The effect of selective loss of auditory nerve fibers on temporal envelope processing: a simulation study

Mengchao Zhang, Jacques Grange, John Culling

 School of Psychology, Cardiff University, Cardiff, UK

Background: Cochlear synaptopathy is a selective loss of auditory nerve fibers with low spontaneous rates (SRs) due to sublimit noise exposure or aging and has been proposed to affect temporal envelope (TE) processing at suprathreshold levels. However, due to insufficiently sensitive measures and highly variable patients’ background, evidence of cochlear synaptopathy in humans has been unconvincing. This study examines the role of low SR fibers in suprathreshold TE processing and the choice of task parameters to manifest cochlear synaptopathy through computational simulation.

Methods: Selective deafferentation of auditory nerves was simulated in a physiologically inspired model of the auditory system. The model output was then transformed into sound and tested on normal-hearing listeners. Loss of low SR fibers was compared to loss of both low and medium SR fibers, loss of high SR fibers, or no fiber loss. TE processing tasks included amplitude modulation detection (500-Hz carrier, modulation rates of 16, 32 and 64 Hz), speech recognition in modulated noise, and recognition of unvoiced speech in modulated noise. 

Results: The performances of the tasks were consistently affected by loss of low SR fibers and loss of low and medium SR fibers, but not by loss of high SR fibers. For amplitude modulation detection, the performance was degraded more severely when the modulation rate was 16 Hz than 64 Hz. For speech tasks, the intelligibility of unvoiced speech was impaired more severely than natural speech. 

Conclusion: The simulation supports the role of low SR fibers in coding suprathreshold TE and shows that a sensitive TE measure requires careful selection of the task parameters. The current study recommends 16 Hz over 64 Hz for amplitude modulation detection and unvoiced speech over natural speech for testing cochlear synaptopathy.

Zhang et al
Average thresholds for amplitude modulation detection task (N = 11) at different modulation rates (left: 16 Hz, middle: 32 Hz, right: 64 Hz). Carrier frequency: 500 Hz.