vestibular nuclei, nucleus tractus solitarius, dorsal motor nucleus of the vagus nerve, and other cardiovascular regulatory centers. Through these pathways, vestibular stimulation may influence autonomic cardiovascular regulation and HRV.

Several laboratory studies have reported that GVS may increase parasympathetic activity, reduce sympathetic influence, and modify cardiovascular reflexes. However, reported autonomic effects vary considerably across studies due to differences in stimulation parameters, waveforms, current intensity, recording conditions, and participant characteristics. Most previous investigations were conducted under highly controlled laboratory conditions, which maximize internal validity but limit ecological validity and clinical translation.

Recent technological advances have facilitated the development of portable GVS systems suitable for ambulatory use. These developments have created opportunities to evaluate vestibular neuromodulation under real-life conditions, where autonomic regulation is continuously challenged by natural fluctuations in cognitive workload, emotional demands, posture, movement, fatigue, and environmental stressors. Despite increasing interest in wearable neuromodulation technologies, the physiological effects of repeated GVS administered during normal occupational activities remain insufficiently investigated.

An additional unresolved question concerns the considerable inter-individual variability in subjective responses to vestibular stimulation. Some individuals experience pronounced sensations of self-motion or dizziness during GVS, whereas others report minimal or no vestibular sensations despite receiving identical stimulation parameters. This variability suggests that autonomic responses may depend not only on the physical characteristics of vestibular stimulation but also on the degree of conscious vestibular perception.

Current concepts of interoception and predictive processing propose that autonomic regulation is dynamically influenced by the brain's interpretation of incoming bodily signals rather than by peripheral sensory input alone. According to these models, physiological responses may be modulated by the subjective significance of sensory information and by prediction errors generated when incoming vestibular information conflicts with visual and proprioceptive signals. Therefore, conscious perception of vestibular stimulation may represent an important determinant of autonomic adaptation.

Detailed Methodology The study was conducted in two occupational settings: rest facilities of Kazakhtelecom JSC and the neurocognitive laboratories of the Brain Institute at Farabi University.

Following enrollment and baseline assessment, participants entered a four-day intervention period with one study visit per day. Each visit followed an identical protocol. Upon arrival, participants were fitted with a Polar H10 chest-mounted heart rate sensor for continuous ambulatory ECG recording. After sensor placement, participants completed the first PANAS assessment. ECG recording continued while participants performed their routine occupational activities. Immediately before stimulation, participants completed a second PANAS assessment. They subsequently received either active or sham GVS while seated comfortably with their eyes closed, listening to standardized meditative music. Immediately following stimulation, participants completed a third PANAS assessment together with a standardized questionnaire evaluating stimulation-related sensations (dizziness, warmth, burning, itching, each rated on a 4-point scale from 0 = "not experienced" to 3 = "strongly experienced"). Approximately one hour after stimulation, the sensor was removed and participants completed the fourth PANAS assessment. On the final intervention day, participants indicated whether they believed they had received active or sham stimulation to evaluate blinding integrity.

One to two days after the intervention course, participants underwent a post-intervention assessment identical to baseline procedures, including ambulatory ECG monitoring and psychometric evaluation.

Heart Rate Variability Assessment Continuous RR interval data were acquired using the Polar H10 sensor. Recordings were exported for offline processing using Kubios HRV Premium software. Artifact correction and interpolation procedures were applied before HRV calculation. HRV indices were calculated for consecutive six-minute epochs according to international recommendations.

Time-domain indices included mean RR intervals and SDNN. Frequency-domain analysis included normalized high-frequency power (HFnu). Low-frequency power was excluded from primary analyses because the stimulation frequency (0.1 Hz) overlapped with the conventional low-frequency HRV band, potentially confounding physiological interpretation. Nonlinear indices included SD1, SD2, the SD2/SD1 ratio, and detrended fluctuation analysis (DFA α1). The Baevsky Stress Index was calculated as an integrative geometric measure of regulatory strain.

HRV was analyzed at three temporal scales: acute (six-minute epochs before, during, and after stimulation), macro-scale (one-hour pre- and post-stimulation recordings), and course-level (baseline versus post-intervention comparisons).

Psychometric Assessment Psycho-emotional functioning was evaluated using standardized self-report questionnaires. The Positive and Negative Affect Schedule (PANAS) was administered repeatedly to assess short-term affective changes. Depressive symptoms were evaluated using the Inventory of Depressive Symptomatology - Self Report (IDS-SR). Anxiety symptoms were assessed using the Generalized Anxiety Disorder 7-item scale (GAD-7). Validated Russian-language versions were used for all questionnaires.

Statistical Analysis Baseline characteristics were compared using parametric or non-parametric tests depending on data distribution. Categorical variables were analyzed using Pearson's chi-square tests.

Psychometric outcomes were analyzed using repeated-measures ANOVA and ANCOVA with baseline adjustment when appropriate. Post hoc comparisons were corrected using Bonferroni adjustment.

Primary autonomic analyses were performed using linear mixed-effects models estimated by restricted maximum likelihood. Separate models were constructed for acute, macro-scale, and cumulative analyses. Mean heart rate was included as a time-varying covariate. Random intercepts were specified for individual participants, and first-order autoregressive covariance structures were applied to model temporal dependence among repeated observations. Model assumptions were evaluated using residual diagnostics. Effect sizes were calculated using partial eta squared (η²p).

Participants were prospectively stratified according to average dizziness ratings using median split classification to create High-Dizziness and Low/No-Dizziness subgroups. Sensitivity analyses evaluated dizziness as a continuous variable to confirm robustness.

Safety Monitoring Participant safety was monitored throughout the intervention. Immediately after each session, participants completed questionnaires documenting stimulation-related sensations. No medications were administered, and no invasive procedures were performed. Participants were free to discontinue their participation at any time without providing a reason. No penalties or consequences were applied for withdrawal, and participants retained the right to withdraw their data upon request.

Scientific Significance This study was designed to extend previous laboratory investigations by evaluating autonomic and psycho-emotional responses to GVS under ecologically valid occupational conditions. By integrating acute, short-term recovery, and cumulative assessments during participants' normal professional activities, the study addresses a gap in the current literature regarding real-world applicability of vestibular neuromodulation. The prospective evaluation of subjective vestibular perception as a potential moderator of autonomic responses represents a conceptual innovation. The findings are expected to contribute to individualized non-invasive neuromodulation approaches and provide additional evidence regarding the physiological safety and tolerability of repeated sinusoidal GVS in healthy adults.