The hearing aid dilemma: amplification, compression, and distortion of the neural code

Hearing aids are the only available treatment for mild-to-moderate sensorineural hearing loss, but often fail to improve perception in difficult listening conditions. To identify the reasons for this failure, we studied the underlying neural code using large-scale single-neuron recordings in gerbils, a common animal model of human hearing. We found that a hearing aid restored the sensitivity of neural responses, but failed to restore their selectivity. The low selectivity of aided responses was not a direct effect of hearing loss per se, but rather a consequence of the strategies used by hearing aids to restore sensitivity: compression, which decreases the spectral and temporal contrast of incoming sounds, and amplification, which produces high intensities that distort the neural code even with normal hearing. To improve future hearing aids, new processing strategies that avoid this tradeoff between neural sensitivity and selectivity must be developed.