Noise Masking in Cochlear Synaptopathy: Auditory Brainstem Response vs. Auditory Nerve Response in Mouse

Area of Science:

• Neuroscience
• Auditory Neuroscience
• Ototoxicity Research

Background:

• Acoustic overexposure causes permanent auditory-nerve fiber (ANF) synapse retraction (synaptopathy).
• Synaptopathy degrades complex sound discrimination, especially in noise, but is difficult to diagnose due to minimal audiometric threshold changes until severe loss.
• Auditory brainstem responses (ABRs) are crucial for assessing auditory pathway function.

Purpose of the Study:

• To investigate an assay based on masking auditory brainstem responses (ABRs) to diagnose synaptopathy.
• To explore the underlying auditory-nerve fiber (ANF) responses at the single-fiber level in synaptopathic ears.

Main Methods:

• Synaptopathy was induced via octave-band noise overexposure, causing ~50% permanent synaptic loss.
• Auditory brainstem responses (ABRs) were measured using a probe tone masked by continuous noise at varied levels.
• Single-fiber recordings of ANF responses were analyzed.

Main Results:

• The normal delay of ABR peaks with increasing masker level was diminished in synaptopathic ears.
• This latency shift is attributed to a change in the dominant cochlear region, not fiber type shift.
• The dynamic range of masking in ABRs was surprisingly enhanced in synaptopathic ears.
• Single-fiber data showed paradoxical enhancement of onset-response synchrony and masking resistance in affected ANFs.

Conclusions:

• The enhanced dynamic range of masking in ABRs mirrors single-fiber findings and could serve as a diagnostic biomarker for synaptopathy.
• This assay offers a potential method for diagnosing synaptic damage in human populations, even with preserved audiometric thresholds.