Functional neural modelling of just noticeable difference in interaural time detection for normal hearing and bilateral cochlear implant users

Authors: Andreas N. Prokopiou, Tom Francart, Jan Wouters. Abstract: Sound localisation is mediated by interaural time differences (ITD) and interaural level differences (ILD). Cochlear implant (CI) users of current clinical systems do not have access to ITD cues because of the loss of phase information after sound processing. This loss of phase is partly caused by the fixed period of the carrier pulse train. However, recent studies have shown that it is possible for bimodal and bilateral CI users to perceive ITDs from interaural phase shifts of the envelope of modulated pulse train stimuli, such as the ones generated by clinical processors. Furthermore the shape of the temporal envelope determines ITD sensitivity. This creates a wide parameter space which has not been fully explored. To aid in the study of this phenomenon a computational model was developed. It amalgamates existing work on acoustic stimulation and electrical stimulation of the auditory nerve with a novel neurometric-psychometric comparison which aims at estimating the just noticeable difference (JND) in ITD for both normal hearing subjects and CI users. The acoustical and the electrical stimulation of the auditory nerve constitute the peripheral processing part of the model. In both cases the output is a train of action potentials generated by the auditory nerve. The model used for electrical stimulation is an already existing model, developed by Bruce et al. [IEEE Tr Biomed. Eng. 1999] and extended by Goldwyn et al. [J Neurophysiol 2012]. Temporal aspects of the generated spike train, such as neuron refractory period, adaptation and response latency to the electric pulse are well predicted by this model. The model used for acoustic stimulation is also an already existing model developed by Zilany et al. [J. Acoust. Soc. Am. 2014]. It is a cascade of phenomenological description of the major functional components of the auditory-periphery, from the middle-ear to the auditory nerve. The neurometric-psychometric comparison developed takes as inputs the action potentials generated by the left and right auditory nerve stimulation models, either electrically or acoustically, thus permitting the modelling of normal hearing, bilateral CI and bimodal listeners' psychometric estimates, specifically the JND in ITD. This is especially useful, as estimated JND values can be validated against experimental investigations of the human binaural performance, and assess the model's performance. Furthermore the JND estimation can be directly used as a performance metric of novel temporal enhancement stimulation strategies, and as such the model can readily be applied as a test-bench for developing strategies for bimodal and bilateral CIs.status: submitte