Age-Related Differences in Early Cortical Representations of Target Speech Masked by Either Steady-State Noise or Competing Speech.

Word in noise identification is facilitated by acoustic differences between target and competing sounds and temporal separation between the onset of the masker and that of the target. Younger and older adults are able to take advantage of onset delay when the masker is dissimilar (Noise) to the target word, but only younger adults are able to do so when the masker is similar (Babble). We examined the neural underpinning of this age difference using cortical evoked responses to words masked by either Babble or Noise when the masker preceded the target word by 100 or 600 ms in younger and older adults, after adjusting the signal-to-noise ratios (SNRs) to equate behavioural performance across age groups and conditions. For the 100 ms onset delay, the word in noise elicited an acoustic change complex (ACC) response that was comparable in younger and older adults. For the 600 ms onset delay, the ACC was modulated by both masker type and age. In older adults, the ACC to a word in babble was not affected by the increase in onset delay whereas younger adults showed a benefit from longer delays. Hence, the age difference in sensitivity to temporal delay is indexed by early activity in the auditory cortex. These results are consistent with the hypothesis that an increase in onset delay improves stream segregation in younger adults in both noise and babble, but only in noise for older adults and that this change in stream segregation is evident in early cortical processes.

Do Age and Linguistic Status Alter the Effect of Sound Source Diffuseness on Speech Recognition in Noise?

One aspect of auditory scenes that has received very little attention is the level of diffuseness of sound sources. This aspect has increasing importance due to growing use of amplification systems. When an auditory stimulus is amplified and presented over multiple, spatially-separated loudspeakers, the signal's timbre is altered due to comb filtering. In a previous study we examined how increasing the diffuseness of the sound sources might affect listeners' ability to recognize speech presented in different types of background noise. Listeners performed similarly when both the target and the masker were presented via a similar number of loudspeakers. However, performance improved when the target was presented using a single speaker (compact) and the masker from three spatially separate speakers (diffuse) but worsened when the target was diffuse, and the masker was compact. In the current study, we extended our research to examine whether the effects of timbre changes with age and linguistic experience. Twenty-four older adults whose first language was English (Old-EFLs) and 24 younger adults whose second language was English (Young-ESLs) were asked to repeat non-sense sentences masked by either Noise, Babble, or Speech and their results were compared with those of the Young-EFLs previously tested. Participants were divided into two experimental groups: (1) A Compact-Target group where the target sentences were presented over a single loudspeaker, while the masker was either presented over three loudspeakers or over a single loudspeaker; (2) A Diffuse-Target group, where the target sentences were diffuse while the masker was either compact or diffuse. The results indicate that the Target Timbre has a negligible effect on thresholds when the timbre of the target matches the timbre of the masker in all three groups. When there is a timbre contrast between target and masker, thresholds are significantly lower when the target is compact than when it is diffuse for all three listening groups in a Noise background. However, while this difference is maintained for the Young and Old-EFLs when the masker is Babble or Speech, speech reception thresholds in the Young-ESL group tend to be equivalent for all four combinations of target and masker timbre.

Can the diffuseness of sound sources in an auditory scene alter speech perception?

When amplification is used, sound sources are often presented over multiple loudspeakers, which can alter their timbre, and introduce comb-filtering effects. Increasing the diffuseness of a sound by presenting it over spatially separated loudspeakers might affect the listeners' ability to form a coherent auditory image of it, alter its perceived spatial position, and may even affect the extent to which it competes for the listener's attention. In addition, it can lead to comb-filtering effects that can alter the spectral profiles of sounds arriving at the ears. It is important to understand how these changes affect speech perception. In this study, young adults were asked to repeat nonsense sentences presented in either noise, babble, or speech. Participants were divided into two groups: (1) A Compact-Target Timbre group where the target sentences were presented over a single loudspeaker (compact target), while the masker was either presented over three loudspeakers (diffuse) or over a single loudspeaker (compact); (2) A Diffuse-Target Timbre group, where the target sentences were diffuse while the masker was either compact or diffuse. Timbre had no significant effect in the absence of a timbre contrast between target and masker. However, when there was a timbre contrast, the signal-to-noise ratios needed for 50% correct recognition of the target speech were higher (worse) when the masker was compact, and lower (better) when the target was compact. These results were consistent with the expected effects from comb filtering, and could also reflect a tendency for attention to be drawn towards compact sound sources.

Do age and linguistic background alter the audiovisual advantage when listening to speech in the presence of energetic and informational masking?

We examined how the type of masker presented in the background affected the extent to which visual information enhanced speech recognition, and whether the effect was dependent on or independent of age and linguistic competence. In the present study, young speakers of English as a first language (YEL1) and English as a second language (YEL2), as well as older speakers of English as a first language (OEL1), were asked to complete an audio (A) and an audiovisual (AV) speech recognition task in which they listened to anomalous target sentences presented against a background of one of three masker types (noise, babble, and competing speech). All three main effects were found to be statistically significant (group, masker type, A vs. AV presentation type). Interesting two-way interactions were found between masker type and group and between masker type and presentation type; however, no interactions were found between group (age and/or linguistic competence) and presentation type (A vs. AV). The results of this study, while they shed light on the effect of masker type on the AV advantage, suggest that age and linguistic competence have no significant effects on the extent to which a listener is able to use visual information to improve speech recognition in background noise.

The effects of the binocular disparity differences between targets and maskers on visual search.

A visual search for targets is facilitated when the target objects are on a different depth plane than other masking objects cluttering the scene. The ability of observers to determine whether one of four letters presented stereoscopically at four symmetrically located positions on the fixation plane differed from the other three was assessed when the target letters were masked by other randomly positioned and oriented letters appearing on the same depth plane as the target letters, or in front, or behind it. Three additional control maskers, derived from the letter maskers, were also presented on the same three depth planes: (1) random-phase maskers (same spectral amplitude composition as the letter masker but with the phase spectrum randomized); (2) random-pixel maskers (the locations of the letter maskers' pixel amplitudes were randomized); (3) letter-fragment maskers (the same letters as in the letter masker but broken up into fragments). Performance improved with target duration when the target-letter plane was in front of the letter-masker plane, but not when the target letters were on the same plane as the masker, or behind it. A comparison of the results for the four different kinds of maskers indicated that maskers consisting of recognizable objects (letters or letter fragments) interfere more with search and comparison judgments than do visual noise maskers having the same spatial frequency profile and contrast. In addition, performance was poorer for letter maskers than for letter-masker fragments, suggesting that the letter maskers interfered more with performance than the letter-fragment maskers because of the lexical activity they elicit.

Does the degree of linguistic experience (native versus nonnative) modulate the degree to which listeners can benefit from a delay between the onset of the maskers and the onset of the target speech?

Background noise has a greater adverse effect on word recognition when people are listening in their second language (L2) as opposed to their first language (L1). The present study investigates the extent to which linguistic experience affects the ability of L2 listeners to benefit from a delay between the onset of a masker and the onset of a word. In a previous study (Ben-David, Tse & Schneider, 2012), word recognition thresholds for young L1s were found to improve with the increase in the delay between the onset of a masker (either a stationary noise or a babble of voices) and the onset of a word. The investigators interpreted this result as reflecting the ability of L1 listeners to rapidly segregate the target words from a masker. Given stream segregation depends, in part, on top-down knowledge-driven processes, we might expect stream segregation to be more "sluggish" for L2 listeners than for L1 listeners, especially when the masker consists of a babble of L2 voices. In the present study, we compared the ability of native English speakers to those who had either recent or long-term immersion in English as L2, to benefit from a delay between masker onset and word onset for English words. Results show that thresholds were higher for the two L2s groups than for the L1s. However, the rate at which word recognition improved with word-onset delay was unaffected by linguistic status, both when words were presented in noise, and in babble. Hence, for young listeners, stream segregation appears to be independent of linguistic status, suggesting that bottom-up sensory mechanisms play a large role in stream segregation in this paradigm. The implications of a failure of older L1 listeners (in Ben-David et al.) to benefit from a word-onset delay when the masker is a babble of voices are discussed.

Does increasing the intelligibility of a competing sound source interfere more with speech comprehension in older adults than it does in younger adults?

A previous study (Schneider, Daneman, Murphy, & Kwong See, 2000) found that older listeners' decreased ability to recognize individual words in a noisy auditory background was responsible for most, if not all, of the comprehension difficulties older adults experience when listening to a lecture in a background of unintelligible babble. The present study investigated whether the use of a more intelligible distracter (a competing lecture) might reveal an increased susceptibility to distraction in older adults. The results from Experiments 1 and 2 showed that both normal-hearing and hearing-impaired older adults performed poorer than younger adults when everyone was tested in identical listening situations. However, when the listening situation was individually adjusted to compensate for age-related differences in the ability to recognize individual words in noise, age-related difference in comprehension disappeared. Experiment 3 compared the masking effects of a single-talker competing lecture to a babble of 12 voices directly after adjusting for word recognition. The results showed that the competing lecture interfered more than did the babble for both younger and older listeners. Interestingly, an increase in the level of noise had a deleterious effect on listening when the distractor was babble but had no effect when it was a competing lecture. These findings indicated that the speech comprehension difficulties of healthy older adults in noisy backgrounds primarily reflect age-related declines in the ability to recognize individual words.

How Age and Linguistic Competence Affect Memory for Heard Information.

The short-term memory performance of a group of younger adults, for whom English was a second language (young EL2 listeners), was compared to that of younger and older adults for whom English was their first language (EL1 listeners). To-be-remembered words were presented in noise and in quiet. When presented in noise, the listening situation was adjusted to ensure that the likelihood of recognizing the individual words was comparable for all groups. Previous studies which used the same paradigm found memory performance of older EL1 adults on this paired-associate task to be poorer than that of their younger EL1 counterparts both in quiet and in a background of babble. The purpose of the present study was to investigate whether the less well-established semantic and linguistic skills of EL2 listeners would also lead to memory deficits even after equating for word recognition as was done for the younger and older EL1 listeners. No significant differences in memory performance were found between young EL1 and EL2 listeners after equating for word recognition, indicating that the EL2 listeners' poorer semantic and linguistic skills had little effect on their ability to memorize and recall paired associates. This result is consistent with the hypothesis that age-related declines in memory are primarily due to age-related declines in higher-order processes supporting stream segregation and episodic memory. Such declines are likely to increase the load on higher-order (possibly limited) cognitive processes supporting memory. The problems that these results pose for the comprehension of spoken language in these three groups are discussed.

Age-related inhibitory deficit, or lack of familiarity benefit? Evidence from letter identification among visual distractors.

Age-related deficits in processing complex visual scenes are often attributed to age-related declines in the cognitive abilities required for such tasks. For example, poorer or slower performance of a complex task in the presence of distractor items is often attributed to an age-related deficit in the ability to inhibit the processing of irrelevant information. To investigate the relative contributions of sensory and cognitive factors in such tasks, younger and older participants were asked to identify a letter presented simultaneously with distractors that were either other letters, pieces of letters, or visual noise controls with identical spatial frequency content or contrast profile. In Experiment 1, older adults performed much worse than younger adults when the masking field consisted of other letters. Surprisingly-and contrary to the predictions of inhibitory deficit or visual "pop-out" phenomena-this effect emerged because younger adults performed much better with letter-containing maskers than with any other type of masker, whereas older adults did not. Experiment 2 revealed that age-related changes in the time required to process the visual display do not appear to account for this effect. In Experiment 3, however, we replicated older adults' task performance in a younger adult sample by filtering the experimental stimuli to match the image contrast typically experienced by an older adult. The results of Experiment 3 suggest that age-related differences in task performance amidst distractors can emerge from age-related declines in contrast sensitivity, which set older adults up to fail at tasks in which younger adults may typically be able to benefit from the familiarity of the target and surrounding objects.

Attentional modulation of informational masking on early cortical representations of speech signals.

To recognize speech in a noisy auditory scene, listeners need to perceptually segregate the target talker's voice from other competing sounds (stream segregation). A number of studies have suggested that the attentional demands placed on listeners increase as the acoustic properties and informational content of the competing sounds become more similar to that of the target voice. Hence we would expect attentional demands to be considerably greater when speech is masked by speech than when it is masked by steady-state noise. To investigate the role of attentional mechanisms in the unmasking of speech sounds, event-related potentials (ERPs) were recorded to a syllable masked by noise or competing speech under both active (the participant was asked to respond when the syllable was presented) or passive (no response was required) listening conditions. The results showed that the long-latency auditory response to a syllable (/bi/), presented at different signal-to-masker ratios (SMRs), was similar in both passive and active listening conditions, when the masker was a steady-state noise. In contrast, a switch from the passive listening condition to the active one, when the masker was two-talker speech, significantly enhanced the ERPs to the syllable. These results support the hypothesis that the need to engage attentional mechanisms in aid of scene analysis increases as the similarity (both acoustic and informational) between the target speech and the competing background sounds increases.

How Spoken Language Comprehension is Achieved by Older Listeners in Difficult Listening Situations.

BACKGROUND/STUDY CONTEXT: Comprehending spoken discourse in noisy situations is likely to be more challenging to older adults than to younger adults due to potential declines in the auditory, cognitive, or linguistic processes supporting speech comprehension. These challenges might force older listeners to reorganize the ways in which they perceive and process speech, thereby altering the balance between the contributions of bottom-up versus top-down processes to speech comprehension. METHODS: The authors review studies that investigated the effect of age on listeners' ability to follow and comprehend lectures (monologues), and two-talker conversations (dialogues), and the extent to which individual differences in lexical knowledge and reading comprehension skill relate to individual differences in speech comprehension. Comprehension was evaluated after each lecture or conversation by asking listeners to answer multiple-choice questions regarding its content. RESULTS: Once individual differences in speech recognition for words presented in babble were compensated for, age differences in speech comprehension were minimized if not eliminated. However, younger listeners benefited more from spatial separation than did older listeners. Vocabulary knowledge predicted the comprehension scores of both younger and older listeners when listening was difficult, but not when it was easy. However, the contribution of reading comprehension to listening comprehension appeared to be independent of listening difficulty in younger adults but not in older adults. CONCLUSION: The evidence suggests (1) that most of the difficulties experienced by older adults are due to age-related auditory declines, and (2) that these declines, along with listening difficulty, modulate the degree to which selective linguistic and cognitive abilities are engaged to support listening comprehension in difficult listening situations. When older listeners experience speech recognition difficulties, their attentional resources are more likely to be deployed to facilitate lexical access, making it difficult for them to fully engage higher-order cognitive abilities in support of listening comprehension.

"Older is always better": Age-related differences in vocabulary scores across 16 years.

Cross-sectional studies of cognitive aging compare age groups at 1 time point. It is unclear from such studies whether age-related cognitive differences remain stable across time. We present a cross-sectional investigation of vocabulary scores of 2,000 younger and older adults collected across 16 years, using the same laboratory and protocol. We found a steady decrease with year of testing and an advantage for older adults. An additive relation between age group and year of testing implied that age-related differences in vocabulary are independent of changes over time, suggesting that younger and older adults are similarly affected by changes in word usage.

How Age, Linguistic Status, and the Nature of the Auditory Scene Alter the Manner in Which Listening Comprehension Is Achieved in Multitalker Conversations.

PURPOSE: We investigated how age and linguistic status affected listeners' ability to follow and comprehend 3-talker conversations, and the extent to which individual differences in language proficiency predict speech comprehension under difficult listening conditions. METHOD: Younger and older L1s as well as young L2s listened to 3-talker conversations, with or without spatial separation between talkers, in either quiet or against moderate or high 12-talker babble background, and were asked to answer questions regarding their contents. RESULTS: After compensating for individual differences in speech recognition, no significant differences in conversation comprehension were found among the groups. As expected, conversation comprehension decreased as babble level increased. Individual differences in reading comprehension skill contributed positively to performance in younger EL1s and in young EL2s to a lesser degree but not in older EL1s. Vocabulary knowledge was significantly and positively related to performance only at the intermediate babble level. CONCLUSION: The results indicate that the manner in which spoken language comprehension is achieved is modulated by the listeners' age and linguistic status.

A cautionary note on the use of the Analysis of Covariance (ANCOVA) in classification designs with and without within-subject factors.

A number of statistical textbooks recommend using an analysis of covariance (ANCOVA) to control for the effects of extraneous factors that might influence the dependent measure of interest. However, it is not generally recognized that serious problems of interpretation can arise when the design contains comparisons of participants sampled from different populations (classification designs). Designs that include a comparison of younger and older adults, or a comparison of musicians and non-musicians are examples of classification designs. In such cases, estimates of differences among groups can be contaminated by differences in the covariate population means across groups. A second problem of interpretation will arise if the experimenter fails to center the covariate measures (subtracting the mean covariate score from each covariate score) whenever the design contains within-subject factors. Unless the covariate measures on the participants are centered, estimates of within-subject factors are distorted, and significant increases in Type I error rates, and/or losses in power can occur when evaluating the effects of within-subject factors. This paper: (1) alerts potential users of ANCOVA of the need to center the covariate measures when the design contains within-subject factors, and (2) indicates how they can avoid biases when one cannot assume that the expected value of the covariate measure is the same for all of the groups in a classification design.

Delayed Stream Segregation in Older Adults: More Than Just Informational Masking.

OBJECTIVE: To determine whether the time course for the buildup of auditory stream segregation differs between younger and older adults. DESIGN: Word recognition thresholds were determined for the first and last keywords in semantically anomalous but syntactically correct sentences (e.g., "A rose could paint a fish") when the target sentences were masked by speech-spectrum noise, 3-band vocoded speech, 16-band vocoded speech, intact and colocated speech, and intact and spatially separated speech. A significant reduction in thresholds from the first to the last keyword was interpreted as indicating that stream segregation improved with time. RESULTS: The buildup of stream segregation is slowed for both age groups when the masker is intact, colocated speech. CONCLUSIONS: Older adults are more disadvantaged; for them, stream segregation is also slowed even when a speech masker is spatially separated, conveys little meaning (3-band vocoding), and vocal fine structure cues are impoverished but envelope cues remain available (16-band vocoding).

How age and linguistic competence alter the interplay of perceptual and cognitive factors when listening to conversations in a noisy environment.

Multi-talker conversations challenge the perceptual and cognitive capabilities of older adults and those listening in their second language (L2). In older adults these difficulties could reflect declines in the auditory, cognitive, or linguistic processes supporting speech comprehension. The tendency of L2 listeners to invoke some of the semantic and syntactic processes from their first language (L1) may interfere with speech comprehension in L2. These challenges might also force them to reorganize the ways in which they perceive and process speech, thereby altering the balance between the contributions of bottom-up vs. top-down processes to speech comprehension. Younger and older L1s as well as young L2s listened to conversations played against a babble background, with or without spatial separation between the talkers and masker, when the spatial positions of the stimuli were specified either by loudspeaker placements (real location), or through use of the precedence effect (virtual location). After listening to a conversation, the participants were asked to answer questions regarding its content. Individual hearing differences were compensated for by creating the same degree of difficulty in identifying individual words in babble. Once compensation was applied, the number of questions correctly answered increased when a real or virtual spatial separation was introduced between babble and talkers. There was no evidence that performance differed between real and virtual locations. The contribution of vocabulary knowledge to dialog comprehension was found to be larger in the virtual conditions than in the real whereas the contribution of reading comprehension skill did not depend on the listening environment but rather differed as a function of age and language proficiency. The results indicate that the acoustic scene and the cognitive and linguistic competencies of listeners modulate how and when top-down resources are engaged in aid of speech comprehension.

Time course and cost of misdirecting auditory spatial attention in younger and older adults.

OBJECTIVES: The effects of directing, switching, and misdirecting auditory spatial attention in a complex listening situation were investigated in 8 younger and 8 older listeners with normal-hearing sensitivity below 4 kHz. DESIGN: In two companion experiments, a target sentence was presented from one spatial location and two competing sentences were presented simultaneously, one from each of two different locations. Pretrial, listeners were informed of the call-sign cue that identified which of the three sentences was the target and of the probability of the target sentence being presented from each of the three possible locations. Four different probability conditions varied in the likelihood of the target being presented at the left, center, and right locations. In Experiment 1, four timing conditions were tested: the original (unedited) sentences (which contained about 300 msec of filler speech between the call-sign cue and the onset of the target words), or modified (edited) sentences with silent pauses of 0, 150, or 300 msec replacing the filler speech. In Experiment 2, when the cued sentence was presented from an unlikely (side) listening location, for half of the trials the listener's task was to report target words from the cued sentence (cue condition); for the remaining trials, the listener's task was to report target words from the sentence presented from the opposite, unlikely (side) listening location (anticue condition). RESULTS: In Experiment 1, for targets presented from the likely (center) location, word identification was better for the unedited than for modified sentences. For targets presented from unlikely (side) locations, word identification was better when there was more time between the call-sign cue and target words. All listeners benefited similarly from the availability of more compared with less time and the presentation of continuous compared with interrupted speech. In Experiment 2, the key finding was that age-related performance deficits were observed in conditions requiring anticue but not cue responses. CONCLUSIONS: The findings from Experiment 1 suggest that for both age groups, stream continuity mediates the process of allocating and maintaining auditory spatial attention when the target originates at an expected location, but that time is needed for the reallocation of auditory spatial attention when the target originates at an unexpected location. The findings from Experiment 2 suggest that when attention is momentarily misdirected, difficulties disengaging attention may help explain why older adults with good hearing report difficulty communicating in multi-talker listening situations.

Aging effects on detection of spectral changes induced by a break in sound correlation.

OBJECTIVES: Previous studies have shown that both younger adults and older adults with clinically normal hearing are able to detect a break in correlation (BIC) between interaurally correlated sounds presented over headphones. This ability to detect a BIC improved when the correlated sounds were presented over left and right loudspeakers rather than over left and right headphones, suggesting that additional spectral cues provided by comb filtering (caused by interference between the two channels) facilitate detection of the BIC. However, older adults receive significantly less benefit than younger adults from a switch to loudspeaker presentation. It is hypothesized that this is a result of an age-related reduction in the sensitivity to the monaural spectral cues provided by comb filtering. DESIGN: Two experiments were conducted in this study. Correlated white noises with a BIC in the temporal middle were presented from two spatially separated loudspeakers (positioned at ±45-degree azimuth) and recorded at the right ear of a Knowles Electronic Manikin for Acoustic Research (KEMAR). In Experiment 1, the waveforms recorded at the KEMAR's right ear were presented to the participant's right ear over a headphone in 14 younger adults and 24 older adults with clinically normal hearing. In Experiment 2, 8 of the 14 younger participants participated. Under the monaurally cueing condition, the waveforms recorded at the KEMAR's right ear were presented to the participant's right ear as Experiment 1. Under the binaurally cueing condition, waveforms delivered from the left loudspeaker and those from the right loudspeaker were recorded at the KEMAR's left and right ear, respectively, thereby eliminating the spectral ripple cue, and were presented to the participant's left and right ears, respectively. For each of the two experiments, the break duration threshold for detecting the BIC was examined when the interloudspeaker interval (delay) (ILI) was 0, 1, 2, or 4 msec (left loudspeaker leading). RESULTS: In Experiment 1, both younger participants and older participants detected the BIC in the waveforms recorded by the right ear of KEMAR, but older participants had higher detection thresholds than younger participants when the ILI was 0, 2, or 4 msec without an effect of SPL shift between 59 and 71 dB. In Experiment 2, each of the eight younger participants was able to detect the occurrence of the BIC in either the monaurally cueing or binaural-cueing condition. In addition, the detection threshold under the monaurally cueing condition was substantially the same as that under the binaurally cueing condition at each of the four ILIs. CONCLUSIONS: Younger adults and older adults with clinically normal hearing are able to detect the monaural spectral changes arising from comb filtering when a sudden drop in intersound correlation is introduced. However, younger adults are more sensitive than older adults are, at detecting the BIC. The findings suggest that older adults are less able than younger adults to detect a periodic ripple in the sound spectrum. This age-related ability reduction may contribute to older adults' difficulties in hearing under noisy, reverberant conditions.