Effects of age on responses of principal cells of the mouse anteroventral cochlear nucleus in quiet and noise

Older listeners often report difficulties understanding speech in noisy environments. It is important to identify where in the auditory pathway hearing-in-noise deficits arise to develop appropriate therapies. We tested how encoding of sounds is affected by masking noise at early stages of the auditory pathway by recording responses of principal cells in the anteroventral cochlear nucleus (AVCN) of aging CBA/CaJ and C57BL/6J mice in vivo. Previous work indicated that masking noise shifts the dynamic range of single auditory nerve fibers (ANFs), leading to elevated tone thresholds. We hypothesized that such threshold shifts could contribute to increased hearing-in-noise deficits with age if susceptibility to masking increased in AVCN units. We tested this by recording the responses of AVCN principal neurons to tones in the presence and absence of masking noise. Surprisingly, we found that masker-induced threshold shifts decreased with age in primary-like units and did not change in choppers. In addition, spontaneous activity decreased in primary-like and chopper units of old mice, with no change in dynamic range or tuning precision. In C57 mice, which undergo early onset hearing loss, units showed similar changes in threshold and spontaneous rate at younger ages, suggesting they were related to hearing loss and not simply aging. These findings suggest that sound information carried by AVCN principal cells remains largely unchanged with age. Therefore, hearing-in-noise deficits may result from other changes during aging, such as distorted across-channel input from the cochlea and changes in sound coding at later stages of the auditory pathway. Significance StatementMiddle age and older listeners commonly experience hearing deficits in the presence of background noise. Central auditory areas have been implicated in hearing-in-noise deficits, but it is not known where these deficits arise. We performed in vivo recordings in mice of different ages at the first stage of the auditory pathway in the brain, the cochlear nucleus, to examine how encoding of sounds is perturbed by masking noise. We found that the responses of individual neurons remain largely intact with age, including the processing of tones in masking noise, despite previously documented structural and physiological degeneration of their auditory nerve inputs. This suggests that problems hearing in masking noise result from changes at other stages of the auditory pathway.