Investigation of the Relationship Between LS Chirp ABR and Comprehension In Noise Skills In Individuals With Suspected Hidden Hearing Loss

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.