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Intelligibility of temporally degraded speech was investigated with locally time-reversed speech (LTR) and its interrupted version (ILTR). Control stimuli comprising interrupted speech (I) were also included. Speech stimuli consisted of 200 Japanese meaningful sentences. In interrupted stimuli, speech segments were alternated with either silent gaps or pink noise bursts. The noise bursts had a level of − 10, 0 or + 10 dB relative to the speech level. Segment duration varied from 20 to 160 ms for ILTR sentences, but was fixed at 160 ms for I sentences. At segment durations between 40 and 80 ms, severe reductions in intelligibility were observed for ILTR sentences, compared with LTR sentences. A substantial improvement in intelligibility (30–33%) was observed when 40-ms silent gaps in ILTR were replaced with 0- and + 10-dB noise. Noise with a level of − 10 dB had no effect on the intelligibility. These findings show that the combined effects of interruptions and temporal reversal of speech segments on intelligibility are greater than the sum of each individual effect. The results also support the idea that illusory continuity induced by high-level noise bursts improves the intelligibility of ILTR and I sentences. Supplementary Information The online version contains supplementary material available at 10.3758/s13414-021-02292-3.

In noisy listening conditions, intelligibility of degraded speech can be enhanced by top-down restoration. Cochlear implant (CI) users have difficulty understanding speech in noisy environments. This could partially be due to reduced top-down restoration of speech, which may be related to the changes that the electrical stimulation imposes on the bottom-up cues. We tested this hypothesis using the phonemic restoration (PhR) paradigm in which speech interrupted with periodic silent intervals is perceived illusorily continuous (continuity illusion or Col) and becomes more intelligible (PhR benefit) when the interruptions are filled with noise bursts. Using meaningful sentences, both Col and PhR benefit were measured in CI users, and compared with those of normal-hearing (NH) listeners presented with normal speech and 8-channel noise-band vocoded speech, acoustically simulating CIs. CI users showed different patterns in both PhR benefit and Col, compared to NH results with or without the noise-band vocoding. However, they were able to use top-down restoration under certain test conditions. This observation supports the idea that changes in bottom-up cues can impose changes to the top down processes needed to enhance intelligibility of degraded speech. The knowledge that CI users seem to be able to do restoration under the right circumstances could be exploited in patient rehabilitation and product development. (C) 2013 Elsevier B.V. All rights reserved.

External degradations in incoming speech reduce understanding, and hearing impairment further compounds the problem. While cognitive mechanisms alleviate some of the difficulties, their effectiveness may change with age. In our research, reviewed here, we investigated cognitive compensation with hearing impairment, cochlear implants, and aging, via (a) phonemic restoration as a measure of top-down filling of missing speech, (b) listening effort and response times as a measure of increased cognitive processing, and (c) visual world paradigm and eye gazing as a measure of the use of context and its time course. Our results indicate that between speech degradations and their cognitive compensation, there is a fine balance that seems to vary greatly across individuals. Hearing impairment or inadequate hearing device settings may limit compensation benefits. Cochlear implants seem to allow the effective use of sentential context, but likely at the cost of delayed processing. Linguistic and lexical knowledge, which play an important role in compensation, may be successfully employed in advanced age, as some compensatory mechanisms seem to be preserved. These findings indicate that cognitive compensation in hearing impairment can be highly complicated—not always absent, but also not easily predicted by speech intelligibility tests only.

The brain is capable of restoring missing parts of speech, a top-down repair mechanism that enhances speech understanding in noisy environments. This enhancement can be quantified using the phonemic restoration paradigm, i.e., the improvement in intelligibility when silent interruptions of interrupted speech are filled with noise. Benefit from top-down repair of speech differs between cochlear implant (CI) users and normal-hearing (NH) listeners. This difference could be due to poorer spectral resolution and/or weaker pitch cues inherent to CI transmitted speech. In CIs, those two degradations cannot be teased apart because spectral degradation leads to weaker pitch representation. A vocoding method was developed to evaluate independently the roles of pitch and spectral resolution for restoration in NH individuals. Sentences were resynthesized with different spectral resolutions and with either retaining the original pitch cues or discarding them all. The addition of pitch significantly improved restoration only at six-bands spectral resolution. However, overall intelligibility of interrupted speech was improved both with the addition of pitch and with the increase in spectral resolution. This improvement may be due to better discrimination of speech segments from the filler noise, better grouping of speech segments together, and/or better bottom-up cues available in the speech segments. (C) 2016 Acoustical Society of America.

Human hearing is constructive. For example, when a voice is partially replaced by an extraneous sound (e.g., on the telephone due to a transmission problem), the auditory system may restore the missing portion so that the voice can be perceived as continuous (Miller and Licklider, 1950; for review, see Bregman, 1990; Warren, 1999). The neural mechanisms underlying this continuity illusion have been studied mostly with schematic stimuli (e.g., simple tones) and are still a matter of debate (for review, see Petkov and Sutter, 2011). The goal of the present study was to elucidate how these mechanisms operate under more natural conditions. Using psychophysics and electroencephalography (EEG), we assessed simultaneously the perceived continuity of a human vowel sound through interrupting noise and the concurrent neural activity. We found that vowel continuity illusions were accompanied by a suppression of the 4 Hz EEG power in auditory cortex (AC) that was evoked by the vowel interruption. This suppression was stronger than the suppression accompanying continuity illusions of a simple tone. Finally, continuity perception and 4 Hz power depended on the intactness of the sound that preceded the vowel (i.e., the auditory context). These findings show that a natural sound may be restored during noise due to the suppression of 4 Hz AC activity evoked early during the noise. This mechanism may attenuate sudden pitch changes, adapt the resistance of the auditory system to extraneous sounds across auditory scenes, and provide a useful model for assisted hearing devices.

An interrupted sound can be perceived as continuous when noise masks the interruption, creating an illusion of continuity. Recent findings have shown that adaptor sounds preceding an ambiguous target sound can influence listeners' rating of target continuity. However, it remains unclear whether these aftereffects on perceived continuity influence sensory processes, decisional processes (i.e., criterion shifts), or both. The present study addressed this question. Results show that the target sound was more likely to be rated as 'continuous' when preceded by adaptors that were perceived as clearly discontinuous than when it was preceded by adaptors that were heard (illusorily or veridically) as continuous. Detection-theory analyses indicated that these contrastive aftereffects reflect a combination of sensory and decisional processes. The contrastive sensory aftereffect persisted even when adaptors and targets were presented to opposite ears, suggesting a neural origin in structures that receive binaural inputs. Finally, physically identical but perceptually ambiguous adaptors that were rated as 'continuous' induced more reports of target continuity than adaptors that were rated as 'discontinuous'. This assimilative aftereffect was purely decisional. These findings confirm that judgments of auditory continuity can be influenced by preceding events, and reveal that these aftereffects have both sensory and decisional components.

In complex listening situations, cognitive restoration mechanisms are commonly used to enhance perception of degraded speech with inaudible segments. Profoundly hearing-impaired people with a cochlear implant (Cl) show less benefit from such mechanisms. However, both normal hearing (NH) listeners and CI users do benefit from visual speech cues in these listening situations. In this study we investigated if an accompanying video of the speaker can enhance the intelligibility of interrupted sentences and the phonemic restoration benefit, measured by an increase in intelligibility when the silent intervals are filled with noise. Similar to previous studies, restoration benefit was observed with interrupted speech without spectral degradations (Experiment 1), but was absent in acoustic simulations of CIs (Experiment 2) and was present again in simulations of electric-acoustic stimulation (Experiment 3). In all experiments, the additional speech information provided by the complementary visual cues lead to overall higher intelligibility, however, these cues did not influence the occurrence or extent of the phonemic restoration benefit of filler noise. Results imply that visual cues do not show a synergistic effect with the filler noise, as adding them equally increased the intelligibility of interrupted sentences with or without the filler noise. (C) 2015 Elsevier B.V. All rights reserved.

Speech reading enhances auditory perception in noise. One means by which this perceptual facilitation comes about is through information from visual networks reinforcing the encoding of the congruent speech signal by ignoring interfering acoustic signals. We tested this hypothesis neurophysiologically by acquiring EEG while individuals listened to words with a fixed portion of each word replaced by white noise. Congruent (meaningful) or incongruent (reversed frames) mouth movements accompanied the words. Individuals judged whether they heard the words as continuous (illusion) or interrupted (illusion failure) through the noise. We hypothesized that congruent, as opposed to incongruent, mouth movements should further enhance illusory perception by suppressing the auditory cortex's response to interruption onsets and offsets. Indeed, we found that the N1 auditory evoked potential (AEP) to noise onsets and offsets was reduced when individuals experienced the illusion during congruent, but not incongruent, audiovisual streams. This N1 inhibitory effect was most prominent at noise offsets, suggesting that visual influences on auditory perception are instigated to a greater extent during noisy periods. These findings suggest that visual context due to speech-reading disengages (inhibits) neural processes associated with interfering sounds (e.g., noisy interruptions) during speech perception.

The comprehension of fluent speech in one's native language requires that listeners integrate the detailed acoustic-phonetic information available in the sound signal with linguistic knowledge. This interplay is especially apparent in the phoneme restoration effect, a phenomenon in which a missing phoneme is "restored" via the influence of top-down information from the lexicon and through bottom-up acoustic processing. Developmental dyslexia is a disorder characterized by an inability to read at the level of one's peers without any clear failure due to environmental influences. In the current study we utilized the phonemic restoration illusion paradigm to examine individual differences in phonemic restoration across a range of reading ability, from very good to dyslexic readers. Results demonstrate that restoration occurs less in those who have high scores on measures of phonological processing. Based on these results, we suggest that the processing or representation of acoustic detail may not be as reliable in poor and dyslexic readers, with the result that lexical information is more likely to override acoustic properties of the stimuli. This pattern of increased restoration could result from a failure of perceptual tuning, in which unstable representations of speech sounds result in the acceptance of non-speech sounds as speech. An additional or alternative theory is that degraded or impaired phonological processing at the speech sound level may reflect architecture that is overly plastic and consequently fails to stabilize appropriately for speech sound representations. Therefore, the inability to separate speech and noise may result as a deficit in separating noise from the acoustic signal.

Cognitive skills, such as processing speed, memory functioning, and the ability to divide attention, are known to diminish with aging. The present study shows that, despite these changes, older adults can successfully compensate for degradations in speech perception. Critically, the older participants of this study were not pre-selected for high performance on cognitive tasks, but only screened for normal hearing. We measured the compensation for speech degradation using phonemic restoration, where intelligibility of degraded speech is enhanced using top-down repair mechanisms. Linguistic knowledge, Gestalt principles of perception, and expectations based on situational and linguistic context are used to effectively fill in the inaudible masked speech portions. A positive compensation effect was previously observed only with young normal hearing people, but not with older hearing-impaired populations, leaving the question whether the lack of compensation was due to aging or due to age-related hearing problems. Older participants in the present study showed poorer intelligibility of degraded speech than the younger group, as expected from previous reports of aging effects. However, in conditions that induce top-down restoration, a robust compensation was observed. Speech perception by the older group was enhanced, and the enhancement effect was similar to that observed with the younger group. This effect was even stronger with slowed-down speech, which gives more time for cognitive processing. Based on previous research, the likely explanations for these observations are that older adults can overcome age-related cognitive deterioration by relying on linguistic skills and vocabulary that they have accumulated over their lifetime. Alternatively, or simultaneously, they may use different cerebral activation patterns or exert more mental effort. This positive finding on top-down restoration skills by the older individuals suggests that new cognitive training methods can teach older adults to effectively use compensatory mechanisms to cope with the complex listening environments of everyday life.

Purpose Fitting a cochlear implant (CI) for optimal speech perception does not necessarily optimize listening effort. This study aimed to show that listening effort may change between CI processing conditions for which speech intelligibility remains constant. Method Nineteen normal-hearing participants listened to CI simulations with varying numbers of spectral channels. A dual-task paradigm combining an intelligibility task with either a linguistic or nonlinguistic visual response-time (RT) task measured intelligibility and listening effort. The simultaneously performed tasks compete for limited cognitive resources; changes in effort associated with the intelligibility task are reflected in changes in RT on the visual task. A separate self-report scale provided a subjective measure of listening effort. Results All measures showed significant improvements with increasing spectral resolution up to 6 channels. However, only the RT measure of listening effort continued improving up to 8 channels. The effects were stronger for RTs recorded during listening than for RTs recorded between listening. Conclusion The results suggest that listening effort decreases with increased spectral resolution. Moreover, these improvements are best reflected in objective measures of listening effort, such as RTs on a secondary task, rather than intelligibility scores or subjective effort measures.

The intelligibility of periodically interrupted speech improves once the silent gaps are filled with noise bursts. This improvement has been attributed to phonemic restoration, a top-down repair mechanism that helps intelligibility of degraded speech in daily life. Two hypotheses were investigated using perceptual learning of interrupted speech. If different cognitive processes played a role in restoring interrupted speech with and without filler noise, the two forms of speech would be learned at different rates and with different perceived mental effort. If the restoration benefit were an artificial outcome of using the ecologically invalid stimulus of speech with silent gaps, this benefit would diminish with training. Two groups of normal-hearing listeners were trained, one with interrupted sentences with the filler noise, and the other without. Feedback was provided with the auditory playback of the unprocessed and processed sentences, as well as the visual display of the sentence text. Training increased the overall performance significantly, however restoration benefit did not diminish. The increase in intelligibility and the decrease in perceived mental effort were relatively similar between the groups, implying similar cognitive mechanisms for the restoration of the two types of interruptions. Training effects were generalizable, as both groups improved their performance also with the other form of speech than that they were trained with, and retainable. Due to null results and relatively small number of participants (10 per group), further research is needed to more confidently draw conclusions. Nevertheless, training with interrupted speech seems to be effective, stimulating participants to more actively and efficiently use the top-down restoration. This finding further implies the potential of this training approach as a rehabilitative tool for hearing-impaired/elderly populations.

The combined effect of low-pass filtering (cut-off frequencies between 500 and 3000 Hz) and periodic interruptions (1.5 and 10 Hz) on speech intelligibility was investigated. When combined, intelligibility was lower than each manipulation alone, even in some conditions where there was no effect from a single manipulation (such as the fast interruption rate of 10 Hz). By using young normal-hearing listeners, potential suprathreshold deficits and aging effects that may occur due to hearing impairment were eliminated. Thus, the results imply that reduced audibility of high-frequency speech components may partially explain the reduced intelligibility of interrupted speech in hearing impaired persons. (C) 2012 Acoustical Society of America

Recognition of periodically interrupted sentences (with an interruption rate of 1.5 Hz, 50% duty cycle) was investigated under conditions of spectral degradation, implemented with a noiseband vocoder, with and without additional unprocessed low-pass filtered speech (cutoff frequency 500 Hz). Intelligibility of interrupted speech decreased with increasing spectral degradation. For all spectral degradation conditions, however, adding the unprocessed low-pass filtered speech enhanced the intelligibility. The improvement at 4 and 8 channels was higher than the improvement at 16 and 32 channels: 19% and 8%, on average, respectively. The Articulation Index predicted an improvement of 0.09, in a scale from 0 to 1. Thus, the improvement at poorest spectral degradation conditions was larger than what would be expected from additional speech information. Therefore, the results implied that the fine temporal cues from the unprocessed low-frequency speech, such as the additional voice pitch cues, helped perceptual integration of temporally interrupted and spectrally degraded speech, especially when the spectral degradations were severe. Considering the vocoder processing as a cochlear implant simulation, where implant users' performance is closest to 4 and 8-channel vocoder performance, the results support additional benefit of low-frequency acoustic input in combined electric-acoustic stimulation for perception of temporally degraded speech. (C) 2010 Elsevier B.V. All rights reserved.

The auditory system is capable of perceptually restoring inaudible portions of speech. This restoration may be compromised as a result of hearing impairment, particularly if it is combined with advanced age, because of degradations in the bottom-up and top-down processes. To test this hypothesis, phonemic restoration was quantitatively measured with hearing-impaired listeners of varying ages and degrees of hearing impairment, as well as with a normal hearing control group. The results showed that the restoration benefit was negatively correlated with both hearing impairment and age, supporting the original hypothesis. Group data showed that listeners with mild hearing loss were able to perceptually restore the missing speech segments as well as listeners with normal hearing. By contrast, the moderately-impaired listeners showed no evidence of perceptual restoration. Further analysis using the articulation index showed that listeners with mild hearing loss were able to increase phonemic restoration with audibility. Moderately-impaired listeners, on the other hand, were unable to do so, even when the articulation index was high. The overall findings suggest that, in addition to insufficient audibility, degradations in the bottom-up and/or top-down mechanisms as a result of hearing loss may limit or entirely prevent phonemic restoration. (C) 2009 Elsevier B.V. All rights reserved.

The brain, using expectations, linguistic knowledge, and context, can perceptually restore inaudible portions of speech. Such top-down repair is thought to enhance speech intelligibility in noisy environments. Hearing-impaired listeners with cochlear implants commonly complain about not understanding speech in noise. We hypothesized that the degradations in the bottom-up speech signals due to the implant signal processing may have a negative effect on the top-down repair mechanisms, which could partially be responsible for this complaint. To test the hypothesis, phonemic restoration of interrupted sentences was measured with young normal-hearing listeners using a noise-band vocoder simulation of implant processing. Decreasing the spectral resolution (by reducing the number of vocoder processing channels from 32 to 4) systematically degraded the speech stimuli. Supporting the hypothesis, the size of the restoration benefit varied as a function of spectral resolution. A significant benefit was observed only at the highest spectral resolution of 32 channels. With eight channels, which resembles the resolution available to most implant users, there was no significant restoration effect. Combined electric-acoustic hearing has been previously shown to provide better intelligibility of speech in adverse listening environments. In a second configuration, combined electric-acoustic hearing was simulated by adding low-pass-filtered acoustic speech to the vocoder processing. There was a slight improvement in phonemic restoration compared to the first configuration; the restoration benefit was observed at spectral resolutions of both 16 and 32 channels. However, the restoration was not observed at lower spectral resolutions (four or eight channels). Overall, the findings imply that the degradations in the bottom-up signals alone (such as occurs in cochlear implants) may reduce the top-down restoration of speech.

Compared with normal-hearing listeners, cochlear implant (CI) users display a loss of intelligibility of speech interrupted by silence or noise, possibly due to reduced ability to integrate and restore speech glimpses across silence or noise intervals. The present study was conducted to establish the extent of the deficit typical CI users have in understanding interrupted high-context sentences as a function of a range of interruption rates (1.5 to 24 Hz) and duty cycles (50 and 75 %). Further, factors such as reduced signal quality of CI signal transmission and advanced age, as well as potentially lower speech intelligibility of CI users even in the lack of interruption manipulation, were explored by presenting young, as well as age-matched, normal-hearing (NH) listeners with full-spectrum and vocoded speech (eight-channel and speech intelligibility baseline performance matched). While the actual CI users had more difficulties in understanding interrupted speech and taking advantage of faster interruption rates and increased duty cycle than the eight-channel noise-band vocoded listeners, their performance was similar to the matched noise-band vocoded listeners. These results suggest that while loss of spectro-temporal resolution indeed plays an important role in reduced intelligibility of interrupted speech, these factors alone cannot entirely explain the deficit. Other factors associated with real CIs, such as aging or failure in transmission of essential speech cues, seem to additionally contribute to poor intelligibility of interrupted speech.