Performing simulations with a realistic biophysical auditory nerve fiber model can be very time-consuming, due to the complex nature of the calculations involved. Here, a surrogate (approximate)... Show morePerforming simulations with a realistic biophysical auditory nerve fiber model can be very time-consuming, due to the complex nature of the calculations involved. Here, a surrogate (approximate) model of such an auditory nerve fiber model was developed using machine learning methods, to perform simulations more efficiently. Several machine learning models were compared, of which a Convolutional Neural Network showed the best performance. In fact, the Convolutional Neural Network was able to emulate the behavior of the auditory nerve fiber model with extremely high similarity ( R 2 > 0 . 99 ), tested under a wide range of experimental conditions, whilst reducing the simulation time by five orders of magnitude. In addition, a method for randomly generating charge-balanced waveforms using hyperplane projection is introduced. In the second part of this paper, the Convolutional Neural Network surrogate model was used by an Evolutionary Algorithm to optimize the shape of the stimulus waveform in terms of energy efficiency. The resulting waveforms resemble a positive Gaussian-like peak, preceded by an elongated negative phase. When comparing the energy of the waveforms generated by the Evolutionary Algorithm with the commonly used square wave, energy decreases of 8%-45% were observed for differ-ent pulse durations. These results were validated with the original auditory nerve fiber model, which demonstrates that the proposed surrogate model can be used as its accurate and efficient replacement.(c) 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ) Show less
Background: The refractory recovery function (RRF) measures the electrically evoked compound action potential (eCAP) in response to a second pulse (probe) after masking by a first pulse (masker).... Show moreBackground: The refractory recovery function (RRF) measures the electrically evoked compound action potential (eCAP) in response to a second pulse (probe) after masking by a first pulse (masker). This RRF is usually used to assess the refractory properties of the electrically stimulated auditory nerve (AN) by recording the eCAP amplitude as a function of the masker probe interval. Instead of assessing eCAP amplitudes only, recorded waveforms can also be described as a combination of a short-latency component (S-eCAP) and a long-latency component (L-eCAP). It has been suggested that these two components originate from two different AN fiber populations with differing refractory properties. The main objective of this study was to explore whether the refractory characteristics revealed by S-eCAP, L-eCAP, and the raw eCAP (R-eCAP) differ from each other. For clinical relevance, we compared these refractory properties between children and adults and examined whether they are related to cochlear implant (CI) outcomes.Design: In this retrospective study, the raw RRF (R-RRF) was obtained from 121 Hi-Focus Mid-Scala or 1 J cochlear implant (Advanced Bionics, Valencia, CA) recipients. Each R-eCAP of the R-RRF was split into an S-eCAP and an L-eCAP using deconvolution to produce two new RRFs: S-RRF and L-RRF. The refractory properties were characterized by fitting an exponential decay function with three parameters: the absolute refractory period (T); the saturation level (A); and the speed of recovery from nerve refractoriness ( Tau), i.e., a measure of the relative refractory period. We compared the parameters of the R-RRF (R T , R A , R Tau) with those obtained from the S-RRF (S T , S A , S Tau) and L-RRF (L T , L A , L Tau) and investigated whether these parameters differed between children and adults. In addition, we examined the associations between these parameters and speech perception in adults with CI. Linear mixed modeling was used for the analyses.Results: We found that T R was significantly longer than S T and L T , and S T was significantly longer than L T . R A was significantly larger than S A and L A , and S A was significantly larger than L A . Also, S Tau was significantly longer in comparison to R Tau and L Tau, but no significant difference was found between R Tau and L Tau. Children presented a significantly larger S A and L A and a shorter R T in comparison to adults. Shorter S Tau was significantly associated with better speech perception in adult CI recipients, but other parameters were not.Conclusion: We demonstrated that the two components of the eCAP have different refractory properties and that these also differ from those of the R-eCAP. In comparison with the R-eCAP, the refractory properties derived from the S-eCAP and L-eCAP can reveal additional clinical implications in terms of the refractory difference between children and adults as well as speech performance after implantation. Thus, it is worthwhile considering the two components of the eCAP in the future when assessing the clinical value of the auditory refractory properties. Show less
Electrically evoked compound action potentials (eCAPs) are measurements of the auditory nerve's response to electrical stimulation. ECAP amplitudes during pulse trains can exhibit temporal... Show moreElectrically evoked compound action potentials (eCAPs) are measurements of the auditory nerve's response to electrical stimulation. ECAP amplitudes during pulse trains can exhibit temporal alternations. The magnitude of this alternation tends to diminish over time during the stimulus. How this pattern relates to the temporal behavior of nerve fibers is not known. We hypothesized that the stochasticity, refractoriness, adaptation of the threshold and spike-times influence pulse-train eCAP responses. Thirty thousand auditory nerve fibers were modeled in a three-dimensional cochlear model incorporating pulse-shape effects, pulse-history effects, and stochasticity in the individual neural responses. ECAPs in response to pulse trains of different rates and amplitudes were modeled for fibers with different stochastic properties (by variation of the relative spread) and different temporal properties (by variation of the refractory periods, adaptation and latency). The model predicts alternation of peak amplitudes similar to available human data. In addition, the peak alternation was affected by changing the refractoriness, adaptation, and relative spread of auditory nerve fibers. As these parameters are related to factors such as the duration of deafness and neural survival, this study suggests that the eCAP pattern in response to pulse trains could be used to assess the underlying temporal and stochastic behavior of the auditory nerve. As these properties affect the nerve's response to pulse trains, they are of uttermost importance to sound perception with cochlear implants. (C) 2019 Elsevier B.V. All rights reserved. Show less
