Advanced simulations for the differential diagnosis of sensorineural pathologies

The MAPsim simulator reconstructs an acoustic signal from the auditory nerve firing pattern predicted by the MATLAB© Auditory Periphery model. Stimuli reconstructed with various simulated sensorineural pathologies were presented to normally hearing (NH) participants in psychophysical tasks. This enabled studying the impact of specific pathologies on performance and developing tasks designed to detect specific pathologies. MAP predicts that efferent reflexes may play an important role in dynamic range adaptation at the auditory-nerve level, the acoustic reflex shifting rate-level functions towards a context level and the medial olivocochlear reflex sharpening auditory nerve sensitivity around that level. A NH implementation of MAPsim was validated for speech reception thresholds in noise (Grange et al., 2022). Disabling efferent reflexes significantly reduced dynamic range adaptation, and due to the saturation of high spontaneous rate fibres, speech intelligibility.

Simulating general deafferentation with stochastic under-sampling alone, 90% of fibres had to be knocked out for speech reception thresholds (SRTs) to measurably degrade. With the correct reduction of efferent signals caused by a decrease in efferent signals, 75% deafferentation yielded a 3 dB SRT elevation from NH. In interaural time delay discrimination, the combined effect of reduced efferent signals elevated thresholds from 19 μs with unprocessed or NH processed stimuli to 60 μs at 75% deafferentation. Selective knockout of low spontaneous rate fibres degraded temporal envelope coding, elevating natural and unvoiced SRTs by 1 and 4.5 dB from the NH condition, respectively. In amplitude-modulation detection, such synaptopathy elevated thresholds more for lower than for higher modulation rates. Overall, synaptopathy severely impaired temporal envelope processing, when the knockout of high spontaneous rate fibres did not. In knocking out outer haircells, while 75% knockout led to an overall 2 dB SRT elevation, the benefits of dip-listening and fundamental-frequency discrimination did not significantly drop. A total knockout of outer haircells removed and halved the benefits of dip-listening and F0 segregation, respectively. A reduction of endocochlear potential from 100 to 12.5 mV led to similar endpoints, but via a gradual reduction in dip-listening and F0-segregation benefits.

Overall, our findings suggest that (1) efferent reflexes are key in adequate simulations of normal and impaired hearing, (2) efferent reflexes interact with deafferentation, (3) synaptopathy may be detectable by probing patients' temporal envelope processing abilities and (4) MAPsim paves the way to revealing the psychophysical signatures of most sensorineural pathologies.