Encoding and decoding amplitude-modulated cochlear implant stimuli—a point process analysis
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Cochlear implant speech processors stimulate the auditory nerve by delivering amplitude-modulated electrical pulse trains to intracochlear electrodes. Studying how auditory nerve cells encode modulation information is of fundamental importance, therefore, to understanding cochlear implant function and improving speech perception in cochlear implant users. In this paper, we analyze simulated responses of the auditory nerve to amplitude-modulated cochlear implant stimuli using a point process model. First, we quantify the information encoded in the spike trains by testing an ideal observer’s ability to detect amplitude modulation in a two-alternative forced-choice task. We vary the amount of information available to the observer to probe how spike timing and averaged firing rate encode modulation. Second, we construct a neural decoding method that predicts several qualitative trends observed in psychophysical tests of amplitude modulation detection in cochlear implant listeners. We find that modulation information is primarily available in the sequence of spike times. The performance of an ideal observer, however, is inconsistent with observed trends in psychophysical data. Using a neural decoding method that jitters spike times to degrade its temporal resolution and then computes a common measure of phase locking from spike trains of a heterogeneous population of model nerve cells, we predict the correct qualitative dependence of modulation detection thresholds on modulation frequency and stimulus level. The decoder does not predict the observed loss of modulation sensitivity at high carrier pulse rates, but this framework can be applied to future models that better represent auditory nerve responses to high carrier pulse rate stimuli. The supplemental material of this article contains the article’s data in an active, re-usable format.
KeywordsPoint process model Cochlear implant Auditory nerve Amplitude modulation Neural coding
The authors thank four anonymous reviewers for providing thoughtful critiques that have led to improvements of this manuscript. The authors also thank Dr. R.V. Shannon and Dr. J.J. Galvin III for granting permission to reproduce their data in Fig. 3. This research has been supported by a National Science Foundation VIGRE Fellowship (J.H.G.), National Institute on Deafness and Other Communication Disorders grants 1F31DC010306-01 (J.H.G.) and R01 DC007525 (J.T.R.), and a Burroughs-Wellcome Fund Career Award at the Scientific Interface (E.S.-B.).
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