How repetition influences speech understanding by younger, middle-aged and older adults.

OBJECTIVE: To examine benefit from immediate repetition of a masked speech message in younger, middle-aged and older adults. DESIGN: Participants listened to sentences in conditions where only the target message was repeated, and when both the target message and its accompanying masker (noise or speech) were repeated. In a follow-up experiment, the effect of repetition was evaluated using a square-wave modulated noise masker to compare benefit when listeners were exposed to the same glimpses of the target message during first and second presentation versus when the glimpses differed. STUDY SAMPLE: Younger, middle-aged and older adults (n = 16/group) for the main experiment; 15 younger adults for the follow-up experiment. RESULTS: Repetition benefit was larger when the target but not the masker was repeated for all groups. This was especially true for older adults, suggesting that these individuals may be more negatively affected when a background message is repeated. Data obtained using noise maskers suggest that it is slightly more beneficial when listeners hear different (versus identical) portions of speech between initial presentation and repetition. CONCLUSIONS: Although subtle age-related differences were found in some conditions, results confirm that repetition is an effective repair strategy for listeners spanning the adult age range.

Can monaural temporal masking explain the ongoing precedence effect?

The precedence effect for transient sounds has been proposed to be based primarily on monaural processes, manifested by asymmetric temporal masking. This study explored the potential for monaural explanations with longer ("ongoing") sounds exhibiting the precedence effect. Transient stimuli were single lead-lag noise burst pairs; ongoing stimuli were trains of 63 burst pairs. Unlike with transients, monaural masking data for ongoing sounds showed no advantage for the lead, and are inconsistent with asymmetric audibility as an explanation for ongoing precedence. This result, along with supplementary measurements of interaural time discrimination, suggests different explanations for transient and ongoing precedence.

Strength of onset and ongoing cues in judgments of lateral position.

This study describes the contributions to auditory image position of an interaural time delay (ITD) cue at onset relative to subsequent ITDs during the ongoing part of a stimulus. Test stimuli were trains of 1-ms binaural noise bursts; lateral position was measured with a wideband acoustic pointer that subjects adjusted to match the intracranial position of test stimuli. In different conditions the ongoing part of the stimulus (the bursts following the first one) either had a consistent ITD (the same ITD on each ongoing burst), or had alternating leading and lagging components with ITDs that opposed one another. As duration of the ongoing part was increased from 4 to 250 ms, with the initial ITD fixed, lateral position changed from being dominated by the onset ITD to being dominated by the ongoing consistent or leading ITD. With alternating ongoing ITDs equal contributions from onset and ongoing parts were obtained at an ongoing duration of about 40 ms; with consistent ongoing ITDs equal contributions were obtained at about 15 ms. The results point up the increased dominance of onset cues when ongoing cues are ambiguous, as they often are in real-world settings.

Temporal effects in priming of masked and degraded speech.

When listeners know the content of the message they are about to hear, the clarity of distorted or partially masked speech increases dramatically. The current experiments investigated this priming phenomenon quantitatively using a same-different task where a typed caption and auditory message either matched exactly or differed by one key word. Four conditions were tested with groups of normal-hearing listeners: (a) natural speech presented in two-talker babble in a non-spatial configuration, (b) same as (a) but with the masker time reversed, (c) same as (a) but with target-masker spatial separation, and (d) vocoded sentences presented in speech-spectrum noise. The primary manipulation was the timing of the caption relative to the auditory message, which varied in 20 steps with a resolution of 200 ms. Across all four conditions, optimal performance was achieved when the initiation of the text preceded the acoustic speech signal by at least 400 ms, driven mostly by a low number of "different" responses to Same stimuli. Performance was slightly poorer with simultaneous delivery and much poorer when the auditory signal preceded the caption. Because priming may be used to facilitate perceptual learning, identifying optimal temporal conditions for priming could help determine the best conditions for auditory training.

Stimulus and listener factors affecting age-related changes in competing speech perception.

The purpose of this study was to examine associations among hearing thresholds, cognitive ability, and speech understanding in adverse listening conditions within and between groups of younger, middle-aged, and older adults. Participants repeated back sentences played in the presence of several types of maskers (syntactically similar and syntactically different competing speech from one or two other talkers, and steady-state speech-shaped noise). They also completed tests of auditory short-term/working memory, processing speed, and inhibitory ability. Results showed that group differences in accuracy of word identification and in error patterns differed depending upon the number of masking voices; specifically, older and middle-aged individuals had particular difficulty, relative to younger subjects, in the presence of a single competing message. However, the effect of syntactic similarity was consistent across subject groups. Hearing loss, short-term memory, processing speed, and inhibitory ability were each related to some aspects of performance by the middle-aged and older participants. Notably, substantial age-related changes in speech recognition were apparent within the group of middle-aged listeners.

Priming of lowpass-filtered speech affects response bias, not sensitivity, in a bandwidth discrimination task.

Priming is demonstrated when prior information about the content of a distorted, filtered, or masked auditory message improves its clarity. The current experiment attempted to quantify aspects of priming by determining its effects on performance and bias in a lowpass-filter-cutoff frequency discrimination task. Nonsense sentences recorded by a female talker were sharply lowpass filtered at a nominal cutoff frequency (F) of 0.5 or 0.75 kHz or at a higher cutoff frequency (F + ΔF). The listeners' task was to determine which interval of a two-interval-forced-choice trial contained the nonsense sentence filtered with F + ΔF. On priming trials, the interval 1 sentence was displayed on a computer screen prior to the auditory portion of the trial. The prime markedly affected bias, increasing the number of correct and incorrect interval 1 responses but did not affect overall discrimination performance substantially. These findings were supported through a second experiment that required listeners to make confidence judgments. The paradigm has the potential to help quantify the limits of speech perception when uncertainty about the auditory message is removed.

Intelligibility of whispered speech in stationary and modulated noise maskers.

This study investigated the role of natural periodic temporal fine structure in helping listeners take advantage of temporal valleys in amplitude-modulated masking noise when listening to speech. Young normal-hearing participants listened to natural, whispered, and/or vocoded nonsense sentences in a variety of masking conditions. Whispering alters normal waveform temporal fine structure dramatically but, unlike vocoding, does not degrade spectral details created by vocal tract resonances. The improvement in intelligibility, or masking release, due to introducing 16-Hz square-wave amplitude modulations in an otherwise steady speech-spectrum noise was reduced substantially with vocoded sentences relative to natural speech, but was not reduced for whispered sentences. In contrast to natural speech, masking release for whispered sentences was observed even at positive signal-to-noise ratios. Whispered speech has a different short-term amplitude distribution relative to natural speech, and this appeared to explain the robust masking release for whispered speech at high signal-to-noise ratios. Recognition of whispered speech was not disproportionately affected by unpredictable modulations created by a speech-envelope modulated noise masker. Overall, the presence or absence of periodic temporal fine structure did not have a major influence on the degree of benefit obtained from imposing temporal fluctuations on a noise masker.

Effect of priming on energetic and informational masking in a same-different task.

OBJECTIVES: The primary goal of this study was to investigate how speech perception is altered by the provision of a preview or "prime" of a sample of speech just before it is presented in masking. A same-different test paradigm was developed which enabled the effect of priming to be measured with energetic maskers in addition to those that most likely produced both energetic and informational masking. Using this paradigm, the benefit of priming in overcoming energetic and informational masking was compared. DESIGN: Twenty-four normal-hearing subjects listened to nonsense sentences presented in a background of competing speech (two-talker babble) or one of two types of speech-shaped noise. Both target and masker were presented via loudspeaker directly in front of the listeners. In the baseline condition, the listeners were then shown a sentence on a computer screen that either matched the auditory target sentence exactly or contained a replacement for one of the three target key words. Their task was to judge whether the printed sentence matched the auditory target and respond via computer keyboard. In the first experimental condition, the printed sentence preceded rather than followed the auditory presentation (the priming condition). In the second experimental condition, the perception of spatial separation was created between target and masker by presenting the masker from two loudspeakers (front and 60° to the right) and imposing a 4-msec delay in the masker coming from the front loudspeaker. This resulted in the target being heard from the front while, because of the precedence effect, the masker was heard well to the right (the spatial condition). In a third experimental condition, spatial separation and priming were combined. A total of five signal-to-noise ratios were tested for each masker. RESULTS: The competing speech masker produced more masking than noise, consistent with previous findings. For the competing speech masker, the signal-to-noise ratio for 80% correct performance was approximately 6.7 dB lower when the listeners read the sentences first (the priming condition) than in the baseline condition. This priming effect was similar to the improvement obtained when the target and masker were separated spatially. Significant priming effects were also observed with speech-shaped noise maskers, and when there was perceived spatial separation between target and masker, conditions in which informational masking was believed to have been minimal. There seemed to be an additive effect of spatial separation and priming in the two-talker babble condition. CONCLUSIONS: (1) Priming was effective in improving speech perception in all conditions, including those consisting of primarily energetic masking. (2) It is not clear how much benefit from priming could be attributed to release from informational masking. (3) Performance on the same-different task was linearly related to performance on an open-set speech recognition task using the same target and masker.

Manipulations of listeners' echo perception are reflected in event-related potentials.

To gain information from complex auditory scenes, it is necessary to determine which of the many loudness, pitch, and timbre changes originate from a single source. Grouping sound into sources based on spatial information is complicated by reverberant energy bouncing off multiple surfaces and reaching the ears from directions other than the source's location. The ability to localize sounds despite these echoes has been explored with the precedence effect: Identical sounds presented from two locations with a short stimulus onset asynchrony (e.g., 1-5 ms) are perceived as a single source with a location dominated by the lead sound. Importantly, echo thresholds, the shortest onset asynchrony at which a listener reports hearing the lag sound as a separate source about half of the time, can be manipulated by presenting sound pairs in contexts. Event-related brain potentials elicited by physically identical sounds in contexts that resulted in listeners reporting either one or two sources were compared. Sound pairs perceived as two sources elicited a larger anterior negativity 100-250 ms after onset, previously termed the object-related negativity, and a larger posterior positivity 250-500 ms. These results indicate that the models of room acoustics listeners form based on recent experience with the spatiotemporal properties of sound modulate perceptual as well as later higher-level processing.

Lateralization of noise-burst trains based on onset and ongoing interaural delays.

The lateralization of 250-ms trains of brief noise bursts was measured using an acoustic pointing technique. Stimuli were designed to assess the contribution of the interaural time delay (ITD) of the onset binaural burst relative to that of the ITDs in the ongoing part of the train. Lateralization was measured by listeners' adjustments of the ITD of a pointer stimulus, a 50-ms burst of noise, to match the lateral position of the target train. Results confirmed previous reports of lateralization dominance by the onset burst under conditions in which the train is composed of frozen tokens and the ongoing part contains multiple ambiguous interaural delays. In contrast, lateralization of ongoing trains in which fresh noise tokens were used for each set of two alternating (left-leading/right-leading) binaural pairs followed the ITD of the first pair in each set, regardless of the ITD of the onset burst of the entire stimulus and even when the onset burst was removed by gradual gating. This clear lateralization of a long-duration stimulus with ambiguous interaural delay cues suggests precedence mechanisms that involve not only the interaural cues at the beginning of a sound, but also the pattern of cues within an ongoing sound.

Aging, spatial cues, and single- versus dual-task performance in competing speech perception.

Older individuals often report difficulty coping in situations with multiple conversations in which they at times need to "tune out" the background speech and at other times seek to monitor competing messages. The present study was designed to simulate this type of interaction by examining the cost of requiring listeners to perform a secondary task in conjunction with understanding a target talker in the presence of competing speech. The ability of younger and older adults to understand a target utterance was measured with and without requiring the listener to also determine how many masking voices were presented time-reversed. Also of interest was how spatial separation affected the ability to perform these two tasks. Older adults demonstrated slightly reduced overall speech recognition and obtained less spatial release from masking, as compared to younger listeners. For both younger and older listeners, spatial separation increased the costs associated with performing both tasks together. The meaningfulness of the masker had a greater detrimental effect on speech understanding for older participants than for younger participants. However, the results suggest that the problems experienced by older adults in complex listening situations are not necessarily due to a deficit in the ability to switch and/or divide attention among talkers.

Release and re-buildup of listeners' models of auditory space.

When listeners hear sound presented repeatedly in a room with reflections, echo threshold rises. The current experiments tested how long this buildup in echo threshold would last when exposure to a different simulated space (designated as room B) intervened before returning to the original space (designated room A). Stimuli were trains of lead-lag click pairs (room A) and trains of clicks with no reflections (room B) in an ABA sequence. After buildup in room A, echo threshold for click pairs in room A decreased in direct relation to amount of intervening exposure to room B. After 11 click pairs of room B, the effect of exposure to room A was gone. A second buildup in echo threshold in room A was not differentially affected by prior exposure to room A or a different simulated room, room C. Listeners appear to form a model when exposed to sound in a particular space, which is lost quickly upon hearing sound in a different space. Storing previous models is inefficient because the processes of buildup and breakdown occur quickly to sound in a new space.

Binaural detection of 500-Hz tones in broadband and in narrowband masking noise: effects of signal/masker duration and forward masking fringes.

NoSpi thresholds for a 500-Hz tonal signal were measured with broadband and with narrowband maskers using a single-interval adaptive matrix procedure [C. Kaernbach, J Acoust. Soc. Am. 88, 2645-2655 (1990)]. The purpose of the study was to investigate and to account for the effects on thresholds of varying the durations of the signals and maskers and the durations of forward masking fringes that preceded the occurrence of signal-plus-noise. For detection in both broadband and narrowband noise, the addition of brief forward fringes of masking noise resulted in elevations in threshold for the shortest signal durations. Longer forward fringes led to larger decreases in threshold when the masker was broadband as compared to when the masker was narrowband. The complex patterning of the data was explained by the operation of: (1) "predetection" temporal integration associated with peripheral auditory filtering; (2) duration-dependent, across-frequency influences that differentially affect broadband and narrowband NoSpi thresholds, (3) "post-detection" temporal integration associated with the central binaural mechanism, and (4) consideration of the detection thresholds in terms of changes in interaural correlation rather than in terms of signal level or signal-to-noise ratio, per se.