Pitch and loudness matching of unmodulated and modulated stimuli in cochlear implantees

Outcomes Using the Optimized Pitch and Language Strategy Versus the Advanced Combination Encoder Strategy in Mandarin-Speaking Cochlear Implant Recipients

OBJECTIVES

The experimental Optimized Pitch and Language (OPAL) strategy enhances coding of fundamental frequency (F0) information in the temporal envelope of electrical signals delivered to channels of a cochlear implant (CI). Previous studies with OPAL have explored performance on speech and lexical tone perception in Mandarin- and English-speaking CI recipients. However, it was not clear which cues to lexical tone (primary and/or secondary) were used by the Mandarin CI listeners. The primary aim of the present study was to investigate whether OPAL provides improved recognition of Mandarin lexical tones in both quiet and noisy environments compared with the Advanced Combination Encoder (ACE) strategy. A secondary aim was to investigate whether, and to what extent, removal of secondary (duration and intensity envelope) cues to lexical tone affected Mandarin tone perception.

DESIGN

Thirty-two CI recipients with an average age of 24 (range 7 to 57) years were enrolled in the study. All recipients had at least 1 year of experience using ACE. Each subject attended two testing sessions, the first to measure baseline performance, and the second to evaluate the effect of strategy after provision of some take-home experience using OPAL. A minimum take-home duration of approximately 4 weeks was prescribed in which subjects were requested to use OPAL as much as possible but were allowed to also use ACE when needed. The evaluation tests included recognition of Mandarin lexical tones in quiet and in noise (signal to noise ratio [SNR] +5 dB) using naturally produced tones and duration/intensity envelope normalized versions of the tones; Mandarin sentence in adaptive noise; Mandarin monosyllabic and disyllabic word in quiet; a subset of Speech, Spatial, and Qualities of hearing questionnaire (SSQ, speech hearing scale); and subjective preference for strategy in quiet and noise.

RESULTS

For both the natural and normalized lexical tone tests, mean scores for OPAL were significantly higher than ACE in quiet by 2.7 and 2.9%-points, respectively, and in noise by 7.4 and 7.2%-points, respectively. Monosyllabic word recognition in quiet using OPAL was significantly higher than ACE by approximately 7.5% points. Average SSQ ratings for OPAL were significantly higher than ACE by approximately 0.5 points on a 10-point scale. In quiet conditions, 14 subjects preferred OPAL, 7 expressed a preference for ACE, and 9 reported no preference. Compared with quiet, in noisy situations, there was a stronger preference for OPAL (19 recipients), a similar preference for ACE (7 recipients), while fewer expressed no preference. Average daily take-home use of ACE and OPAL was 4.9 and 7.1 hr, respectively.

CONCLUSIONS

For Mandarin-speaking CI recipients, OPAL provided significant improvements to lexical tone perception for natural and normalized tones in quiet and noise, monosyllabic word recognition in quiet, and subjective ratings of speech intelligibility. Subjects accessed both primary and secondary cues to lexical tone for perception in quiet and noise conditions. The benefits of lexical tone recognition were attributed to enhanced F0 rate cues encoded by OPAL, especially in a noisy environment. The OPAL strategy was well accepted by many of the Mandarin-speaking CI recipients.

Pitch discrimination in electric hearing with inconsistent and consistent amplitude-modulation and inter-pulse rate cues

Users of cochlear implants (CIs) struggle in situations that require selective hearing to focus on a target source while ignoring other sources. One major reason for that is the limited access to timing cues such as temporal pitch or interaural time differences (ITDs). Various approaches to improve timing-cue sensitivity while maintaining speech understanding have been proposed, among them inserting extra pulses with short inter-pulse intervals (SIPIs) into amplitude-modulated (AM) high-rate pulse trains. Indeed, SIPI rates matching the naturally occurring AM rates improve pitch discrimination. For ITD, however, low SIPI rates are required, potentially mismatching the naturally occurring AM rates and thus creating unknown pitch effects. In this study, we investigated the perceptual contribution of AM and SIPI rate to pitch discrimination in five CI listeners and with two AM depths (0.1 and 0.5). Our results show that the SIPI-rate cue generally dominated the percept for both consistent and inconsistent cues. When tested with inconsistent cues, also the AM rate contributed, however, at the large AM depth only. These findings have implications when aiming at jointly improving temporal-pitch and ITD sensitivity in a future mixed-rate stimulation approach.

Pre-attentive fundamental frequency processing in Mandarin-speaking children with cochlear implants as revealed by the peak latency of positive mismatch response

Introduction

Fundamental frequency (F0) serves as the primary acoustic cue for Mandarin tone perception. Recent behavioral studies suggest that F0 information may be differently processed between Mandarin-speaking normal-hearing (NH) children and children with cochlear implants (CIs), which may partially explain the unsatisfactory outcome of lexical tone recognition using CIs with tonal language-oriented speech processing strategies. The aim of the current study was to provide neural evidence of F0 processing in Mandarin-speaking kindergarten-aged children with CIs compared with NH children.

Methods

Positive mismatch responses (p-MMRs) to the change of the two acoustic dimensions of F0 (F0 contour and F0 level) in Mandarin-speaking kindergarten-aged children with CIs (n = 19) and their age-matched NH peers (n = 21).

Results

The two groups of children did not show any significant difference on the mean amplitude of p-MMR to either F0 contour or F0 level change. While the CI group exhibited a significantly shorter peak latency of p-MMR to F0 contour change than to F0 level change, an opposite pattern was observed in the NH group.

Discussion

This study revealed a higher sensitivity to F0 contour change than to F0 level change in children with CIs, which was different from that in NH children. The neural evidence of discrepant F0 processing between children with CIs and NH children in this study was consistent with the previously reported behavioral findings and may serve as a reference for the development and improvement of tonal language-oriented speech processing strategies.

Asymmetric temporal envelope sensitivity: Within- and across-ear envelope comparisons in listeners with bilateral cochlear implants

For listeners with bilateral cochlear implants (BiCIs), patient-specific differences in the interface between cochlear implant (CI) electrodes and the auditory nerve can lead to degraded temporal envelope information, compromising the ability to distinguish between targets of interest and background noise. It is unclear how comparisons of degraded temporal envelope information across spectral channels (i.e., electrodes) affect the ability to detect differences in the temporal envelope, specifically amplitude modulation (AM) rate. In this study, two pulse trains were presented simultaneously via pairs of electrodes in different places of stimulation, within and/or across ears, with identical or differing AM rates. Results from 11 adults with BiCIs indicated that sensitivity to differences in AM rate was greatest when stimuli were paired between different places of stimulation in the same ear. Sensitivity from pairs of electrodes was predicted by the poorer electrode in the pair or the difference in fidelity between both electrodes in the pair. These findings suggest that electrodes yielding poorer temporal fidelity act as a bottleneck to comparisons of temporal information across frequency and ears, limiting access to the cues used to segregate sounds, which has important implications for device programming and optimizing patient outcomes with CIs.

Modulation Depth Discrimination by Cochlear Implant Users

Cochlear implants (CIs) convey the amplitude envelope of speech by modulating high-rate pulse trains. However, not all of the envelope may be necessary to perceive amplitude modulations (AMs); the effective envelope depth may be limited by forward and backward masking from the envelope peaks. Three experiments used modulated pulse trains to measure which portions of the envelope can be effectively processed by CI users as a function of AM frequency. Experiment 1 used a three-interval forced-choice task to test the ability of CI users to discriminate less-modulated pulse trains from a fully modulated standard, without controlling for loudness. The stimuli in experiment 2 were identical, but a two-interval task was used in which participants were required to choose the less-modulated interval, ignoring loudness. Catch trials, in which judgements based on level or modulation depth would give opposing answers, were included. Experiment 3 employed novel stimuli whose modulation envelope could be modified below a variable point in the dynamic range, without changing the loudness of the stimulus. Overall, results showed that substantial portions of the envelope are not accurately encoded by CI users. In experiment 1, where loudness cues were available, participants on average were insensitive to changes in the bottom 30% of their dynamic range. In experiment 2, where loudness was controlled, participants appeared insensitive to changes in the bottom 50% of the dynamic range. In experiment 3, participants were insensitive to changes in the bottom 80% of the dynamic range. We discuss potential reasons for this insensitivity and implications for CI speech-processing strategies.

Just-Noticeable Differences of Fundamental Frequency Change in Mandarin-Speaking Children with Cochlear Implants

Fundamental frequency (F0) provides the primary acoustic cue for lexical tone perception in tonal languages but remains poorly represented in cochlear implant (CI) systems. Currently, there is still a lack of understanding of sensitivity to F0 change in CI users who speak tonal languages. In the present study, just-noticeable differences (JNDs) of F0 contour and F0 level changes in Mandarin-speaking children with CIs were measured and compared with those in their age-matched normal-hearing (NH) peers. Results showed that children with CIs demonstrated significantly larger JND of F0 contour (JND-C) change and F0 level (JND-L) change compared to NH children. Further within-group comparison revealed that the JND-C change was significantly smaller than the JND-L change among children with CIs, whereas the opposite pattern was observed among NH children. No significant correlations were seen between JND-C change/JND-L change and age at implantation /duration of CI use. The contrast between children with CIs and NH children in sensitivity to F0 contour and F0 level change suggests different mechanisms of F0 processing in these two groups as a result of different hearing experiences.

Advantages of Pulse Rate Compared to Modulation Frequency for Temporal Pitch Perception in Cochlear Implant Users

Most cochlear implants encode the fundamental frequency of periodic sounds by amplitude modulation of constant-rate pulsatile stimulation. Pitch perception provided by such stimulation strategies is markedly poor. Two experiments are reported here that consider potential advantages of pulse rate compared to modulation frequency for providing stimulation timing cues for pitch. The first experiment examines beat frequency distortion that occurs when modulating constant-rate pulsatile stimulation. This distortion has been reported on previously, but the results presented here indicate that distortion occurs for higher stimulation rates than previously reported. The second experiment examines pitch resolution as provided by pulse rate compared to modulation frequency. The results indicate that pitch discrimination is better with pulse rate than with modulation frequency. The advantage was large for rates near what has been suggested as the upper limit of temporal pitch perception conveyed by cochlear implants. The results are relevant to sound processing design for cochlear implants particularly for algorithms that encode fundamental frequency into deep envelope modulations or into precisely timed pulsatile stimulation.

Formant frequency discrimination with a fine structure sound coding strategy for cochlear implants

Recent sound coding strategies for cochlear implants (CI) have focused on the transmission of temporal fine structure to the CI recipient. To date, knowledge about the effects of fine structure coding in electrical hearing is poorly charactarized. The aim of this study was to examine whether the presence of temporal fine structure coding affects how the CI recipient perceives sound. This was done by comparing two sound coding strategies with different temporal fine structure coverage in a longitudinal cross-over setting. The more recent FS4 coding strategy provides fine structure coding on typically four apical stimulation channels compared to FSP with usually one or two fine structure channels. 34 adult CI patients with a minimum CI experience of one year were included. All subjects were fitted according to clinical routine and used both coding strategies for three months in a randomized sequence. Formant frequency discrimination thresholds (FFDT) were measured to assess the ability to resolve timbre information. Further outcome measures included a monosyllables test in quiet and the speech reception threshold of an adaptive matrix sentence test in noise (Oldenburger sentence test). In addition, the subjective sound quality was assessed using visual analogue scales and a sound quality questionnaire after each three months period. The extended fine structure range of FS4 yields FFDT similar to FSP for formants occurring in the frequency range only covered by FS4. There is a significant interaction (p = 0.048) between the extent of fine structure coverage in FSP and the improvement in FFDT in favour of FS4 for these stimuli. FS4 Speech perception in noise and quiet was similar with both coding strategies. Sound quality was rated heterogeneously showing that both strategies represent valuable options for CI fitting to allow for best possible individual optimization.

Improving Interaural Time Difference Sensitivity Using Short Inter-pulse Intervals with Amplitude-Modulated Pulse Trains in Bilateral Cochlear Implants

Interaural time differences (ITDs) at low frequencies are important for sound localization and spatial speech unmasking. These ITD cues are not encoded in commonly used envelope-based stimulation strategies for cochlear implants (CIs) using high pulse rates. However, ITD sensitivity can be improved by adding extra pulses with short inter-pulse intervals (SIPIs) in unmodulated high-rate trains. Here, we investigated whether this improvement also applies to amplitude-modulated (AM) high-rate pulse trains. To this end, we systematically varied the temporal position of SIPI pulses within the envelope cycle (SIPI phase), the fundamental frequency (F0) of AM (125 Hz and 250 Hz), and AM depth (from 0.1 to 0.9). Stimuli were presented at an interaurally place-matched electrode pair at a reference pulse rate of 1000 pulses/s. Participants performed an ITD-based left/right discrimination task. SIPI insertion resulted in improved ITD sensitivity throughout the range of modulation depths and for both male and female F0s. The improvements were largest for insertion at and around the envelope peak. These results are promising for conveying salient ITD cues at high pulse rates commonly used to encode speech information.

Temporal-pitch sensitivity in electric hearing with amplitude modulation and inserted pulses with short inter-pulse intervals

Listeners with cochlear implants (CIs) typically show poor sensitivity to the temporal-envelope pitch of high-rate pulse trains. Sensitivity to interaural time differences improves when adding pulses with short inter-pulse intervals (SIPIs) to high-rate pulse trains. In the current study, monaural temporal-pitch sensitivity with SIPI pulses was investigated for six CI listeners. Amplitude-modulated single-electrode stimuli, representing the coding of the fundamental frequency (F0) in the envelope of a high-rate carrier, were used. Two SIPI-insertion approaches, five modulation depths, two typical speech-F0s, and two carrier rates were tested. SIPI pulses were inserted either in every amplitude-modulation period (full-rate SIPI) to support the F0 cue or in every other amplitude-modulation period (half-rate SIPI) to circumvent a potential rate limitation at higher F0s. The results demonstrate that full-rate SIPI pulses improve temporal-pitch sensitivity across F0s and particularly at low modulation depths where envelope-pitch cues are weak. The half-rate SIPI pulses did not circumvent the limitation and further increased variability across listeners. Further, no effect of the carrier rate was found. Thus, the SIPI approach appears to be a promising approach to enhance CI listeners' access to temporal-envelope pitch cues at pulse rates used clinically.

Place and Temporal Cues in Cochlear Implant Pitch and Melody Perception

The present study compared pitch and melody perception using cochlear place of excitation and temporal cues in six adult nucleus cochlear implant (CI) recipients. The stimuli were synthesized tones presented through a loudspeaker, and recipients used the Advanced Combinational Encoder (ACE) sound coding strategy on their own sound processors. Three types of tones were used, denoted H3, H4, and P5. H3 tones were harmonic tones with fundamental frequencies in the range C3-C4 (131-262 Hz), providing temporal pitch cues alone. H4 tones were harmonic tones with fundamental frequencies in the range C4-C5 (262-523 Hz), providing a mixture of temporal and place cues. P5 tones were pure tones with fundamental frequencies in the range C5-C6 (523-1046 Hz), providing place pitch cues alone. Four experimental procedures were used: pitch discrimination, pitch ranking, backward modified melodies, and warped modified melodies. In each trial of the modified melodies tests, subjects heard a familiar melody and a version with modified pitch (in randomized order), and had to select the unmodified melody. In all four procedures, many scores were much lower than would be expected for normal hearing listeners, implying that the strength of the perceived pitch was weak. Discrimination and ranking with H3 and P5 tones was poor for two-semitone intervals, but near perfect for intervals of five semitones and larger. H4 tones provided the lowest group mean scores in all four procedures, with some pitch reversals observed in pitch ranking. Group mean scores for P5 tones (place cues alone) were at least as high as those for H3 tones (temporal cues alone). The relatively good scores on the melody tasks with P5 tones were surprising, given the lack of temporal cues, raising the possibility of musical pitch using place cues alone. However, the alternative possibility that the CI recipients perceived the place cues as brightness, rather than musical pitch per se, cannot be excluded. These findings show that pitch perception models need to incorporate neural place representations alongside temporal cues if they are to predict pitch and melody perception in the absence of temporal cues.

Perceptual Differences Between Low-Frequency Analog and Pulsatile Stimulation as Shown by Single- and Multidimensional Scaling

Cochlear-implant users who have experienced both analog and pulsatile sound coding strategies often have strong preferences for the sound quality of one over the other. This suggests that analog and pulsatile stimulation may provide different information or sound quality to an implant listener. It has been well documented that many implant listeners both prefer and perform better with multichannel analog than multichannel pulsatile strategies, although the reasons for these differences remain unknown. Here, we examine the perceptual differences between analog and pulsatile stimulation on a single electrode. A multidimensional scaling task, analyzed across two dimensions, suggested that pulsatile stimulation was perceived to be considerably different from analog stimulation. Two associated tasks using single-dimensional scaling showed that analog stimulation was perceived to be less Clean on average than pulsatile stimulation and that the perceptual differences were not related to pitch. In a follow-up experiment, it was determined that the perceptual differences between analog and pulsatile stimulation were not dependent on the interpulse gap present in pulsatile stimulation. Although the results suggest that there is a large perceptual difference between analog and pulsatile stimulation, further work is needed to determine the nature of these differences.

Effects of Stimulation Rate With the FS4 and HDCIS Coding Strategies in Cochlear Implant Recipients

OBJECTIVE

The aim of the present study was to evaluate the effect of stimulation rate on speech perception and sound quality for the fine structure strategy FS4 and the envelope-based strategy high definition continuous interleaved sampling (HDCIS).

STUDY DESIGN

Randomized crossover trial with four conditions.

SETTING

Tertiary referral.

PATIENTS

Twenty-six postlingually deafened adult cochlear implant (CI) recipients were included.

INTERVENTION

All subjects were equipped with four coding strategies: FS4 with high rate on the envelope channels (on average 1376 pps/ch), FS4 low rate (750 pps/ch), and HDCIS with the same high and low rates. A "flat-charge map" was used for all four strategies. Only the loudness was balanced between programs. All tests were performed acutely in a double blind manner and a randomized sequence.

MAIN OUTCOME MEASURES

Monosyllables in quiet and subjective sound quality.

RESULTS

Mean monosyllables scores at 65 dB in quiet were 25.5% correct with HDCIS low rate, 27.2% correct with HDCIS high rate, 25.2% with FS4 low rate, and 33.1% with FS4 high rate. Performance with high stimulation rates was significantly higher than with the low rate settings. Subjective sound quality measured with visual analogue scales showed that for naturalness of speech, the improvement with a high rate version was only evident with the FS4 strategy. In both FS4 and HDCIS, higher stimulation rates elicited a higher pitch and were perceived as less dull than lower rates.

CONCLUSION

A high rate of stimulation resulted in better speech recognition in both strategies and a favorable subjective sound quality for FS4 in all tested settings.

Envelope Interactions in Multi-Channel Amplitude Modulation Frequency Discrimination by Cochlear Implant Users

Rationale

Previous cochlear implant (CI) studies have shown that single-channel amplitude modulation frequency discrimination (AMFD) can be improved when coherent modulation is delivered to additional channels. It is unclear whether the multi-channel advantage is due to increased loudness, multiple envelope representations, or to component channels with better temporal processing. Measuring envelope interference may shed light on how modulated channels can be combined.

Methods

In this study, multi-channel AMFD was measured in CI subjects using a 3-alternative forced-choice, non-adaptive procedure (“which interval is different?”). For the reference stimulus, the reference AM (100 Hz) was delivered to all 3 channels. For the probe stimulus, the target AM (101, 102, 104, 108, 116, 132, 164, 228, or 256 Hz) was delivered to 1 of 3 channels, and the reference AM (100 Hz) delivered to the other 2 channels. The spacing between electrodes was varied to be wide or narrow to test different degrees of channel interaction.

Results

Results showed that CI subjects were highly sensitive to interactions between the reference and target envelopes. However, performance was non-monotonic as a function of target AM frequency. For the wide spacing, there was significantly less envelope interaction when the target AM was delivered to the basal channel. For the narrow spacing, there was no effect of target AM channel. The present data were also compared to a related previous study in which the target AM was delivered to a single channel or to all 3 channels. AMFD was much better with multiple than with single channels whether the target AM was delivered to 1 of 3 or to all 3 channels. For very small differences between the reference and target AM frequencies (2–4 Hz), there was often greater sensitivity when the target AM was delivered to 1 of 3 channels versus all 3 channels, especially for narrowly spaced electrodes.

Conclusions

Besides the increased loudness, the present results also suggest that multiple envelope representations may contribute to the multi-channel advantage observed in previous AMFD studies. The different patterns of results for the wide and narrow spacing suggest a peripheral contribution to multi-channel temporal processing. Because the effect of target AM frequency was non-monotonic in this study, adaptive procedures may not be suitable to measure AMFD thresholds with interfering envelopes. Envelope interactions among multiple channels may be quite complex, depending on the envelope information presented to each channel and the relative independence of the stimulated channels.

Modulation frequency discrimination with single and multiple channels in cochlear implant users

Temporal envelope cues convey important speech information for cochlear implant (CI) users. Many studies have explored CI users' single-channel temporal envelope processing. However, in clinical CI speech processors, temporal envelope information is processed by multiple channels. Previous studies have shown that amplitude modulation frequency discrimination (AMFD) thresholds are better when temporal envelopes are delivered to multiple rather than single channels. In clinical fitting, current levels on single channels must often be reduced to accommodate multi-channel loudness summation. As such, it is unclear whether the multi-channel advantage in AMFD observed in previous studies was due to coherent envelope information distributed across the cochlea or to greater loudness associated with multi-channel stimulation. In this study, single- and multi-channel AMFD thresholds were measured in CI users. Multi-channel component electrodes were either widely or narrowly spaced to vary the degree of overlap between neural populations. The reference amplitude modulation (AM) frequency was 100 Hz, and coherent modulation was applied to all channels. In Experiment 1, single- and multi-channel AMFD thresholds were measured at similar loudness. In this case, current levels on component channels were higher for single-than for multi-channel AM stimuli, and the modulation depth was approximately 100% of the perceptual dynamic range (i.e., between threshold and maximum acceptable loudness). Results showed no significant difference in AMFD thresholds between similarly loud single- and multi-channel modulated stimuli. In Experiment 2, single- and multi-channel AMFD thresholds were compared at substantially different loudness. In this case, current levels on component channels were the same for single- and multi-channel stimuli ("summation-adjusted" current levels) and the same range of modulation (in dB) was applied to the component channels for both single- and multi-channel testing. With the summation-adjusted current levels, loudness was lower with single than with multiple channels and the AM depth resulted in substantial stimulation below single-channel audibility, thereby reducing the perceptual range of AM. Results showed that AMFD thresholds were significantly better with multiple channels than with any of the single component channels. There was no significant effect of the distribution of electrodes on multi-channel AMFD thresholds. The results suggest that increased loudness due to multi-channel summation may contribute to the multi-channel advantage in AMFD, and that overall loudness may matter more than the distribution of envelope information in the cochlea.

Perception of binaural cues develops in children who are deaf through bilateral cochlear implantation

There are significant challenges to restoring binaural hearing to children who have been deaf from an early age. The uncoordinated and poor temporal information available from cochlear implants distorts perception of interaural timing differences normally important for sound localization and listening in noise. Moreover, binaural development can be compromised by bilateral and unilateral auditory deprivation. Here, we studied perception of both interaural level and timing differences in 79 children/adolescents using bilateral cochlear implants and 16 peers with normal hearing. They were asked on which side of their head they heard unilaterally or bilaterally presented click- or electrical pulse- trains. Interaural level cues were identified by most participants including adolescents with long periods of unilateral cochlear implant use and little bilateral implant experience. Interaural timing cues were not detected by new bilateral adolescent users, consistent with previous evidence. Evidence of binaural timing detection was, for the first time, found in children who had much longer implant experience but it was marked by poorer than normal sensitivity and abnormally strong dependence on current level differences between implants. In addition, children with prior unilateral implant use showed a higher proportion of responses to their first implanted sides than children implanted simultaneously. These data indicate that there are functional repercussions of developing binaural hearing through bilateral cochlear implants, particularly when provided sequentially; nonetheless, children have an opportunity to use these devices to hear better in noise and gain spatial hearing.

Electrophysiological properties of neurosensory progenitors derived from human embryonic stem cells

In severe cases of sensorineural hearing loss where the numbers of auditory neurons are significantly depleted, stem cell-derived neurons may provide a potential source of replacement cells. The success of such a therapy relies upon producing a population of functional neurons from stem cells, to enable precise encoding of sound information to the brainstem. Using our established differentiation assay to produce sensory neurons from human stem cells, patch-clamp recordings indicated that all neurons examined generated action potentials and displayed both transient sodium and sustained potassium currents. Stem cell-derived neurons reliably entrained to stimuli up to 20 pulses per second (pps), with 50% entrainment at 50 pps. A comparison with cultured primary auditory neurons indicated similar firing precision during low-frequency stimuli, but significant differences after 50 pps due to differences in action potential latency and width. The firing properties of stem cell-derived neurons were also considered relative to time in culture (31-56 days) and revealed no change in resting membrane potential, threshold or firing latency over time. Thus, while stem cell-derived neurons did not entrain to high frequency stimulation as effectively as mammalian auditory neurons, their electrical phenotype was stable in culture and consistent with that reported for embryonic auditory neurons.