Dau, Torsten

Epp, Bastian

Joshi, Suyash Narendra

Online Research Database In Technology

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Nov 08, 2017What affects envelope coding in theelectrically stimulated auditory nerve?Joshi, Suyash Narendra; Dau, Torsten; Epp, BastianPublication date:2016Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Joshi, S. N., Dau, T., & Epp, B. (2016). What affects envelope coding in theelectrically stimulated auditorynerve?. Poster session presented at 39th midwinter meeting of Association of Research in Otolaryngology, SanDiego, CA, United States.What affects envelope coding in theelectrically stimulated auditory nerve?SuyashN. Joshi, TorstenDau, Bastian EppHearing Systems, Technical University of DenmarkBackgroundCarrier Pulse RateLow HighHypothesisRubinstein et al, 1999• Cochlear implants (CI) elec-trically stimulate the auditorynerve fibers (ANF) with a trainof biphasic pulses modulatedwith an envelope of a desiredacoustic signal.• It has been suggested thathigher pulse rates would re-duce phase-locking to the car-rier pulse train such that theresponse will only be domi-nated by the envelope (Rubin-stein et al, 1999).• However, psychophysicalmeasures of the amplitudemodulation detection (AMD)thresholds showed detrimen-tal effects of the higher pulserate, particularly at low stim-ulus levels (Galvin and Fu,2005).Which potential factors impair the tempo-ral coding in CI listeners?ANF modelThis study explores the factors affecting temporal coding using amodel of ANF responses to electrical stimulation (Joshi et al, 2015).Fig.1 The model of ANF responses proposed by Joshi et al (2015) was used to simulatethe ANF responses to modulated and unmodulated pulse trains. The model consists of twoneurons, representing a peripheral and a central axon of the AN fiber. The model receivesstimulus waveform as an input and provides a spike train output.This model -• has been developed based on the observation that the extracellular stim-ulation with CI can lead to excitation at multiple sites along the ANF.• The site of excitation largely determines the delay at which a spike arrivesat the next processing stage and therefore may be a dominant source inlimiting the temporal coding.• also accounts for the excitation produced by both anodic and cathodicphase of the biphasic pulse and the spike time differences between theirresponses.• can also account for sub-threshold adaptation such as facilitation and ac-commodation as well as the supra-threshold adaptation such as refractori-ness and spike-rate adaptation.• has been shown to predict the correct response statistics including thespike times for various pulse shapes, pulse rates and stimulus levels (Joshiet al, 2015).Stimuli• Trains of symmetric biphasic pulses (cathodic-anodic) with a pulse-phaseduration of 100  s/phase and inter-phase gap of 45  s• Carrier pulse rates - 250 or 2000 pulses per second• 20 Hz sinusoidal amplitude modulation was applied using Eq. 1.St m   s = P  se tr  n ⇥ [1 + m ⇥ (2⇥   ⇥ ƒ ⇥ t)] (1)• The modulation depth - from 0 to 100 % modulation• Stimulation levels that produced firing rates from 20 spikes/second to230 spikes/secondAnalysis• Vector strength (VS) was used to measure spike time synchrony to themodulation frequency• The receiver operating characteristic (ROC) was built using distributions ofVS for unmodulated and modulated pulse trains• The area under the ROC was plotted as a function of the modulation depthand was fitted with a logistic function (Fig 2).• The modulation depth which resulted in 79.7 % correct performance wasconsidered as the AMD threshold.Fig.2 An example of the ROC curves (A) and the neurometric function (B) obtained usingthe vector strength for modulated and modulated pulse trains at various modulation depths.The area under the ROC curve is equivalent to the percent correct performance of an unbi-ased observer (Middlebrooks, 2008) and is used to build a neurometric function.ResultsFig.3 AMD thresholds predicted by the ANF model (A) and from human listener (B) as func-tion of the stimulus level and carrier pulse rate (Subject 1 digitized from Galvin and Fu,2005). The data shows that AMD thresholds are better for a low carrier pulse rate than forhigh pulse rate. AMD thresholds get significantly better with increasing stimulus level. Pre-vious models could not predict the effect of pulse rate or stimulus level on AMD thresholds(Goldwyn et al, 2010; 2012).Contributions of the two axonsSpike time differences between the peripheral and central axons are approxi-mately 250  s in cats (Miller et al, 1999) and 400  s in humans (Rattay et al,2001). Do multiple sites of excitation limit the precise temporal codingin electrically stimulated ANF?Fig.4 Proportion of total number of spikes produced at peripheral axon measured for un-modulated (A) and modulated (B) pulse train as function of pulse rate. At low stimulus leveland high pulse rate, the central axon dominates the response. The increase in stimulus levelincreases the proportion of spikes produced at the peripheral axon for unmodulated pulsetrains. Responses to modulated pulse trains show more uncertainty of the site of excitation.These results indicate that the site of excitation does indeed limit the temporal coding inelectrically stimulated ANF.Fig.5 The AMD thresholds obtained from single individual axons of the model. To further testthe hypothesis that multiple sites of excitation does contribute to distortions in the temporalcoding in ANF, the AMD thresholds were obtained from simulations of only peripheral or thecentral axon of the model. The results from individual axons do not show any effect of pulserate on AMD thresholds.Contribution of stochasticity0 50 100 150 200 2504035302520151050Modulation detection thresholds = 0250 PPS2000 PPS0 50 100 150 200 250spikes/secondFig.6 The effect of spectrum of the ion channel noise on predicted AMD thresholds. Thespectrum of the noise is shown in the insets. Rubinstein et al (1999) suggested that athigh pulse rates, the temporal responses of the ANF will be dominated by ion channel noiseand therefore will be better than at low pulse rates. This analysis suggests that the tempo-ral coding at high pulse rate would only improve if the spectrum of the ion channel noiseresembles white noise. [The original model uses noise of shape 1/f0.78.]Contribution of sub-threshold currentFig.7 The effect of sub-threshold adaptive current on AMD thresholds predicted by themodel. In model version (A), the facilitation (summation of charge of succeeding sub-threshold pulses) was removed. Version (B) of the model was modified to remove anyeffects of accommodation (masking by sub-threshold pulses). These simulations show thatintegration of successive sub-threshold pulses (facilitation) impairs the temporal coding atlow stimulus levels.Summary and insights• The recently proposed model of ANF responses to electrical stimulation(Joshi et al, 2015) can successfully capture the effect of pulse rates onenvelope coding in the CI listeners.• The uncertainty regarding the site of excitation at high pulse rates impairsthe temporal coding in electrically stimulated ANF. This might explain whythe AMD thresholds of the CI listeners are worse for high pulse rates.• The spectrum of ion channel noise largely determines the effect of pulserates on the temporal coding in the ANF.• The ability of ANF to integrate the charge of successive sub-thresholdpulses impairs the temporal coding at low stimulus levels.• These results suggest that a stimulation strategy that reduces the un-certainty regarding the site of excitation along the ANF will improve thetemporal coding in CI listeners.Fig.8 Extra-cellular stimulation of the AN with CI. An electrode inserts the charge in thecochlea and the resulting charge gradient can stimulate multiple sites along the ANF. Thespikes produced at the peripheral axon must travel through the soma to reach the brain-stem. The spikes produced at the central axon reach the brainstem sooner than thoseproduced at the peripheral axon.AcknowledgementsThe work has been funded by grant from the People Programme (Marie Curie Actions) of the European Union’s 7th Frame-work Programme FP7/2007-2013/ under REA grant agreement number PITN-GA-2012-317521.References• Brette and Gerstner (2005) J Neurophysiology, 94, 3637-3642.• Galvin and Fu (2005) JARO, 6(2), 180-189.• Goldwyn et al. (2010) J Comp Neurosci, 28, 405-424.• Goldwyn et al. (2012) J Neurophysiology, 108, 1430-1452.• Joshi et al. (2015) Mid-winter meeting ARO, Baltimore, MD• Joshi et al. (2015) CIAP, Lake Tahoe, California.• Middlebrooks (2008). J Neurophysiology, 100, 92-107.• Miller et al. (1999) Hear Res, 130, 197-218.• Rattay et al. (2001) Hear Res, 153, 43-63.• Rubinstein et al. (1999) Hear Res, 127, 108-118.

What affects envelope coding in theelectrically stimulated auditory nerve?

http://orbit.dtu.dk/ws/files/121887780/2016_ARO_Joshi_et_al_1_.pdf

What affects envelope coding in theelectrically stimulated auditory nerve?

Abstract

Similar works

Full text

Available Versions

Online Research Database In Technology