Authors: Marina Saiz-Alia1 and Tobias Reichenbach1
1Department of Bioengineering and Centre for Neurotechnology, Imperial College London, South Kensington Campus, SW7 2AZ, London, U.K.
Background: The auditory brainstem’s response to many repeated short clicks serves as an important clinical assessment of hearing. Recently we have proposed a method for detecting the response of the auditory brainstem to the fundamental frequency of continuous speech, without repetitions. However, different parts of the speech signal as well as several parts of the brainstem contribute to this response. Here we employ a computational model of the brainstem to elucidate the influence of these different factors.
Methods: We combined existing models of the middle and inner ear with a neural network model of the globular bushy cells in the cochlear nuclei and with a phenomenological model of the inferior colliculus, to develop a common simulation framework. We then employed the model to investigate the neural response to continuous speech at different stages in the brainstem, following the methodology developed recently by ourselves for detecting the brainstem response to running speech from scalp recordings. Simulations were compared with recordings from volunteers.
Results: We found that the different parts of the brainstem model contributed to the speech-evoked brainstem response, although the dominant source was the inferior colliculus. The delay of the
response corresponded to that observed in the experiments, suggesting that the scalp-recorded brainstem response at the fundamental frequency of speech originates predominantly in the inferior colliculus. We further showed that a large number of harmonics contributed to the response, and that it declined with increasing fundamental frequency.
Conclusion: Our results suggest that the scalp-recorded brainstem response at the fundamental frequency of speech originates predominantly in the inferior colliculus. They further show that the response is shaped by a large number of higher harmonics of the fundamental frequency, reflecting highly nonlinear processing in the auditory periphery and illustrating the complexity of the response.