Description

Hidden Hearing Loss (HHL) is defined as the presence of normal pure-tone audiometric thresholds accompanied by self-reported listening difficulties, most notably problems understanding speech in noisy environments. Recent neurophysiological and histopathological evidence suggests that this condition is associated with cochlear synaptopathy-selective loss or dysfunction of the synapses between inner hair cells and low-spontaneous-rate (low-SR) type I auditory nerve fibers. Low-SR fibers are crucial for encoding suprathreshold acoustic signals and preserving speech perception in noise. Damage to these fibers can occur from noise exposure, aging, or other metabolic factors even when outer hair cells remain intact, thereby leaving conventional audiometry within normal limits.

Auditory Brainstem Response (ABR) testing provides an objective measure of neural synchrony along the auditory pathway. The amplitude and latency of early ABR waves, particularly Waves I, III, and V, reflect the temporal integrity of auditory nerve and brainstem transmission. Among these, Wave I amplitude is considered a peripheral marker of auditory nerve fiber activity, while Wave V represents the summated output of higher brainstem nuclei. A reduction in Wave I amplitude with preserved Wave V amplitude, or a decreased I/V ratio, is interpreted as an electrophysiological signature of cochlear synaptopathy.

Traditional ABR using click stimuli synchronizes neural responses primarily from the basal cochlear region. The introduction of chirp stimuli aimed to improve neural synchrony by compensating for cochlear travel-time delays. The Level-Specific (LS) CE-Chirp stimulus designed by Claus Elberling and Don allows the amplitude of Wave V to remain robust across intensities while providing frequency-specific timing. Yet, at high levels (80-100 dB nHL), LS CE-Chirp responses may demonstrate altered growth functions and latency patterns, potentially revealing subtle differences in suprathreshold neural coding compared to conventional clicks.

The current study adopts an observational, cross-sectional, correlational design to investigate whether electrophysiological indices derived from LS CE-Chirp-stimulated ABR are associated with behavioral speech-in-noise performance in individuals suspected of HHL. Participants with normal pure-tone thresholds will undergo both ABR and the Hearing In Noise Test (HINT), enabling exploration of the relationship between amplitude and latency measures of ABR waves and the critical signal-to-noise ratio (SNR-50) obtained behaviorally.

The scientific rationale for the study stems from the premise that reduced neural output from low-SR auditory fibers manifests as smaller Wave I amplitudes at high stimulation levels, producing diminished I/V ratios and flattened amplitude-growth slopes. These physiological patterns are expected to parallel poorer speech-in-noise perception, reflected by elevated SNR-50 values on HINT. Demonstrating a significant correlation between ABR parameters and HINT outcomes would thus support the hypothesis that LS CE-Chirp ABR can serve as an objective marker of suprathreshold auditory coding deficits in HHL.

Participants will be recruited from audiology clinics and university hearing research laboratories. Three subgroups will be studied:

1. A control group with normal hearing and no tinnitus or listening complaints;
2. A tinnitus group with chronic subjective tinnitus (≥ 3 months) but normal hearing thresholds; and
3. A speech-in-noise complaint group reporting difficulty understanding speech in background noise despite normal thresholds and no tinnitus.

Eligibility criteria include ages 20-60 years, bilateral air-conduction thresholds ≤ 25 dB HL at 0.5-4 kHz, normal immittance results, and MoCA ≥ 21. Individuals with peripheral or central hearing loss, somatosensory tinnitus, neurological disorders, or significant emotional/psychiatric conditions will be excluded. Approximately 63 participants (21 per group) are planned, as determined by G\*Power analysis (effect size 0.5, α = 0.05, power = 0.80). Recruitment status is active.

All participants will provide written informed consent. Testing will occur in a sound-attenuated booth. The assessment sequence includes:

Montreal Cognitive Assessment (MoCA) for cognitive screening;

Speech, Spatial and Qualities of Hearing Scale (SSQ) or its Turkish adaptation for subjective speech-perception profiling;

Pure-tone audiometry to confirm normal thresholds;

Threshold Equalizing Noise (TEN) test to screen for cochlear dead regions (control measure, not analyzed);

Immittance and acoustic reflexes to confirm middle-ear integrity;

LS CE-Chirp and Click ABR recordings; and

HINT administration in free-field conditions.

ABR recordings will be obtained monaurally using the Neurosoft Neuro-MEP-4 system. Disposable surface electrodes will be applied after skin preparation with Nuprep gel: active (Cz or Fz), reference (ipsilateral mastoid A1/A2 or earlobe), and ground (FPz or AFz). Electrode impedance will be maintained below 3 kΩ and balanced across channels. Insert earphones will deliver alternating-polarity stimuli to minimize cochlear microphonic artifacts.

For both Click and LS CE-Chirp stimuli, recordings will be made at 80, 90, and 100 dB nHL using a 21.1 stimuli/s repetition rate; additional short blocks may use 11.1 or 61.1 /s to examine neural adaptation. The recording filter will be 100-3000 Hz (12 dB/octave), artifact rejection ± 35-40 µV, and analysis window 0-15 ms (pre-stimulus -2 ms). Each condition will comprise 2000-4000 sweeps averaged twice for replicability; recordings with residual noise \> 20 nV rms or inter-replicate discrepancy \> 15 % will be repeated. Insert-earphone tubing delay (≈ 0.9 ms) will be compensated in latency calculations.

Wave peaks I, III, and V will be identified independently by two blinded examiners. Amplitude and latency measures will be extracted for each wave at each intensity, and V/I amplitude ratios will be computed. LS CE-Chirp responses are expected to show larger Wave V amplitudes at lower intensities and potential amplitude saturation at higher intensities, providing a basis for evaluating suprathreshold neural synchronization.

The HINT will consist of 25 sentence lists (10 sentences each, total 250 sentences). Speech will be presented at variable levels while background noise is held constant at 65 dB SPL. The adaptive algorithm adjusts sentence level according to participant accuracy, converging on the critical SNR (dB) where 50 % of sentences are correctly repeated. Testing will be conducted with the participant seated 1 meter from the loudspeaker at 0° azimuth in a calibrated free-field environment.

Data from ABR and HINT will be linked for correlational analysis. For each participant, "better-ear" ABR metrics (based on higher Wave I amplitude) will be paired with binaural HINT performance to reflect real-world auditory integration.

Data Analysis Plan

Data will be analyzed using the Statistical Package for the Social Sciences (SPSS v27). The normality of variable distributions will be verified using the Kolmogorov-Smirnov test. Depending on the results, parametric or non-parametric statistics will be applied. For group comparisons, one-way analysis of variance (ANOVA) will be used for normally distributed variables, and the Kruskal-Wallis test will be applied for non-normal distributions. When significant effects are found, appropriate post-hoc pairwise comparisons with Bonferroni or Dunn corrections will be conducted.

For correlation analyses, Pearson's correlation coefficient will be used for normally distributed variables and Spearman's rho for non-normal distributions. The primary analytic focus will be the correlation between LS CE-Chirp ABR amplitudes, latencies, and V/I amplitude ratios and the HINT critical signal-to-noise ratio (SNR-50). Secondary analyses will replicate these correlations for click-evoked ABR parameters. Exploratory analyses will evaluate Wave I amplitude growth functions (µV/dB) and I/V ratio changes across stimulus levels to assess their potential as physiological markers of cochlear synaptopathy.

Effect sizes (Cohen's r or partial η²) and 95 % confidence intervals will accompany all key results. The alpha level will be set at 0.05. Outliers beyond ± 3 SD will be verified and, if necessary, analyzed using robust estimators. Correlation strength will be interpreted according to Cohen's conventions (0.1 = small, 0.3 = medium, 0.5 = large).

Ethical Considerations and Data Handling

All procedures conform to the principles of the Declaration of Helsinki and national ethical guidelines for non-interventional clinical research. Participation is voluntary, and written informed consent will be obtained prior to data collection. Each participant will be assigned a unique numeric code; identifiable information will be stored separately from research data and accessible only to the principal investigator. Data will be encrypted and stored on password-protected institutional drives. Participants may withdraw at any time without penalty. Adverse events or significant discomfort are not anticipated, as ABR and HINT are non-invasive audiological assessments with minimal risk.

Quality Control and Reliability

To ensure data integrity, ABR recordings will be checked for reproducibility by collecting two independent averages per condition. Recordings with residual noise greater than 20 nV or poor inter-replicate correlation (ICC \< 0.80) will be excluded. Wave identification will be performed by two trained examiners blinded to group status. Inter-rater reliability for amplitude and latency marking will be calculated. All HINT procedures will follow standardized instructions with the same ambient conditions and calibrated equipment throughout the study.

Expected Outcomes and Significance

The study is expected to demonstrate measurable associations between suprathreshold auditory brainstem response metrics and behavioral speech-in-noise performance. Individuals with suspected hidden hearing loss-particularly those in the speech-in-noise complaint group-are hypothesized to exhibit reduced Wave I amplitudes, lower I/V ratios, and flatter amplitude-growth slopes compared with control participants. These physiological features are predicted to correlate with poorer HINT performance (higher SNR-50 values).

Confirmation of these hypotheses would support the use of LS CE-Chirp ABR as a non-invasive objective tool for identifying neural encoding deficits underlying HHL. The integration of electrophysiological and behavioral outcomes could refine diagnostic criteria for patients who report "I can hear but not understand," despite normal audiograms. Additionally, results may inform future intervention trials aimed at improving neural synchrony or auditory training protocols for individuals with subclinical hearing difficulties.

Study Limitations

The cross-sectional observational design precludes causal inference; however, it is appropriate for hypothesis generation and establishing physiological-behavioral associations. The absence of otoacoustic emission measurements is a potential limitation, but middle-ear status, TEN testing, and normal audiometric thresholds minimize the likelihood of undetected outer-hair-cell dysfunction. Participant numbers are based on power calculations for medium effect sizes; replication with larger cohorts is recommended.

Dissemination Plan

Study findings will be presented at audiology and neuroscience conferences and submitted to peer-reviewed journals indexed in the Science Citation Index Expanded (SCIE). De-identified datasets may be shared upon reasonable request following publication, in accordance with institutional data-sharing policies.

Conclusion

This observational, recruiting study explores how LS CE-Chirp- and Click-evoked ABR parameters relate to speech-in-noise understanding in individuals with suspected hidden hearing loss. By combining objective neural response metrics with subjective and behavioral assessments, the research aims to bridge the gap between physiological evidence of cochlear synaptopathy and functional listening complaints. The expected outcome is an improved framework for early detection of HHL and more precise differentiation of peripheral versus central contributions to speech-in-noise deficits.