Modeling the Anti-masking Effects of the Olivocochlear Reflex in Auditory Nerve Responses to Tones in Sustained Noise

Abstract

The medial olivocochlear reflex (MOCR) has been hypothesized to provide benefit for listening in noise. Strong physiological support for an anti-masking role for the MOCR has come from the observation that auditory nerve (AN) fibers exhibit reduced firing to sustained noise and increased sensitivity to tones when the MOCR is elicited. The present study extended a well-established computational model for normal-hearing and hearing-impaired AN responses to demonstrate that these anti-masking effects can be accounted for by reducing outer hair cell (OHC) gain, which is a primary effect of the MOCR. Tone responses in noise were examined systematically as a function of tone level, noise level, and OHC gain. Signal detection theory was used to predict detection and discrimination for different spontaneous rate fiber groups. Decreasing OHC gain decreased the sustained noise response and increased maximum discharge rate to the tone, thus modeling the ability of the MOCR to decompress AN fiber rate-level functions. Comparing the present modeling results with previous data from AN fibers in decerebrate cats suggests that the ipsilateral masking noise used in the physiological study may have elicited up to 20 dB of OHC gain reduction in addition to that inferred from the contralateral noise effects. Reducing OHC gain in the model also extended the dynamic range for discrimination over a wide range of background noise levels. For each masker level, an optimal OHC gain reduction was predicted (i.e., where maximum discrimination was achieved without increased detection threshold). These optimal gain reductions increased with masker level and were physiologically realistic. Thus, reducing OHC gain can improve tone-in-noise discrimination even though it may produce a "hearing loss" in quiet. Combining MOCR effects with the sensorineural hearing loss effects already captured by this computational AN model will be beneficial for exploring the implications of their interaction for the difficulties hearing-impaired listeners have in noisy situations.

Level-Dependent Changes in Perception of Speech Envelope Cues

“…As signal levels increase further, discharge rates would approach saturation and discriminability would decline again, thus mapping out a quadratic pattern (best performance at mid levels and poorer performance at lower and higher levels). These predicted level-dependent changes in discriminability of intensity increments (Chintanpalli et al 2012) have not been applied to the encoding of speech in the auditory nerve at a range of levels (e.g., Young and Sachs 1979;Young 1979, 1980;Delgutte 1980Delgutte , 1995. Therefore, further studies are needed to determine if the shape of auditory nerve rate-level functions and a resultant optimal range or "sweet spot" of neural encoding may relate to level-dependent improvements and declines in perception of envelope cues in speech that are unrelated to cochlear nonlinearities.…”

Computational Model Predictions of Cues for Concurrent Vowel Identification

“…The medial olivocochlear (MOC) reflex has also been shown to enhance the response of AN fibers to pure tones in the presence of background noise (i.e., anti-masking effect; see Kawase et al 1993;Chintanpalli et al 2012;Smalt et al 2014). In realworld listening, an anti-masking effect could play a role in understanding target speech in the presence of interfering speech or noise.…”

Roles of the Contralateral Efferent Reflex in Hearing Demonstrated with Cochlear Implants

“…This idea is based on the fact that MOC efferent activation restores the dynamic range of auditory nerve fibre responses in noisy backgrounds to values observed in quiet (Fig. 5 in Guinan 2006), something that probably improves the neural coding of speech embedded in noise (Brown et al 2010;Chintanpalli et al 2012;Clark et al 2012). The evidence in support for this unmasking role of the MOCR during natural listening is, however, still indirect (Kim et al 2006).…”