Encoding and decoding amplitude-modulated cochlear implant stimuli—a point process analysis

Abstract

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.

Effect of stimulus level on the temporal response properties of the auditory nerve in cochlear implants

“…Several studies have evaluated ECAP (Tejani et al, 2017; Wilson et al, 1997) or single-fiber (Hu et al, 2010) responses to amplitude-modulated pulse trains, or modeled spike activity for amplitude-modulated stimuli (Goldwyn et al, 2010). As with the unmodulated pulse-train data, results show that neural activity changes over the course of the modulated pulse train.…”

A point process framework for modeling electrical stimulation of the auditory nerve

“…After computing the likelihood function (Eq. 23) for all possible pairings of spike trains and stimuli, we used a likelihood ratio test to discriminate between the two spike trains (Green and Swets 1966;Pillow et al 2005;Goldwyn et al 2010). If the true pairings of stimulus and response produce the highest likelihood values, then the ideal ideal observer is said to correctly detect the modulated stimulus.…”

Psychophysically based site selection coupled with dichotic stimulation improves speech recognition in noise with bilateral cochlear implants

“…Neural processing of amplitude modulated acoustical and electrical signals is based on a temporal mechanism, where spike timing in the auditory nerve encodes the modulation information, with the synchronization magnitude varying with modulation depth, frequency, or stimulus level (Joris and Yin, 1992;Joris et al, 2004;Goldwyn et al, 2010). The amount of masking, although not completely independent of the MDT measures, is probably determined by the interplay of a number of factors that are primarily spatial.…”