Do Cochlear Mechanisms Explain the Noise-Disruption of the Auditory Brainstem Response to Speech?

In background noise, the timing precision of the auditory brainstem response to speech (speech-ABR) is disrupted and the response latency increases. The severity of the disruption has been correlated with listeners’ ability to understand speech-in-noise. To date, although a central mechanism is assumed, the locus of the speech-ABR timing disruption is not clear. The present study aimed to investigate the contribution of different cochlear mechanisms to noise-induced latency increases. A first experiment examined the “cochlear place” mechanism, by which the latency of the response increases as cochlear origin moves towards lower frequency regions. The results showed that the speech-ABR reflects an average over responses from a broad range of cochlear regions, which respond with substantial relative delays. This implies that cochlear place can potentially have large effects on masked speech-ABR latency. Another mechanism that is known to be involved in noise-induced ABR latency increases is neural adaptation. This is presumed to occur at the inner hair cell-nerve junction and is thought to reflect cochlear masking. Thus, if this mechanism contributes to speech-ABR latency increases in noise, we would expect this contribution to depend on cochlear frequency selectivity and amplification gain. This hypothesis is tested in the second experiment.