Effect of age, presentation method, and learning on identification of noise-vocoded words

Spectral degradation and carrier sentences increase age-related temporal processing deficits in a cue-specific manner

Advancing age is associated with decreased sensitivity to temporal cues in word segments, particularly when target words follow non-informative carrier sentences or are spectrally degraded (e.g., vocoded to simulate cochlear-implant stimulation). This study investigated whether age, carrier sentences, and spectral degradation interacted to cause undue difficulty in processing speech temporal cues. Younger and older adults with normal hearing performed phonemic categorization tasks on two continua: a Buy/Pie contrast with voice onset time changes for the word-initial stop and a Dish/Ditch contrast with silent interval changes preceding the word-final fricative. Target words were presented in isolation or after non-informative carrier sentences, and were unprocessed or degraded via sinewave vocoding (2, 4, and 8 channels). Older listeners exhibited reduced sensitivity to both temporal cues compared to younger listeners. For the Buy/Pie contrast, age, carrier sentence, and spectral degradation interacted such that the largest age effects were seen for unprocessed words in the carrier sentence condition. This pattern differed from the Dish/Ditch contrast, where reducing spectral resolution exaggerated age effects, but introducing carrier sentences largely left the patterns unchanged. These results suggest that certain temporal cues are particularly susceptible to aging when placed in sentences, likely contributing to the difficulties of older cochlear-implant users in everyday environments.

The Costs (and Benefits?) of Effortful Listening for Older Adults: Insights from Simultaneous Electrophysiology, Pupillometry, and Memory

Although the impact of acoustic challenge on speech processing and memory increases as a person ages, older adults may engage in strategies that help them compensate for these demands. In the current preregistered study, older adults (n = 48) listened to sentences-presented in quiet or in noise-that were high constraint with either expected or unexpected endings or were low constraint with unexpected endings. Pupillometry and EEG were simultaneously recorded, and subsequent sentence recognition and word recall were measured. Like young adults in prior work, we found that noise led to increases in pupil size, delayed and reduced ERP responses, and decreased recall for unexpected words. However, in contrast to prior work in young adults where a larger pupillary response predicted a recovery of the N400 at the cost of poorer memory performance in noise, older adults did not show an associated recovery of the N400 despite decreased memory performance. Instead, we found that in quiet, increases in pupil size were associated with delays in N400 onset latencies and increased recognition memory performance. In conclusion, we found that transient variation in pupil-linked arousal predicted trade-offs between real-time lexical processing and memory that emerged at lower levels of task demand in aging. Moreover, with increased acoustic challenge, older adults still exhibited costs associated with transient increases in arousal without the corresponding benefits.

How to vocode: Using channel vocoders for cochlear-implant research

The channel vocoder has become a useful tool to understand the impact of specific forms of auditory degradation-particularly the spectral and temporal degradation that reflect cochlear-implant processing. Vocoders have many parameters that allow researchers to answer questions about cochlear-implant processing in ways that overcome some logistical complications of controlling for factors in individual cochlear implant users. However, there is such a large variety in the implementation of vocoders that the term "vocoder" is not specific enough to describe the signal processing used in these experiments. Misunderstanding vocoder parameters can result in experimental confounds or unexpected stimulus distortions. This paper highlights the signal processing parameters that should be specified when describing vocoder construction. The paper also provides guidance on how to determine vocoder parameters within perception experiments, given the experimenter's goals and research questions, to avoid common signal processing mistakes. Throughout, we will assume that experimenters are interested in vocoders with the specific goal of better understanding cochlear implants.

Effects of training length on adaptation to noise-vocoded speech

Listeners show rapid perceptual learning of acoustically degraded speech, though the amount of exposure required to maximize speech adaptation is unspecified. The current work used a single-session design to examine the length of auditory training on perceptual learning for normal hearing listeners exposed to eight-channel noise-vocoded speech. Participants completed short, medium, or long training using a two-alternative forced choice sentence identification task with feedback. To assess learning and generalization, a 40-trial pre-test and post-test transcription task was administered using trained and novel sentences. Training results showed all groups performed near ceiling with no reliable differences. For test data, we evaluated changes in transcription accuracy using separate linear mixed models for trained or novel sentences. In both models, we observed a significant improvement in transcription at post-test relative to pre-test. Critically, the three training groups did not differ in the magnitude of improvement following training. Subsequent Bayes factors analysis evaluating the test by group interaction provided strong evidence in support of the null hypothesis. For these stimuli and procedure, results suggest increased training does not necessarily maximize learning outcomes; both passive and trained experience likely supported adaptation. Findings may contribute to rehabilitation recommendations for listeners adapting to degraded speech signals.

Checkerboard and interrupted speech: Intelligibility contrasts related to factor-analysis-based frequency bandsa)

It has been shown that the intelligibility of checkerboard speech stimuli, in which speech signals were periodically interrupted in time and frequency, drastically varied according to the combination of the number of frequency bands (2-20) and segment duration (20-320 ms). However, the effects of the number of frequency bands between 4 and 20 and the frequency division parameters on intelligibility have been largely unknown. Here, we show that speech intelligibility was lowest in four-band checkerboard speech stimuli, except for the 320-ms segment duration. Then, temporally interrupted speech stimuli and eight-band checkerboard speech stimuli came in this order (N = 19 and 20). At the same time, U-shaped intelligibility curves were observed for four-band and possibly eight-band checkerboard speech stimuli. Furthermore, different parameters of frequency division resulted in small but significant intelligibility differences at the 160- and 320-ms segment duration in four-band checkerboard speech stimuli. These results suggest that factor-analysis-based four frequency bands, representing groups of critical bands correlating with each other in speech power fluctuations, work as speech cue channels essential for speech perception. Moreover, a probability summation model for perceptual units, consisting of a sub-unit process and a supra-unit process that receives outputs of the speech cue channels, may account for the U-shaped intelligibility curves.

Between alpha and gamma oscillations: Neural signatures of linguistic predictions and listener's attention to speaker's communication intention

When listeners hear a message produced by their interlocutor, they can predict upcoming words thanks to the sentential context and their attention can be focused on the speaker's communication intention. In two electroencephalographical (EEG) studies, we investigated the oscillatory correlates of prediction in spoken-language comprehension and how they are modulated by the listener's attention. Sentential contexts which were strongly predictive of a particular word were ended by a possessive adjective either matching the gender of the predicted word or not. Alpha, beta and gamma oscillations were studied as they were considered to play a crucial role in the predictive process. While evidence of word prediction was related to alpha fluctuations when listeners focused their attention on sentence meaning, changes in high-gamma oscillations were triggered by word prediction when listeners focused their attention on the speaker's communication intention. Independently of the endogenous attention to a level of linguistic information, the oscillatory correlates of word predictions in language comprehension were sensitive to the prosodic emphasis produced by the speaker at a late stage. These findings thus bear major implications for understanding the neural mechanisms that support predictive processing in spoken-language comprehension.

Recognition of Accented Speech by Cochlear-Implant Listeners: Benefit of Audiovisual Cues

OBJECTIVES

When auditory and visual speech information are presented together, listeners obtain an audiovisual (AV) benefit or a speech understanding improvement compared with auditory-only (AO) or visual-only (VO) presentations. Cochlear-implant (CI) listeners, who receive degraded speech input and therefore understand speech using primarily temporal information, seem to readily use visual cues and can achieve a larger AV benefit than normal-hearing (NH) listeners. It is unclear, however, if the AV benefit remains relatively large for CI listeners when trying to understand foreign-accented speech when compared with unaccented speech. Accented speech can introduce changes to temporal auditory cues and visual cues, which could decrease the usefulness of AV information. Furthermore, we sought to determine if the AV benefit was relatively larger in CI compared with NH listeners for both unaccented and accented speech.

DESIGN

AV benefit was investigated for unaccented and Spanish-accented speech by presenting English sentences in AO, VO, and AV conditions to 15 CI and 15 age- and performance-matched NH listeners. Performance matching between NH and CI listeners was achieved by varying the number of channels of a noise vocoder for the NH listeners. Because of the differences in age and hearing history of the CI listeners, the effects of listener-related variables on speech understanding performance and AV benefit were also examined.

RESULTS

AV benefit was observed for both unaccented and accented conditions and for both CI and NH listeners. The two groups showed similar performance for the AO and AV conditions, and the normalized AV benefit was relatively smaller for the accented than the unaccented conditions. In the CI listeners, older age was associated with significantly poorer performance with the accented speaker compared with the unaccented speaker. The negative impact of age was somewhat reduced by a significant improvement in performance with access to AV information.

CONCLUSIONS

When auditory speech information is degraded by CI sound processing, visual cues can be used to improve speech understanding, even in the presence of a Spanish accent. The AV benefit of the CI listeners closely matched that of the NH listeners presented with vocoded speech, which was unexpected given that CI listeners appear to rely more on visual information to communicate. This result is perhaps due to the one-to-one age and performance matching of the listeners. While aging decreased CI listener performance with the accented speaker, access to visual cues boosted performance and could partially overcome the age-related speech understanding deficits for the older CI listeners.

Binaural Optimization of Cochlear Implants: Discarding Frequency Content Without Sacrificing Head-Shadow Benefit

OBJECTIVES

Single-sided deafness cochlear-implant (SSD-CI) listeners and bilateral cochlear-implant (BI-CI) listeners gain near-normal levels of head-shadow benefit but limited binaural benefits. One possible reason for these limited binaural benefits is that cochlear places of stimulation tend to be mismatched between the ears. SSD-CI and BI-CI patients might benefit from a binaural fitting that reallocates frequencies to reduce interaural place mismatch. However, this approach could reduce monaural speech recognition and head-shadow benefit by excluding low- or high-frequency information from one ear. This study examined how much frequency information can be excluded from a CI signal in the poorer-hearing ear without reducing head-shadow benefits and how these outcomes are influenced by interaural asymmetry in monaural speech recognition.

DESIGN

Speech-recognition thresholds for sentences in speech-shaped noise were measured for 6 adult SSD-CI listeners, 12 BI-CI listeners, and 9 normal-hearing listeners presented with vocoder simulations. Stimuli were presented using nonindividualized in-the-ear or behind-the-ear head-related impulse-response simulations with speech presented from a 70° azimuth (poorer-hearing side) and noise from 70° (better-hearing side), thereby yielding a better signal-to-noise ratio (SNR) at the poorer-hearing ear. Head-shadow benefit was computed as the improvement in bilateral speech-recognition thresholds gained from enabling the CI in the poorer-hearing, better-SNR ear. High- or low-pass filtering was systematically applied to the head-related impulse-response-filtered stimuli presented to the poorer-hearing ear. For the SSD-CI listeners and SSD-vocoder simulations, only high-pass filtering was applied, because the CI frequency allocation would never need to be adjusted downward to frequency-match the ears. For the BI-CI listeners and BI-vocoder simulations, both low and high pass filtering were applied. The normal-hearing listeners were tested with two levels of performance to examine the effect of interaural asymmetry in monaural speech recognition (vocoder synthesis-filter slopes: 5 or 20 dB/octave).

RESULTS

Mean head-shadow benefit was smaller for the SSD-CI listeners (~7 dB) than for the BI-CI listeners (~14 dB). For SSD-CI listeners, frequencies <1236 Hz could be excluded; for BI-CI listeners, frequencies <886 or >3814 Hz could be excluded from the poorer-hearing ear without reducing head-shadow benefit. Bilateral performance showed greater immunity to filtering than monaural performance, with gradual changes in performance as a function of filter cutoff. Real and vocoder-simulated CI users with larger interaural asymmetry in monaural performance had less head-shadow benefit.

CONCLUSIONS

The "exclusion frequency" ranges that could be removed without diminishing head-shadow benefit are interpreted in terms of low importance in the speech intelligibility index and a small head-shadow magnitude at low frequencies. Although groups and individuals with greater performance asymmetry gained less head-shadow benefit, the magnitudes of these factors did not predict the exclusion frequency range. Overall, these data suggest that for many SSD-CI and BI-CI listeners, the frequency allocation for the poorer-ear CI can be shifted substantially without sacrificing head-shadow benefit, at least for energetic maskers. Considering the two ears together as a single system may allow greater flexibility in discarding redundant frequency content from a CI in one ear when considering bilateral programming solutions aimed at reducing interaural frequency mismatch.

Recovery from forward masking in cochlear implant listeners: Effects of age and the electrode-neuron interface

Older adults exhibit deficits in auditory temporal processing relative to younger listeners. These age-related temporal processing difficulties may be further exacerbated in older adults with cochlear implant (CIs) when CI electrodes poorly interface with their target auditory neurons. The aim of this study was to evaluate the potential interaction between chronological age and the estimated quality of the electrode-neuron interface (ENI) on psychophysical forward masking recovery, a measure that reflects single-channel temporal processing abilities. Fourteen CI listeners (age 15 to 88 years) with Advanced Bionics devices participated. Forward masking recovery was assessed on two channels in each ear (i.e., the channels with the lowest and highest signal detection thresholds). Results indicated that the rate of forward masking recovery declined with advancing age, and that the effect of age was more pronounced on channels estimated to interface poorly with the auditory nerve. These findings indicate that the quality of the ENI can influence the time course of forward masking recovery for older CI listeners. Channel-to-channel variability in the ENI likely interacts with central temporal processing deficits secondary to auditory aging, warranting further study of programming and rehabilitative approaches tailored to older listeners.

Recognition of vocoded words and sentences in quiet and multi-talker babble with children and adults

A vocoder is used to simulate cochlear-implant sound processing in normal-hearing listeners. Typically, there is rapid improvement in vocoded speech recognition, but it is unclear if the improvement rate differs across age groups and speech materials. Children (8–10 years) and young adults (18–26 years) were trained and tested over 2 days (4 hours) on recognition of eight-channel noise-vocoded words and sentences, in quiet and in the presence of multi-talker babble at signal-to-noise ratios of 0, +5, and +10 dB. Children achieved poorer performance than adults in all conditions, for both word and sentence recognition. With training, vocoded speech recognition improvement rates were not significantly different between children and adults, suggesting that improvement in learning how to process speech cues degraded via vocoding is absent of developmental differences across these age groups and types of speech materials. Furthermore, this result confirms that the acutely measured age difference in vocoded speech recognition persists after extended training.

Age-Related Compensation Mechanism Revealed in the Cortical Representation of Degraded Speech

Older adults understand speech with comparative ease in quiet, but signal degradation can hinder speech understanding much more than it does in younger adults. This difficulty may result, in part, from temporal processing deficits related to the aging process and/or high-frequency hearing loss that can occur in listeners who have normal- or near-normal-hearing thresholds in the speech frequency range. Temporal processing deficits may manifest as degraded neural representation in peripheral and brainstem/midbrain structures that lead to compensation, or changes in response strength in auditory cortex. Little is understood about the process by which the neural representation of signals is improved or restored by age-related cortical compensation mechanisms. Therefore, we used vocoding to simulate spectral degradation to compare the behavioral and neural representation of words that contrast on a temporal dimension. Specifically, we used the closure duration of the silent interval between the vowel and the final affricate /t∫/ or fricative /ʃ/ of the words DITCH and DISH, respectively. We obtained perceptual identification functions and electrophysiological neural measures (frequency-following responses (FFR) and cortical auditory-evoked potentials (CAEPs)) to unprocessed and vocoded versions of these words in young normal-hearing (YNH), older normal- or near-normal-hearing (ONH), and older hearing-impaired (OHI) listeners. We found that vocoding significantly reduced the slope of the perceptual identification function in only the OHI listeners. In contrast to the limited effects of vocoding on perceptual performance, vocoding had robust effects on the FFRs across age groups, such that stimulus-to-response correlations and envelope magnitudes were significantly lower for vocoded vs. unprocessed conditions. Increases in the P1 peak amplitude for vocoded stimuli were found for both ONH and OHI listeners, but not for the YNH listeners. These results suggest that while vocoding substantially degrades early neural representation of speech stimuli in the midbrain, there may be cortical compensation in older listeners that is not seen in younger listeners.

Spectral-Temporal Trade-Off in Vocoded Sentence Recognition: Effects of Age, Hearing Thresholds, and Working Memory

OBJECTIVES

Cochlear implant (CI) signal processing degrades the spectral components of speech. This requires CI users to rely primarily on temporal cues, specifically, amplitude modulations within the temporal envelope, to recognize speech. Auditory temporal processing ability for envelope modulations worsens with advancing age, which may put older CI users at a disadvantage compared with younger users. To evaluate how potential age-related limitations for processing temporal envelope modulations impact spectrally degraded sentence recognition, noise-vocoded sentences were presented to younger and older normal-hearing listeners in quiet. Envelope modulation rates were varied from 10 to 500 Hz by adjusting the low-pass filter cutoff frequency (LPF). The goal of this study was to evaluate if age impacts recognition of noise-vocoded speech and if this age-related limitation existed for a specific range of envelope modulation rates.

DESIGN

Noise-vocoded sentence recognition in quiet was measured as a function of number of spectral channels (4, 6, 8, and 12 channels) and LPF (10, 20, 50, 75, 150, 375, and 500 Hz) in 15 younger normal-hearing listeners and 15 older near-normal-hearing listeners. Hearing thresholds and working memory were assessed to determine the extent to which these factors were related to recognition of noise-vocoded sentences.

RESULTS

Younger listeners achieved significantly higher sentence recognition scores than older listeners overall. Performance improved in both groups as the number of spectral channels and LPF increased. As the number of spectral channels increased, the differences in sentence recognition scores between groups decreased. A spectral-temporal trade-off was observed in both groups in which performance in the 8- and 12-channel conditions plateaued with lower-frequency amplitude modulations compared with the 4- and 6-channel conditions. There was no interaction between age group and LPF, suggesting that both groups obtained similar improvements in performance with increasing LPF. The lack of an interaction between age and LPF may be due to the nature of the task of recognizing sentences in quiet. Audiometric thresholds were the only significant predictor of vocoded sentence recognition. Although performance on the working memory task declined with advancing age, working memory scores did not predict sentence recognition.

CONCLUSIONS

Younger listeners outperformed older listeners for recognizing noise-vocoded sentences in quiet. The negative impact of age was reduced when ample spectral information was available. Age-related limitations for recognizing vocoded sentences were not affected by the temporal envelope modulation rate of the signal, but instead, appear to be related to a generalized task limitation or to reduced audibility of the signal.

The effects of distractor set-size on neural tracking of attended speech

Attention is crucial to speech comprehension in real-world, noisy environments. Selective phase-tracking between low-frequency brain dynamics and the envelope of target speech is a proposed mechanism to reject competing distractors. Studies have supported this theory in the case of a single distractor, but have not considered how tracking is systematically affected by varying distractor set sizes. We recorded electroencephalography (EEG) during selective listening to both natural and vocoded speech as distractor set-size varied from two to six voices. Increasing set-size reduced performance and attenuated EEG tracking of target speech. Further, we found that intrusions of distractor speech into perception were not accompanied by sustained tracking of the distractor stream. Our results support the theory that tracking of speech dynamics is a mechanism for selective attention, and that the mechanism of distraction is not simple stimulus-driven capture of sustained entrainment of auditory mechanisms by the acoustics of distracting speech.

How does aging affect recognition of spectrally degraded speech?

OBJECTIVES/HYPOTHESIS

Cochlear implants (CIs) restore auditory sensation to patients with moderate-to-profound sensorineural hearing loss. However, the benefits to speech recognition vary considerably among patients. Advancing age contributes to this variability in postlingual adult CI users. Similarly, older individuals with normal hearing (NH) perform more poorly on tasks of recognition of spectrally degraded speech. The overarching hypothesis of this study was that the detrimental effects of advancing age on speech recognition can be attributed both to declines in auditory spectral resolution as well as declines in cognitive functions.

STUDY DESIGN

Case-control study.

METHODS

Speech recognition was assessed in CI users (in the clear) and NH controls (spectrally degraded using noise-vocoding), along with auditory spectral resolution using the Spectral-Temporally Modulated Ripple Test. Cognitive skills were assessed using nonauditory visual measures of working memory, inhibitory control, speed of lexical/phonological access, nonverbal reasoning, and perceptual closure. Linear regression models were tested for mediation to explain aging effects on speech recognition performance.

RESULTS

For both groups, older age predicted poorer sentence and word recognition. The detrimental effects of advancing age on speech recognition were partially mediated by declines in spectral resolution and in some measures of cognitive function.

CONCLUSIONS

Advancing age contributes to poorer recognition of degraded speech for CI users and NH controls through declines in both auditory spectral resolution and cognitive functions. Findings suggest that improvements in spectral resolution as well as cognitive improvements may serve as therapeutic targets to optimize CI speech recognition outcomes.

LEVEL OF EVIDENCE

3b Laryngoscope, 2018.

Age-Related Differences in the Processing of Temporal Envelope and Spectral Cues in a Speech Segment

OBJECTIVES

As people age, they experience reduced temporal processing abilities. This results in poorer ability to understand speech, particularly for degraded input signals. Cochlear implants (CIs) convey speech information via the temporal envelopes of a spectrally degraded input signal. Because there is an increasing number of older CI users, there is a need to understand how temporal processing changes with age. Therefore, the goal of this study was to quantify age-related reduction in temporal processing abilities when attempting to discriminate words based on temporal envelope information from spectrally degraded signals.

DESIGN

Younger normal-hearing (YNH) and older normal-hearing (ONH) participants were presented a continuum of speech tokens that varied in silence duration between phonemes (0 to 60 ms in 10-ms steps), and were asked to identify whether the stimulus was perceived more as the word "dish" or "ditch." Stimuli were vocoded using tonal carriers. The number of channels (1, 2, 4, 8, 16, and unprocessed) and temporal envelope low-pass filter cutoff frequency (50 and 400 Hz) were systematically varied.

RESULTS

For the unprocessed conditions, the YNH participants perceived the word ditch for smaller silence durations than the ONH participants, indicating that aging affects temporal processing abilities. There was no difference in performance between the unprocessed and 16-channel, 400-Hz vocoded stimuli. Decreasing the number of spectral channels caused decreased ability to distinguish dish and ditch. Decreasing the envelope cutoff frequency also caused decreased ability to distinguish dish and ditch. The overall pattern of results revealed that reductions in spectral and temporal information had a relatively larger effect on the ONH participants compared with the YNH participants.

CONCLUSIONS

Aging reduces the ability to utilize brief temporal cues in speech segments. Reducing spectral information-as occurs in a channel vocoder and in CI speech processing strategies-forces participants to use temporal envelope information; however, older participants are less capable of utilizing this information. These results suggest that providing as much spectral and temporal speech information as possible would benefit older CI users relatively more than younger CI users. In addition, the present findings help set expectations of clinical outcomes for speech understanding performance by adult CI users as a function of age.

Robust Neuronal Discrimination in Primary Auditory Cortex Despite Degradations of Spectro-temporal Acoustic Details: Comparison Between Guinea Pigs with Normal Hearing and Mild Age-Related Hearing Loss

This study investigated to which extent the primary auditory cortex of young normal-hearing and mild hearing-impaired aged animals is able to maintain invariant representation of critical temporal-modulation features when sounds are submitted to degradations of fine spectro-temporal acoustic details. This was achieved by recording ensemble of cortical responses to conspecific vocalizations in guinea pigs with either normal hearing or mild age-related sensorineural hearing loss. The vocalizations were degraded using a tone vocoder. The neuronal responses and their discrimination capacities (estimated by mutual information) were analyzed at single recording and population levels. For normal-hearing animals, the neuronal responses decreased as a function of the number of the vocoder frequency bands, so did their discriminative capacities at the single recording level. However, small neuronal populations were found to be robust to the degradations induced by the vocoder. Similar robustness was obtained when broadband noise was added to exacerbate further the spectro-temporal distortions produced by the vocoder. A comparable pattern of robustness to degradations in fine spectro-temporal details was found for hearing-impaired animals. However, the latter showed an overall decrease in neuronal discrimination capacities between vocalizations in noisy conditions. Consistent with previous studies, these results demonstrate that the primary auditory cortex maintains robust neural representation of temporal envelope features for communication sounds under a large range of spectro-temporal degradations.

Age-Related Differences in Listening Effort During Degraded Speech Recognition

OBJECTIVES

The purpose of the present study was to quantify age-related differences in executive control as it relates to dual-task performance, which is thought to represent listening effort, during degraded speech recognition.

DESIGN

Twenty-five younger adults (YA; 18-24 years) and 21 older adults (OA; 56-82 years) completed a dual-task paradigm that consisted of a primary speech recognition task and a secondary visual monitoring task. Sentence material in the primary task was either unprocessed or spectrally degraded into 8, 6, or 4 spectral channels using noise-band vocoding. Performance on the visual monitoring task was assessed by the accuracy and reaction time of participants' responses. Performance on the primary and secondary task was quantified in isolation (i.e., single task) and during the dual-task paradigm. Participants also completed a standardized psychometric measure of executive control, including attention and inhibition. Statistical analyses were implemented to evaluate changes in listeners' performance on the primary and secondary tasks (1) per condition (unprocessed vs. vocoded conditions); (2) per task (single task vs. dual task); and (3) per group (YA vs. OA).

RESULTS

Speech recognition declined with increasing spectral degradation for both YA and OA when they performed the task in isolation or concurrently with the visual monitoring task. OA were slower and less accurate than YA on the visual monitoring task when performed in isolation, which paralleled age-related differences in standardized scores of executive control. When compared with single-task performance, OA experienced greater declines in secondary-task accuracy, but not reaction time, than YA. Furthermore, results revealed that age-related differences in executive control significantly contributed to age-related differences on the visual monitoring task during the dual-task paradigm.

CONCLUSIONS

OA experienced significantly greater declines in secondary-task accuracy during degraded speech recognition than YA. These findings are interpreted as suggesting that OA expended greater listening effort than YA, which may be partially attributed to age-related differences in executive control.

Older and younger adults' identification of sentences filtered with amplitude and frequency modulations in quiet and noise

Adding frequency modulations (FM) cues to vocoded (AM) speech aids speech recognition for younger listeners. However, this may not be true for older listeners since they have poorer FM detection thresholds. We measured FM detection thresholds of young and older adults; and in a sentence context examined whether adding FM cues to vocoded speech would assist older adults. Young and old participants were presented vocoded sentences in quiet and multitalker-babble with/without FM cues. Older adults had elevated FM detection thresholds but received the same-size FM benefit as younger adults, showing that they have the capacity to benefit from FM speech cues.

Speech Rate Normalization and Phonemic Boundary Perception in Cochlear-Implant Users

PURPOSE

Normal-hearing (NH) listeners rate normalize, temporarily remapping phonemic category boundaries to account for a talker's speech rate. It is unknown if adults who use auditory prostheses called cochlear implants (CI) can rate normalize, as CIs transmit degraded speech signals to the auditory nerve. Ineffective adjustment to rate information could explain some of the variability in this population's speech perception outcomes.

METHOD

Phonemes with manipulated voice-onset-time (VOT) durations were embedded in sentences with different speech rates. Twenty-three CI and 29 NH participants performed a phoneme identification task. NH participants heard the same unprocessed stimuli as the CI participants or stimuli degraded by a sine vocoder, simulating aspects of CI processing.

RESULTS

CI participants showed larger rate normalization effects (6.6 ms) than the NH participants (3.7 ms) and had shallower (less reliable) category boundary slopes. NH participants showed similarly shallow slopes when presented acoustically degraded vocoded signals, but an equal or smaller rate effect in response to reductions in available spectral and temporal information.

CONCLUSION

CI participants can rate normalize, despite their degraded speech input, and show a larger rate effect compared to NH participants. CI participants may particularly rely on rate normalization to better maintain perceptual constancy of the speech signal.

An acoustic key to eight languages/dialects: Factor analyses of critical-band-filtered speech

The peripheral auditory system functions like a frequency analyser, often modelled as a bank of non-overlapping band-pass filters called critical bands; 20 bands are necessary for simulating frequency resolution of the ear within an ordinary frequency range of speech (up to 7,000 Hz). A far smaller number of filters seemed sufficient, however, to re-synthesise intelligible speech sentences with power fluctuations of the speech signals passing through them; nevertheless, the number and frequency ranges of the frequency bands for efficient speech communication are yet unknown. We derived four common frequency bands—covering approximately 50–540, 540–1,700, 1,700–3,300, and above 3,300 Hz—from factor analyses of spectral fluctuations in eight different spoken languages/dialects. The analyses robustly led to three factors common to all languages investigated—the low & mid-high factor related to the two separate frequency ranges of 50–540 and 1,700–3,300 Hz, the mid-low factor the range of 540–1,700 Hz, and the high factor the range above 3,300 Hz—in these different languages/dialects, suggesting a language universal.

Effects of Age on Long Term Memory for Degraded Speech

Prior research suggests that acoustical degradation impacts encoding of items into memory, especially in elderly subjects. We here aimed to investigate whether acoustically degraded items that are initially encoded into memory are more prone to forgetting as a function of age. Young and old participants were tested with a vocoded and unvocoded serial list learning task involving immediate and delayed free recall. We found that degraded auditory input increased forgetting of previously encoded items, especially in older participants. We further found that working memory capacity predicted forgetting of degraded information in young participants. In old participants, verbal IQ was the most important predictor for forgetting acoustically degraded information. Our data provide evidence that acoustically degraded information, even if encoded, is especially vulnerable to forgetting in old age.

Lexical Influences on Spoken Spondaic Word Recognition in Hearing-Impaired Patients

Top-down contextual influences play a major part in speech understanding, especially in hearing-impaired patients with deteriorated auditory input. Those influences are most obvious in difficult listening situations, such as listening to sentences in noise but can also be observed at the word level under more favorable conditions, as in one of the most commonly used tasks in audiology, i.e., repeating isolated words in silence. This study aimed to explore the role of top-down contextual influences and their dependence on lexical factors and patient-specific factors using standard clinical linguistic material. Spondaic word perception was tested in 160 hearing-impaired patients aged 23-88 years with a four-frequency average pure-tone threshold ranging from 21 to 88 dB HL. Sixty spondaic words were randomly presented at a level adjusted to correspond to a speech perception score ranging between 40 and 70% of the performance intensity function obtained using monosyllabic words. Phoneme and whole-word recognition scores were used to calculate two context-influence indices (the j factor and the ratio of word scores to phonemic scores) and were correlated with linguistic factors, such as the phonological neighborhood density and several indices of word occurrence frequencies. Contextual influence was greater for spondaic words than in similar studies using monosyllabic words, with an overall j factor of 2.07 (SD = 0.5). For both indices, context use decreased with increasing hearing loss once the average hearing loss exceeded 55 dB HL. In right-handed patients, significantly greater context influence was observed for words presented in the right ears than for words presented in the left, especially in patients with many years of education. The correlations between raw word scores (and context influence indices) and word occurrence frequencies showed a significant age-dependent effect, with a stronger correlation between perception scores and word occurrence frequencies when the occurrence frequencies were based on the years corresponding to the patients' youth, showing a "historic" word frequency effect. This effect was still observed for patients with few years of formal education, but recent occurrence frequencies based on current word exposure had a stronger influence for those patients, especially for younger ones.

Weighting of Acoustic Cues to a Manner Distinction by Children With and Without Hearing Loss

PURPOSE

Children must develop optimal perceptual weighting strategies for processing speech in their first language. Hearing loss can interfere with that development, especially if cochlear implants are required. The three goals of this study were to measure, for children with and without hearing loss: (a) cue weighting for a manner distinction, (b) sensitivity to those cues, and (c) real-world communication functions.

METHOD

One hundred and seven children (43 with normal hearing [NH], 17 with hearing aids [HAs], and 47 with cochlear implants [CIs]) performed several tasks: labeling of stimuli from /bɑ/-to-/wɑ/ continua varying in formant and amplitude rise time (FRT and ART), discrimination of ART, word recognition, and phonemic awareness.

RESULTS

Children with hearing loss were less attentive overall to acoustic structure than children with NH. Children with CIs, but not those with HAs, weighted FRT less and ART more than children with NH. Sensitivity could not explain cue weighting. FRT cue weighting explained significant amounts of variability in word recognition and phonemic awareness; ART cue weighting did not.

CONCLUSION

Signal degradation inhibits access to spectral structure for children with CIs, but cannot explain their delayed development of optimal weighting strategies. Auditory training could strengthen the weighting of spectral cues for children with CIs, thus aiding spoken language acquisition.

Factors limiting vocal-tract length discrimination in cochlear implant simulations

Perception of voice characteristics allows normal hearing listeners to identify the gender of a speaker, and to better segregate speakers from each other in cocktail party situations. This benefit is largely driven by the perception of two vocal characteristics of the speaker: The fundamental frequency (F0) and the vocal-tract length (VTL). Previous studies have suggested that cochlear implant (CI) users have difficulties in perceiving these cues. The aim of the present study was to investigate possible causes for limited sensitivity to VTL differences in CI users. Different acoustic simulations of CI stimulation were implemented to characterize the role of spectral resolution on VTL, both in terms of number of channels and amount of channel interaction. The results indicate that with 12 channels, channel interaction caused by current spread is likely to prevent CI users from perceiving VTL differences typically found between male and female speakers.

Age-group differences in speech identification despite matched audiometrically normal hearing: contributions from auditory temporal processing and cognition

Hearing loss with increasing age adversely affects the ability to understand speech, an effect that results partly from reduced audibility. The aims of this study were to establish whether aging reduces speech intelligibility for listeners with normal audiograms, and, if so, to assess the relative contributions of auditory temporal and cognitive processing. Twenty-one older normal-hearing (ONH; 60-79 years) participants with bilateral audiometric thresholds ≤ 20 dB HL at 0.125-6 kHz were matched to nine young (YNH; 18-27 years) participants in terms of mean audiograms, years of education, and performance IQ. Measures included: (1) identification of consonants in quiet and in noise that was unmodulated or modulated at 5 or 80 Hz; (2) identification of sentences in quiet and in co-located or spatially separated two-talker babble; (3) detection of modulation of the temporal envelope (TE) at frequencies 5-180 Hz; (4) monaural and binaural sensitivity to temporal fine structure (TFS); (5) various cognitive tests. Speech identification was worse for ONH than YNH participants in all types of background. This deficit was not reflected in self-ratings of hearing ability. Modulation masking release (the improvement in speech identification obtained by amplitude modulating a noise background) and spatial masking release (the benefit obtained from spatially separating masker and target speech) were not affected by age. Sensitivity to TE and TFS was lower for ONH than YNH participants, and was correlated positively with speech-in-noise (SiN) identification. Many cognitive abilities were lower for ONH than YNH participants, and generally were correlated positively with SiN identification scores. The best predictors of the intelligibility of SiN were composite measures of cognition and TFS sensitivity. These results suggest that declines in speech perception in older persons are partly caused by cognitive and perceptual changes separate from age-related changes in audiometric sensitivity.

The effects of selective attention and speech acoustics on neural speech-tracking in a multi-talker scene

Attending to one speaker in multi-speaker situations is challenging. One neural mechanism proposed to underlie the ability to attend to a particular speaker is phase-locking of low-frequency activity in auditory cortex to speech's temporal envelope ("speech-tracking"), which is more precise for attended speech. However, it is not known what brings about this attentional effect, and specifically if it reflects enhanced processing of the fine structure of attended speech. To investigate this question we compared attentional effects on speech-tracking of natural versus vocoded speech which preserves the temporal envelope but removes the fine structure of speech. Pairs of natural and vocoded speech stimuli were presented concurrently and participants attended to one stimulus and performed a detection task while ignoring the other stimulus. We recorded magnetoencephalography (MEG) and compared attentional effects on the speech-tracking response in auditory cortex. Speech-tracking of natural, but not vocoded, speech was enhanced by attention, whereas neural tracking of ignored speech was similar for natural and vocoded speech. These findings suggest that the more precise speech-tracking of attended natural speech is related to processing its fine structure, possibly reflecting the application of higher-order linguistic processes. In contrast, when speech is unattended its fine structure is not processed to the same degree and thus elicits less precise speech-tracking more similar to vocoded speech.

Using auditory pre-information to solve the cocktail-party problem: electrophysiological evidence for age-specific differences

Speech understanding in complex and dynamic listening environments requires (a) auditory scene analysis, namely auditory object formation and segregation, and (b) allocation of the attentional focus to the talker of interest. There is evidence that pre-information is actively used to facilitate these two aspects of the so-called “cocktail-party” problem. Here, a simulated multi-talker scenario was combined with electroencephalography to study scene analysis and allocation of attention in young and middle-aged adults. Sequences of short words (combinations of brief company names and stock-price values) from four talkers at different locations were simultaneously presented, and the detection of target names and the discrimination between critical target values were assessed. Immediately prior to speech sequences, auditory pre-information was provided via cues that either prepared auditory scene analysis or attentional focusing, or non-specific pre-information was given. While performance was generally better in younger than older participants, both age groups benefited from auditory pre-information. The analysis of the cue-related event-related potentials revealed age-specific differences in the use of pre-cues: Younger adults showed a pronounced N2 component, suggesting early inhibition of concurrent speech stimuli; older adults exhibited a stronger late P3 component, suggesting increased resource allocation to process the pre-information. In sum, the results argue for an age-specific utilization of auditory pre-information to improve listening in complex dynamic auditory environments.

Relationship between perceptual learning in speech and statistical learning in younger and older adults

Within a few sentences, listeners learn to understand severely degraded speech such as noise-vocoded speech. However, individuals vary in the amount of such perceptual learning and it is unclear what underlies these differences. The present study investigates whether perceptual learning in speech relates to statistical learning, as sensitivity to probabilistic information may aid identification of relevant cues in novel speech input. If statistical learning and perceptual learning (partly) draw on the same general mechanisms, then statistical learning in a non-auditory modality using non-linguistic sequences should predict adaptation to degraded speech. In the present study, 73 older adults (aged over 60 years) and 60 younger adults (aged between 18 and 30 years) performed a visual artificial grammar learning task and were presented with 60 meaningful noise-vocoded sentences in an auditory recall task. Within age groups, sentence recognition performance over exposure was analyzed as a function of statistical learning performance, and other variables that may predict learning (i.e., hearing, vocabulary, attention switching control, working memory, and processing speed). Younger and older adults showed similar amounts of perceptual learning, but only younger adults showed significant statistical learning. In older adults, improvement in understanding noise-vocoded speech was constrained by age. In younger adults, amount of adaptation was associated with lexical knowledge and with statistical learning ability. Thus, individual differences in general cognitive abilities explain listeners' variability in adapting to noise-vocoded speech. Results suggest that perceptual and statistical learning share mechanisms of implicit regularity detection, but that the ability to detect statistical regularities is impaired in older adults if visual sequences are presented quickly.

Top-down influences of written text on perceived clarity of degraded speech

An unresolved question is how the reported clarity of degraded speech is enhanced when listeners have prior knowledge of speech content. One account of this phenomenon proposes top-down modulation of early acoustic processing by higher-level linguistic knowledge. Alternative, strictly bottom-up accounts argue that acoustic information and higher-level knowledge are combined at a late decision stage without modulating early acoustic processing. Here we tested top-down and bottom-up accounts using written text to manipulate listeners’ knowledge of speech content. The effect of written text on the reported clarity of noise-vocoded speech was most pronounced when text was presented before (rather than after) speech (Experiment 1). Fine-grained manipulation of the onset asynchrony between text and speech revealed that this effect declined when text was presented more than 120 ms after speech onset (Experiment 2). Finally, the influence of written text was found to arise from phonological (rather than lexical) correspondence between text and speech (Experiment 3). These results suggest that prior knowledge effects are time-limited by the duration of auditory echoic memory for degraded speech, consistent with top-down modulation of early acoustic processing by linguistic knowledge.

Toddlers' recognition of noise-vocoded speech

Despite their remarkable clinical success, cochlear-implant listeners today still receive spectrally degraded information. Much research has examined normally hearing adult listeners' ability to interpret spectrally degraded signals, primarily using noise-vocoded speech to simulate cochlear implant processing. Far less research has explored infants' and toddlers' ability to interpret spectrally degraded signals, despite the fact that children in this age range are frequently implanted. This study examines 27-month-old typically developing toddlers' recognition of noise-vocoded speech in a language-guided looking study. Children saw two images on each trial and heard a voice instructing them to look at one item ("Find the cat!"). Full-spectrum sentences or their noise-vocoded versions were presented with varying numbers of spectral channels. Toddlers showed equivalent proportions of looking to the target object with full-speech and 24- or 8-channel noise-vocoded speech; they failed to look appropriately with 2-channel noise-vocoded speech and showed variable performance with 4-channel noise-vocoded speech. Despite accurate looking performance for speech with at least eight channels, children were slower to respond appropriately as the number of channels decreased. These results indicate that 2-yr-olds have developed the ability to interpret vocoded speech, even without practice, but that doing so requires additional processing. These findings have important implications for pediatric cochlear implantation.

Gender identification in younger and older adults: use of spectral and temporal cues in noise-vocoded speech

OBJECTIVE

The aim of this study was to investigate potential effects of age on the ability of normal-hearing (NH) adult listeners to utilize spectral and temporal cues when performing a voice gender identification task.

DESIGN

Ten younger and 10 older NH adult listeners were measured on their ability to correctly identify the speaker gender of six different vowel tokens (H-/vowel/-D) when spoken by eight speakers (four male and four female). Spectral (number of channels) and temporal cues (low-pass cut-off frequency for temporal envelope extraction) were systematically manipulated using noiseband vocoding techniques; stimuli contained 1, 4, 8, 16, or 32 spectral channels, while the low-pass cut-off frequency of the temporal envelope filter was 20, 50, 100, 200, or 400 Hz. Furthermore, the fundamental frequencies (F0s) of the vowel tokens were manipulated to create two conditions: "Expanded" (large range of F0 values) and "Compressed" (small range of F0 values).

RESULTS

In general, younger listeners performed better than the older listeners but only when stimuli were spectrally degraded. For both the Expanded and Compressed conditions, the overall performance of the younger listeners was better than that of the older listeners, suggesting age-related deficits in both spectral and temporal processing. Furthermore, a significant interaction between age group and temporal envelope cues revealed that older listeners received less benefit from increasing temporal envelope information compared with the benefit observed among younger listeners. In particular, the performance of the younger NH group (collapsed across number of channels), but not the older NH group, improved as the temporal envelope cut-off frequency was increased from 50 to 400 Hz.

CONCLUSIONS

The results reported here support previous findings of senescent declines in perceiving spectrally reduced speech and temporal amplitude modulation processing. These results suggest that when F0 values are similar to one another, younger listeners can use temporal cues alone to glean voice-pitch information but older listeners exhibit a lessened ability to use such cues. Previous studies have demonstrated the importance of temporal envelope cues in periodicity perception (e.g., gender recognition) by cochlear implant listeners. The results of this study suggest that aging affects the use of such cues, and consequently gender recognition might be poorer among older cochlear implant recipients.

Level-dependent changes in perception of speech envelope cues

Level-dependent changes in temporal envelope fluctuations in speech and related changes in speech recognition may reveal effects of basilar-membrane nonlinearities. As a result of compression in the basilar-membrane response, the "effective" magnitude of envelope fluctuations may be reduced as speech level increases from lower level (more linear) to mid-level (more compressive) regions. With further increases to a more linear region, speech envelope fluctuations may become more pronounced. To assess these effects, recognition of consonants and key words in sentences was measured as a function of speech level for younger adults with normal hearing. Consonant-vowel syllables and sentences were spectrally degraded using "noise vocoder" processing to maximize perceptual effects of changes to the speech envelope. Broadband noise at a fixed signal-to-noise ratio maintained constant audibility as speech level increased. Results revealed significant increases in scores and envelope-dependent feature transmission from 45 to 60 dB SPL and decreasing scores and feature transmission from 60 to 85 dB SPL. This quadratic pattern, with speech recognition maximized at mid levels and poorer at lower and higher levels, is consistent with a role of cochlear nonlinearities in perception of speech envelope cues.

Acoustic cue integration in speech intonation recognition with cochlear implants

The present article reports on the perceptual weighting of prosodic cues in question-statement identification by adult cochlear implant (CI) listeners. Acoustic analyses of normal-hearing (NH) listeners' production of sentences spoken as questions or statements confirmed that in English the last bisyllabic word in a sentence carries the dominant cues (F0, duration, and intensity patterns) for the contrast. Furthermore, these analyses showed that the F0 contour is the primary cue for the question-statement contrast, with intensity and duration changes conveying important but less reliable information. On the basis of these acoustic findings, the authors examined adult CI listeners' performance in two question-statement identification tasks. In Task 1, 13 CI listeners' question-statement identification accuracy was measured using naturally uttered sentences matched for their syntactic structures. In Task 2, the same listeners' perceptual cue weighting in question-statement identification was assessed using resynthesized single-word stimuli, within which fundamental frequency (F0), intensity, and duration properties were systematically manipulated. Both tasks were also conducted with four NH listeners with full-spectrum and noise-band-vocoded stimuli. Perceptual cue weighting was assessed by comparing the estimated coefficients in logistic models fitted to the data. Of the 13 CI listeners, 7 achieved high performance levels in Task 1. The results of Task 2 indicated that multiple sources of acoustic cues for question-statement identification were utilized to different extents depending on the listening conditions (e.g., full spectrum vs. spectrally degraded) or the listeners' hearing and amplification status (e.g., CI vs. NH).

Spatial selective auditory attention in the presence of reverberant energy: individual differences in normal-hearing listeners

Listeners can selectively attend to a desired target by directing attention to known target source features, such as location or pitch. Reverberation, however, reduces the reliability of the cues that allow a target source to be segregated and selected from a sound mixture. Given this, it is likely that reverberant energy interferes with selective auditory attention. Anecdotal reports suggest that the ability to focus spatial auditory attention degrades even with early aging, yet there is little evidence that middle-aged listeners have behavioral deficits on tasks requiring selective auditory attention. The current study was designed to look for individual differences in selective attention ability and to see if any such differences correlate with age. Normal-hearing adults, ranging in age from 18 to 55 years, were asked to report a stream of digits located directly ahead in a simulated rectangular room. Simultaneous, competing masker digit streams were simulated at locations 15° left and right of center. The level of reverberation was varied to alter task difficulty by interfering with localization cues (increasing localization blur). Overall, performance was best in the anechoic condition and worst in the high-reverberation condition. Listeners nearly always reported a digit from one of the three competing streams, showing that reverberation did not render the digits unintelligible. Importantly, inter-subject differences were extremely large. These differences, however, were not significantly correlated with age, memory span, or hearing status. These results show that listeners with audiometrically normal pure tone thresholds differ in their ability to selectively attend to a desired source, a task important in everyday communication. Further work is necessary to determine if these differences arise from differences in peripheral auditory function or in more central function.

Lists of Spanish Sentences with Equivalent Predictability, Phonetic Content, Length, and Frequency of the Last Word

This paper presents a pool of Spanish sentences designed for use in cognitive research and speech processing in circumstances in which the effects of context are relevant. These lists of sentences are divided into six lists of 25 equivalent high-predictability sentences and six lists of 25 low-predictability sentences according to the extent to which the last word can be predicted by the preceding context. These lists were also equivalent in phonetic content, length and frequency of the last word. These lists are intended for use in psycholinguistic research with Spanish-speaking listeners.

Recognition of spectrally degraded phonemes by younger, middle-aged, and older normal-hearing listeners

The effects of spectral degradation on vowel and consonant recognition abilities were measured in young, middle-aged, and older normal-hearing (NH) listeners. Noise-band vocoding techniques were used to manipulate the number of spectral channels and frequency-to-place alignment, thereby simulating cochlear implant (CI) processing. A brief cognitive test battery was also administered. The performance of younger NH listeners exceeded that of the middle-aged and older listeners, when stimuli were severely distorted (spectrally shifted); the older listeners performed only slightly worse than the middle-aged listeners. Significant intragroup variability was present in the middle-aged and older groups. A hierarchical multiple-regression analysis including data from all three age groups suggested that age was the primary factor related to shifted vowel recognition performance, but verbal memory abilities also contributed significantly to performance. A second regression analysis (within the middle-aged and older groups alone) revealed that verbal memory and speed of processing abilities were better predictors of performance than age alone. The overall results from the current investigation suggested that both chronological age and cognitive capacities contributed to the ability to recognize spectrally degraded phonemes. Such findings have important implications for the counseling and rehabilitation of adult CI recipients.