Normal-hearing Listeners Research Articles

A model framework for simulating spatial hearing of bilateral cochlear implant users

Bilateral cochlear implants (CIs) greatly improve spatial hearing acuity for CI users, but substantial gaps still exist compared to normal-hearing listeners. For example, CI users have poorer localization skills, little or no binaural unmasking, and reduced spatial release from masking. Multiple factors have been identified that limit binaural hearing with CIs. These include degradation of cues due to the various sound processing stages, the viability of the electrode-neuron interface, impaired brainstem neurons, and deterioration in connectivity between different cortical layers. To help quantify the relative importance and inter-relationship between these factors, computer models can and arguably should be employed. While models exploring single stages are often in good agreement with selected experimental data, their combination often does not yield a comprehensive and accurate simulation of perception. Here, we combine information from CI sound processing with computational auditory model stages in a modular and open-source framework, resembling an artificial bilateral CI user. The main stages are (a) binaural signal generation with optional head-related impulse response filtering, (b) generic CI sound processing not restricted to a specific manufacturer, (c) electrode-to-neuron transmission, (d) binaural interaction, and (e) a decision model. The function and the outputs of different model stages are demonstrated with examples of localization experiments. However, the model framework is not tailored to a specific dataset. It offers a selection of sound coding strategies and allows for third-party model extensions or substitutions; thus, it is possible to employ the model for a wide range of binaural applications and even for educational purposes.

Investigating the Reliability of Pupillometry as a Measure of Individualized Listening Effort

Recordings of the pupillary response have been used in numerous studies to assess listening effort during a speech-in-noise task. Most studies focused on averaged responses across listeners, whereas less is known about pupil dilation as an indicator of the individuals’ listening effort. The present study investigated the reliability of several pupil features as potential indicators of individual listening effort and the impact of different normalization procedures on the reliability. The pupil diameters of 31 normal-hearing listeners were recorded during multiple visits while performing a speech-in-noise task. The signal-to-noise ratios (SNRs) of the stimuli ranged from [Formula: see text]12 dB to [Formula: see text]4 dB. All listeners were measured twice at separate visits, and 11 were re-tested at a third visit. To examine the reliability of the pupil responses across visits, the intraclass correlation coefficient was applied to the peak and mean pupil dilation and to the temporal features of the pupil response, extracted using growth curve analysis. The reliability of the pupillary response was assessed in relation to SNR and different normalization procedures over multiple visits. The most reliable pupil features were the traditional mean and peak pupil dilation. The highest reliability results were obtained when the data were baseline-corrected and normalized to the individual pupil response range across all visits. Moreover, the present study results showed only a minor impact of the SNR and the number of visits on the reliability of the pupil response. Overall, the results may provide an important basis for developing a standardized test for pupillometry in the clinic.

Effect of Realistic Test Conditions on Perception of Speech, Music, and Binaural Cues in Normal-Hearing Listeners.

The purpose of this study was to determine the feasibility of online testing in a quiet room for three auditory perception experiments in normal-hearing listeners: speech, music, and binaural cue. Under Experiment 1, sentence perception was measured using fixed signal-to-noise ratios (SNRs: +10 dB, 0 dB, and -10 dB) and using adaptive speech reception threshold (SRT) procedures. The correct scores were compared between quiet room and soundproof booth listening environments. Experiment 2 was designed to compare melodic contour identification between the two listening environments. Melodic contour identification was assessed with 1, 2, and 4 semitone spacings. Under Experiment 3, interaural level difference (ILD) and interaural time differences (ITD) were measured as a function of carrier frequency. For both measures, two modulated tones (400-ms duration and 100-Hz modulation rate) were sequentially presented through headphones to both ears, and subjects were asked to indicate whether the sound moved to the left or right ear. The measured ITD and ILD were then compared between the two listening environments. There were no significant differences in any outcome measures (SNR- and SRT-based speech perception, melodic contour identification, and ITD/ILD) between the two listening environments. These results suggest that normal-hearing listeners may not require a controlled listening environment in any of the three auditory assessments. As comparable data can be obtained via the online testing tool, using the online auditory experiments is recommended.

Acoustic deprivation modulates central gain in human auditory brainstem and cortex

Beyond reduced audibility, there is convincing evidence that the auditory system adapts according to the principles of homeostatic plasticity in response to a hearing loss. Such compensatory changes include modulation of central auditory gain mechanisms. Earplugging is a common experimental method that has been used to introduce a temporary, reversible hearing loss that induces changes consistent with central gain modulation. In the present study, young, normal-hearing adult participants wore a unilateral earplug for two weeks, during which we measured changes in the acoustic reflex threshold (ART), loudness perception, and cortically-evoked (40 Hz) auditory steady-state response (ASSR) to assess potential modulation in central gain with reduced peripheral input. The ART decreased on average by 8 to 10 dB during the treatment period, with modest increases in loudness perception after one week but not after two weeks of earplug use. Significant changes in both the magnitude and hemispheric laterality of source-localized cortical ASSR measures revealed asymmetrical changes in stimulus-driven cortical activity over time. The ART results following unilateral earplugging are consistent with the literature and suggest that homeostatic plasticity is evident in the brainstem. The novel findings from the cortical ASSR in the present study indicates that reduced peripheral input induces adaptive homeostatic plasticity reflected as both an increase in central gain in the auditory brainstem and reduced cortical activity ipsilateral to the deprived ear. Both the ART and the novel use of the 40-Hz ASSR provide sensitive measures of central gain modulation in the brainstem and cortex of young, normal hearing listeners, and thus may be useful in future studies with other clinical populations.

Age effects on cognitive functions and speech-in-noise processing: An event-related potential study with cochlear-implant users and normal-hearing listeners.

A cochlear implant (CI) can partially restore hearing in individuals with profound sensorineural hearing loss. However, electrical hearing with a CI is limited and highly variable. The current study aimed to better understand the different factors contributing to this variability by examining how age affects cognitive functions and cortical speech processing in CI users. Electroencephalography (EEG) was applied while two groups of CI users (young and elderly; N = 13 each) and normal-hearing (NH) listeners (young and elderly; N = 13 each) performed an auditory sentence categorization task, including semantically correct and incorrect sentences presented either with or without background noise. Event-related potentials (ERPs) representing earlier, sensory-driven processes (N1-P2 complex to sentence onset) and later, cognitive-linguistic integration processes (N400 to semantically correct/incorrect sentence-final words) were compared between the different groups and speech conditions. The results revealed reduced amplitudes and prolonged latencies of auditory ERPs in CI users compared to NH listeners, both at earlier (N1, P2) and later processing stages (N400 effect). In addition to this hearing-group effect, CI users and NH listeners showed a comparable background-noise effect, as indicated by reduced hit rates and reduced (P2) and delayed (N1/P2) ERPs in conditions with background noise. Moreover, we observed an age effect in CI users and NH listeners, with young individuals showing improved specific cognitive functions (working memory capacity, cognitive flexibility and verbal learning/retrieval), reduced latencies (N1/P2), decreased N1 amplitudes and an increased N400 effect when compared to the elderly. In sum, our findings extend previous research by showing that the CI users' speech processing is impaired not only at earlier (sensory) but also at later (semantic integration) processing stages, both in conditions with and without background noise. Using objective ERP measures, our study provides further evidence of strong age effects on cortical speech processing, which can be observed in both the NH listeners and the CI users. We conclude that elderly individuals require more effortful processing at sensory stages of speech processing, which however seems to be at the cost of the limited resources available for the later semantic integration processes.

Interaural speech asymmetry predicts bilateral speech intelligibility but not listening effort in adults with bilateral cochlear implants.

Bilateral cochlear implants (BiCIs) can facilitate improved speech intelligibility in noise and sound localization abilities compared to a unilateral implant in individuals with bilateral severe to profound hearing loss. Still, many individuals with BiCIs do not benefit from binaural hearing to the same extent that normal hearing (NH) listeners do. For example, binaural redundancy, a speech intelligibility benefit derived from having access to duplicate copies of a signal, is highly variable among BiCI users. Additionally, patients with hearing loss commonly report elevated listening effort compared to NH listeners. There is some evidence to suggest that BiCIs may reduce listening effort compared to a unilateral CI, but the limited existing literature has not shown this consistently. Critically, no studies to date have investigated this question using pupillometry to quantify listening effort, where large pupil sizes indicate high effort and small pupil sizes indicate low effort. Thus, the present study aimed to build on existing literature by investigating the potential benefits of BiCIs for both speech intelligibility and listening effort. Twelve BiCI adults were tested in three listening conditions: Better Ear, Poorer Ear, and Bilateral. Stimuli were IEEE sentences presented from a loudspeaker at 0° azimuth in quiet. Participants were asked to repeat back the sentences, and responses were scored by an experimenter while changes in pupil dilation were measured. On average, participants demonstrated similar speech intelligibility in the Better Ear and Bilateral conditions, and significantly worse speech intelligibility in the Poorer Ear condition. Despite similar speech intelligibility in the Better Ear and Bilateral conditions, pupil dilation was significantly larger in the Bilateral condition. These results suggest that the BiCI users tested in this study did not demonstrate binaural redundancy in quiet. The large interaural speech asymmetries demonstrated by participants may have precluded them from obtaining binaural redundancy, as shown by the inverse relationship between the two variables. Further, participants did not obtain a release from effort when listening with two ears versus their better ear only. Instead, results indicate that bilateral listening elicited increased effort compared to better ear listening, which may be due to poor integration of asymmetric inputs.

Electrophysiological and Psychophysical Measures of Temporal Pitch Sensitivity in Normal-hearing Listeners

To obtain combined behavioural and electrophysiological measures of pitch perception, we presented harmonic complexes, bandpass filtered to contain only high-numbered harmonics, to normal-hearing listeners. These stimuli resemble bandlimited pulse trains and convey pitch using a purely temporal code. A core set of conditions consisted of six stimuli with baseline pulse rates of 94, 188 and 280 pps, filtered into a HIGH (3365–4755 Hz) or VHIGH (7800–10,800 Hz) region, alternating with a 36% higher pulse rate. Brainstem and cortical processing were measured using the frequency following response (FFR) and auditory change complex (ACC), respectively. Behavioural rate change difference limens (DLs) were measured by requiring participants to discriminate between a stimulus that changed rate twice (up-down or down-up) during its 750-ms presentation from a constant-rate pulse train. FFRs revealed robust brainstem phase locking whose amplitude decreased with increasing rate. Moderate-sized but reliable ACCs were obtained in response to changes in purely temporal pitch and, like the psychophysical DLs, did not depend consistently on the direction of rate change or on the pulse rate for baseline rates between 94 and 280 pps. ACCs were larger and DLs lower for stimuli in the HIGH than in the VHGH region. We argue that the ACC may be a useful surrogate for behavioural measures of rate discrimination, both for normal-hearing listeners and for cochlear-implant users. We also showed that rate DLs increased markedly when the baseline rate was reduced to 48 pps, and compared the behavioural and electrophysiological findings to recent cat data obtained with similar stimuli and methods.

Defining functional spatial boundaries using a spatial release from masking task.

The classic spatial release from masking (SRM) task measures speech recognition thresholds for discrete separation angles between a target and masker. Alternatively, this study used a modified SRM task that adaptively measured the spatial-separation angle needed between a continuous male target stream (speech with digits) and two female masker streams to achieve a specific SRM. On average, 20 young normal-hearing listeners needed less spatial separation for 6 dB release than 9 dB release, and the presence of background babble reduced across-listener variability on the paradigm. Future work is needed to better understand the psychometric properties of this adaptive procedure.

Factors underlying masking release by voice-gender differences and spatial separation cues in multi-talker listening environments in listeners with and without hearing loss.

Voice-gender differences and spatial separation are important cues for auditory object segregation. The goal of this study was to investigate the relationship of voice-gender difference benefit to the breadth of binaural pitch fusion, the perceptual integration of dichotic stimuli that evoke different pitches across ears, and the relationship of spatial separation benefit to localization acuity, the ability to identify the direction of a sound source. Twelve bilateral hearing aid (HA) users (age from 30 to 75 years) and eleven normal hearing (NH) listeners (age from 36 to 67 years) were tested in the following three experiments. First, speech-on-speech masking performance was measured as the threshold target-to-masker ratio (TMR) needed to understand a target talker in the presence of either same- or different-gender masker talkers. These target-masker gender combinations were tested with two spatial configurations (maskers co-located or 60° symmetrically spatially separated from the target) in both monaural and binaural listening conditions. Second, binaural pitch fusion range measurements were conducted using harmonic tone complexes around a 200-Hz fundamental frequency. Third, absolute localization acuity was measured using broadband (125-8000 Hz) noise and one-third octave noise bands centered at 500 and 3000 Hz. Voice-gender differences between target and maskers improved TMR thresholds for both listener groups in the binaural condition as well as both monaural (left ear and right ear) conditions, with greater benefit in co-located than spatially separated conditions. Voice-gender difference benefit was correlated with the breadth of binaural pitch fusion in the binaural condition, but not the monaural conditions, ruling out a role of monaural abilities in the relationship between binaural fusion and voice-gender difference benefits. Spatial separation benefit was not significantly correlated with absolute localization acuity. In addition, greater spatial separation benefit was observed in NH listeners than in bilateral HA users, indicating a decreased ability of HA users to benefit from spatial release from masking (SRM). These findings suggest that sharp binaural pitch fusion may be important for maximal speech perception in multi-talker environments for both NH listeners and bilateral HA users.

Adaptive Syllable Training Improves Phoneme Identification in Older Listeners with and without Hearing Loss.

Acoustic-phonetic speech training mitigates confusion between consonants and improves phoneme identification in noise. A novel training paradigm addressed two principles of perceptual learning. First, training benefits are often specific to the trained material; therefore, stimulus variability was reduced by training small sets of phonetically similar consonant-vowel-consonant syllables. Second, the training is most efficient at an optimal difficulty level; accordingly, the noise level was adapted to the participant's competency. Fifty-two adults aged between sixty and ninety years with normal hearing or moderate hearing loss participated in five training sessions within two weeks. Training sets of phonetically similar syllables contained voiced and voiceless stop and fricative consonants, as well as voiced nasals and liquids. Listeners identified consonants at the onset or the coda syllable position by matching the syllables with their orthographic equivalent within a closed set of three alternative symbols. The noise level was adjusted in a staircase procedure. Pre-post-training benefits were quantified as increased accuracy and a decrease in the required signal-to-noise ratio (SNR) and analyzed with regard to the stimulus sets and the participant's hearing abilities. The adaptive training was feasible for older adults with various degrees of hearing loss. Normal-hearing listeners performed with high accuracy at lower SNR after the training. Participants with hearing loss improved consonant accuracy but still required a high SNR. Phoneme identification improved for all stimulus sets. However, syllables within a set required noticeably different SNRs. Most significant gains occurred for voiced and voiceless stop and (af)fricative consonants. The training was beneficial for difficult consonants, but the easiest to identify consonants improved most prominently. The training enabled older listeners with different capabilities to train and improve at an individual 'edge of competence'.

Dichotic spectral integration range for consonant recognition in listeners with normal hearing.

Dichotic spectral integration range, or DSIR, was measured for consonant recognition with normal-hearing listeners. DSIR is defined as a frequency range needed from 0 to 8,000 Hz band in one ear for consonant recognition when low-frequency information of the same consonant was presented to the opposite ear. DSIR was measured under the three signal processing conditions: (1) unprocessed, (2) target: intensified target spectro-temporal regions by 6 dB responsible for consonant recognition, and (3) target minus conflicting: intensified target regions minus spectro-temporal regions that increase confusion. Each consonant was low-pass filtered with a cutoff frequency of 250, 500, 750, and 1,000 Hz, and then was presented in the left ear or low-frequency (LF) ear. To create dichotic listening, the same consonant was simultaneously presented to the right ear or high-frequency (HF) ear. This was high-pass filtered with an initial cutoff frequency of 7,000 Hz, which was adjusted using an adaptive procedure to find the maximum high-pass cutoff for 99.99% correct consonant recognition. Mean DSIRs spanned from 3,198-8,000 Hz to 4,668-8,000 Hz (i.e., mid-to-high frequencies were unnecessary), depending on low-frequency information in the LF ear. DSIRs narrowed (i.e., required less frequency information) with increasing low-frequency information in the LF ear. However, the mean DSIRs were not significantly affected by the signal processing except at the low-pass cutoff frequency of 250 Hz. The individual consonant analyses revealed that /ta/, /da/, /sa/, and /za/ required the smallest DSIR, while /ka/, /ga/, /fa/, and /va/ required the largest DSIRs. DSIRs also narrowed with increasing low-frequency information for the two signal processing conditions except for 250 vs. 1,000 Hz under the target-conflicting condition. The results suggest that consonant recognition is possible with large amounts of spectral information missing if complementary spectral information is integrated across ears. DSIR is consonant-specific and relatively consistent, regardless of signal processing. The results will help determine the minimum spectral range needed in one ear for consonant recognition if limited low spectral information is available in the opposite ear.

Cochlear-implant Mandarin tone recognition with a disyllabic word corpus.

Despite pitch being considered the primary cue for discriminating lexical tones, there are secondary cues such as loudness contour and duration, which may allow some cochlear implant (CI) tone discrimination even with severely degraded pitch cues. To isolate pitch cues from other cues, we developed a new disyllabic word stimulus set (Di) whose primary (pitch) and secondary (loudness) cue varied independently. This Di set consists of 270 disyllabic words, each having a distinct meaning depending on the perceived tone. Thus, listeners who hear the primary pitch cue clearly may hear a different meaning from listeners who struggle with the pitch cue and must rely on the secondary loudness contour. A lexical tone recognition experiment was conducted, which compared Di with a monosyllabic set of natural recordings. Seventeen CI users and eight normal-hearing (NH) listeners took part in the experiment. Results showed that CI users had poorer pitch cues encoding and their tone recognition performance was significantly influenced by the "missing" or "confusing" secondary cues with the Di corpus. The pitch-contour-based tone recognition is still far from satisfactory for CI users compared to NH listeners, even if some appear to integrate multiple cues to achieve high scores. This disyllabic corpus could be used to examine the performance of pitch recognition of CI users and the effectiveness of pitch cue enhancement based Mandarin tone enhancement strategies. The Di corpus is freely available online: https://github.com/BetterCI/DiTone.

A biologically oriented algorithm for spatial sound segregation.

Listening in an acoustically cluttered scene remains a difficult task for both machines and hearing-impaired listeners. Normal-hearing listeners accomplish this task with relative ease by segregating the scene into its constituent sound sources, then selecting and attending to a target source. An assistive listening device that mimics the biological mechanisms underlying this behavior may provide an effective solution for those with difficulty listening in acoustically cluttered environments (e.g., a cocktail party). Here, we present a binaural sound segregation algorithm based on a hierarchical network model of the auditory system. In the algorithm, binaural sound inputs first drive populations of neurons tuned to specific spatial locations and frequencies. The spiking response of neurons in the output layer are then reconstructed into audible waveforms via a novel reconstruction method. We evaluate the performance of the algorithm with a speech-on-speech intelligibility task in normal-hearing listeners. This two-microphone-input algorithm is shown to provide listeners with perceptual benefit similar to that of a 16-microphone acoustic beamformer. These results demonstrate the promise of this biologically inspired algorithm for enhancing selective listening in challenging multi-talker scenes.

Within- and across-frequency temporal processing and speech perception in cochlear implant users.

Cochlear implant (CI) recipient's speech perception performance is highly variable and is influenced by temporal processing abilities. Temporal processing is commonly assessed using a behavioral task that requires the participant to detect a silent gap with the pre- and post-gap stimuli of the same frequency (within-frequency gap detection) or of different frequencies (across-frequency gap detection). The purpose of the study was to evaluate behavioral and electrophysiological measures of within- and across-frequency temporal processing and their correlations with speech perception performance in CI users. Participants included 11 post-lingually deafened adult CI users (n = 15 ears; Mean Age = 50.2 yrs) and 11 age- and gender-matched normal hearing (NH) individuals (n = 15 ears; Mean Age = 49.0 yrs). Speech perception was assessed with Consonant-Nucleus-Consonant Word Recognition (CNC), Arizona Biomedical Sentence Recognition (AzBio), and Bamford-Kowal-Bench Speech-in-Noise Test (BKB-SIN) tests. Within- and across-frequency behavioral gap detection thresholds (referred to as the GDTwithin and GDTacross) were measured using an adaptive, two-alternative, forced-choice procedure. Cortical auditory evoked potentials (CAEPs) were elicited using within- and across-frequency gap stimuli under four gap duration conditions (no gap, GDT, sub-threshold GDT, and supra-threshold GDT). Correlations among speech perception, GDTs, and CAEPs were examined. CI users had poorer speech perception scores compared to NH listeners (p < 0.05), but the GDTs were not different between groups (p > 0.05). Compared to NH peers, CI users showed increased N1 latency in the CAEPs evoked by the across-frequency gap stimuli (p < 0.05). No group difference was observed for the CAEPs evoked by the within-frequency gap (p > 0.05). Three CI ears showing the longest GDTwithin also showed the poorest performance in speech in noise. The within-frequency CAEP increased in amplitude with the increase of gap duration; while the across-frequency CAEP displayed a similar amplitude for all gap durations. There was a significant correlation between speech scores and within-frequency CAEP measures for the supra-threshold GDT condition, with CI users with poorer speech performance having a smaller N1-P2 amplitude and longer N1 latency. No correlations were found among GDTacross, speech perception, and across-frequency CAEP measures. Within- and across-frequency gap detection may involve different neural mechanisms. The within-frequency gap detection task can help identify CI users with poor speech performance for rehabilitation. The within-frequency CAEP is a better predictor for speech perception performance than the across-frequency CAEP.

Spatial cues on normal hearing and cochlear implant simulation with different coding strategies

Cochlear implant users are known to have limited access to spatial cues. This study investigated the perception of spatial cues in normal-hearing listeners and cochlear implant simulation users. Perception of spatial cues is assessed for performance in determining the direction of the sound and understanding the speech. The results show that cochlear implant simulation users still have access to spatial cues, just like normal- hearing listeners. Normal-hearing listeners and cochlear implant simulation users can perceive spatial cues in ILD and ITD. Both can accurately identify the direction of the sound (slope ≈ 1.00 and of set ≈ 0.00°). Cochlear implant simulation users can understand sentences as well as normal-hearing listeners (PCW = 113.64 rau) by using the coding strategy SPEAK in all channels or CIS with channel above 8. Perception of spatial cues in normal-hearing listeners and cochlear implant users can be improved by listening with two ears and spatially separating the target-masker position. The largest improvement in spatial cue perception was obtained from the head shadow effect (normal-hearing (NH) = 12.96, cochlear implant simulation users (CI) = 59.02), followed by binaural summation (NH = 5.72, CI = 19, 86) and binaural squelch (NH = 3.76, CI = 7.66).

Suppressing reverberation in cochlear implant stimulus patterns using time-frequency masks based on phoneme groups

Cochlear implant (CI) users experience considerable difficulty in understanding speech in reverberant listening environments. This issue is commonly addressed with time-frequency masking, where a time-frequency decomposed reverberant signal is multiplied by a matrix of gain values to suppress reverberation. However, mask estimation is challenging in reverberant environments due to the large spectro-temporal variations in the speech signal. To overcome this variability, we previously developed a phoneme-based algorithm that selects a different mask estimation model based on the underlying phoneme. In the ideal case where knowledge of the phoneme was assumed, the phoneme-based approach provided larger benefits than a phoneme-independent approach when tested in normal-hearing listeners using an acoustic model of CI processing. The current work investigates the phoneme-based mask estimation algorithm in the real-time feasible case where the prediction from a phoneme classifier is used to select the phoneme-specific mask. To further ensure real-time feasibility, both the phoneme classifier and mask estimation algorithm use causal features extracted from within the CI processing framework. We conducted experiments in normal-hearing listeners using an acoustic model of CI processing, and the results showed that the phoneme-specific algorithm benefitted the majority of subjects. [Work supported by NIH through Grant No. R01DC014290-05.]

Lexical effects on talker discrimination in adult cochlear implant users

The lexical and phonological content of an utterance impacts the processing of talker-specific details in normal-hearing (NH) listeners. For example, previous research has shown that NH listeners display stronger perception of talker-specific details for real words compared to nonwords, and for nonwords with high phonotactic probability compared to nonwords with low phonotactic probability. For adult cochlear implant (CI) users, limitations in the talker-specific details conveyed by their devices may alter the reliance on lexical information in talker discrimination. The current study examined the impact of lexical content on talker discrimination in adult CI users. In a same-different talker discrimination task, word pairs – produced either by the same or different talkers – were either lexically easy (high frequency, low neighborhood density) or lexically hard (low frequency, high neighborhood density). Results showed a significant interaction between lexical content and talker pair. For same-talker pairs, accuracy was higher for lexically easy words than lexically hard words. For different-talker pairs, accuracy was higher for lexically hard words than lexically easy words. Despite limitations in talker discrimination, these preliminary results suggest that adult CI users use lexical information in the processing of talker-specific details, and additionally may rely less on acoustic-phonetic details when processing is easy.

Processing of linguistic and indexical information in adult cochlear implant users

Normal-hearing (NH) listeners simultaneously make use of both linguistic (speech content) and indexical (talker-specific) information in speech perception. For adult cochlear implant (CI) users, limitations in the talker-specific details conveyed by their devices may impact processing dependences between linguistic and indexical information. The current study examined the extent to which linguistic and indexical information are processed interdependently in adult CI users, using speeded phoneme and talker gender classification tasks. Higher-performing adult CI users (open-set word recognition &amp;gt;65% correct) reported the target vowel (phoneme classification) or target talker gender (gender classification). The non-target dimension varied by the number of talkers (phoneme classification) and the number of words (gender classification). Largely consistent with NH findings, adult CI users displayed asymmetric interference between linguistic and indexical dimensions. Phoneme classification was less accurate and slower with high talker variability. In contrast, gender classification was not affected by word variability. Processing dependencies also varied across individual CI users and related to auditory and cognitive-linguistic abilities. These preliminary findings suggest that adult CI users may make use of degraded linguistic and indexical information during speech processing. However, the extent to which they do so may depend on individual auditory and cognitive-linguistic factors.

Engaging effort improves efficiency during word recognition in cochlear implant users

Cochlear Implant (CI) users must cope with the degraded spectral input received through their device. This reduced quality leads to changes in how words are recognized: lexical access is delayed, leading to differences in competition between lexical candidates. CI users also report increased effort and fatigue during language processing, but effort is not clearly related to improved accuracy. We investigated the role that listening effort plays in the dynamics of word recognition. We related lexical competition and listening effort in 77 post-lingually deaf CI users with Visual World Paradigm and pupillometry tasks. Subjects also completed measures of peripheral auditory function (spectral ripple discrimination and temporal modulation detection). We used the difference in pupil size during the recognition of spoken and written words to isolate effort for spoken word recognition. Difference in pupil size predicted efficiency in the spoken word recognition task, over and above age and peripheral fidelity. That is, CI users who engage more effort were faster to recognize words and showed lexical competition similar to normal hearing listeners. This interacted with spectral resolution: better discrimination offsets the need to engage effort. This suggests that listening effort can be engaged to improve efficiency during word recognition.

The possible range of interaural cross correlation after cochlear implant processing

Normal hearing listeners rely on their binaural auditory system to combine signals from both ears into single auditory objects, which aids source segregation and sound localization. The ability to combine signals from the two ears is influenced by interaural symmetry - the similarity of the place of stimulation along the left and right cochlea—and the interaural cross-correlation (ICC) of the signal, calculated as the peak correlation between the signal for the two ears across unilateral temporal offsets. The ability of bilateral cochlear implant (CI) users to combine signals from the two ears can be worse than that of NH listeners. This is partly attributed to decreases in interaural symmetry related to factors such as differences in electrode array insertion depth. A secondary effect of interaural asymmetry is that it can result in reduced ICC at interaurally symmetric locations, mediated by aspects of CI processing. This study aims to investigate how CI processing and interaural asymmetry affect the ICC available to CI users for interaurally symmetric locations, using electrodograms recorded from a CI processor placed on a Knowles Electronics Manikin for Acoustic Research (KEMAR). Inferences on how ICC is altered with interaural asymmetry for CI users will be discussed.