Evaluation of a Bio-inspired Coding Strategy for Cochlear Implant Users

Overview

This study will determine the facilitation, refractoriness and spatial spread effects of auditory nerve fiber responses to electrical stimulation via a cochlear implant. The performance of CI users in melody contour and speech recognition in noise tests with their own clinical sound processor and a MATLAB implementation of their coding strategy will be compared and a bioinspired coding strategy will be evaluated in comparison with the conventional ACE coding strategy.

The advanced combination encoder (ACE) strategy, one of the most widely used clinical speech processing strategies available to cochlear implant (CI) users, attempts to optimize transmission of input acoustic signals, but does not explicitly consider the auditory nerve fibers' (ANFs) capacity for conveying this information. The ACE strategy decomposes temporal frames of the incoming sound signal into frequency bands with a bank of band-pass Fast Fourier Transform (FFT) filters. The temporal envelopes in each band are then extracted and typically eight to ten bands with the highest energy content are selected to amplitude modulate the biphasic pulses. This selection is performed irrespective of the ANFs responses to the electrical stimulation. However, some studies have shown that temporal response properties of ANFs impose limitations for electrical stimulation. Apart from that, the degree of spread of neural excitation which is typically larger than in the acoustic case, may result in responses which can diminish information provided on the individual channels. Thus, a coding strategy taking into account temporal properties of ANFs as well as spatial spread of the electric field could be beneficial for CI users. One of the prominent temporal characteristics of ANFs in response to the electrical stimulation is refractoriness. This phenomenon can be defined as a reduction in the excitability of ANFs immediately following an action potential and has been observed in CI recipients via measuring the electrically evoked compound action potential (ECAP). The refractory period can be divided into an absolute refractory period (ARP) during which the auditory nerve is incapable of responding to the following pulse and a relative refractory period (RRP) during which a response from the neuron is possible under specific circumstances. Refractoriness can impose limitations on the maximum stimulation rate of CIs since ANFs cannot respond to a stimulus presented during the ARP. Apart from refractoriness, an ongoing high rate pulse train produces spike rate adaptation (SRA) in ANFs in which the neurons progressively lose their ability to respond to every pulse. This decrement in neural excitability is even larger than can be explained by refractoriness. Animal studies of ANFs at high rate pulse trains have shown SRA and adaptation has also been observed in the ECAP amplitude of human CI users in which the amplitude decreased as the stimulation rate increased. In parallel to spike rate adaptation, accommodation can contribute to the spike rate decrement. Accommodation reduces excitability for the second pulse (probe) response when there is a subthreshold response to the first pulse (masker) and the masker-probe interval (MPI) is large enough to allow the membrane potential to decay back near or below the resting potential. Accommodation in addition to SRA is considered as reduction in the excitability of ANFs and its effect accumulates over sequential non-spiking responses. Electrical stimulation of ANFs in animals with pairs of pulses has also shown facilitation which is defined as an increase in nerve excitability caused by sub-threshold stimulation in short intervals. Apart from animal studies, the facilitation effect has also been observed in human CI recipients. Facilitation happens when the neuron does not respond to the first pulse, but if the membrane potential remains near the threshold long enough, the second pulse can produce a response. It was shown that facilitation is more evident at a low MPI and for a masker with an intensity equal or less than that of a probe. Although the facilitation effect was observed in human CI users, systematic ECAP measurement to quantify this effect were not yet performed. Thus, neural response telemetry (NRT) measurements with negative masker offset and short MPIs need to be done to determine a non-monotonic behaviour of facilitation and confirm the predictions by Cohen in his model simulations. Apart from ANFs temporal considerations, electrical current spreads out widely along the cochlea and excites a wide range of populations of ANFs which leads to a decrease in the selectivity and the number of effective channels. Thus, spatial spread of the electric field has a major impact on the spectral resolution of CI users and decreases the excitability of the affected ANF population. In the planned study refractoriness, spatial spread and facilitation effects will first be determined from NRT measurements with CI participants. Then, all the aforementioned phenomena are integrated in a bio-inspired coding strategy for a better selection of channels with highest energy content. This new strategy will be compared to the conventional ACE coding strategy.

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