The Binaural Interaction Component of the Auditory Brainstem Response Under Precedence Effect Conditions.

The purpose of this study was to measure the binaural interaction component (BIC) derived from click-evoked auditory brainstem responses (ABRs) using stimuli configured to elicit the Precedence Effect. The hypothesis was that the contribution of binaural processing to echo suppression can be evidenced by a diminished or absent BIC associated with the echo. Ten normal-hearing young adults provided ABRs generated by sequences of click pairs. Results showed that BICs elicited by diotic clicks in isolation were obliterated when those diotic clicks were preceded by a click pair having an interaural time difference of 400 µs and where the interclick interval was 8.4 ms. The presence of the leading click pair increased the latency of the ABR generated by the lagging diotic click pair but did not decrease its amplitude. The results were interpreted as indicating a contribution of binaural processing at the level of the brainstem to echo suppression, at least for the conditions tested here.

Masking Release for Speech in Modulated Maskers: Electrophysiological and Behavioral Measures.

OBJECTIVES: The purpose of this study was to obtain an electrophysiological analog of masking release using speech-evoked cortical potentials in steady and modulated maskers and to relate this masking release to behavioral measures for the same stimuli. The hypothesis was that the evoked potentials can be tracked to a lower stimulus level in a modulated masker than in a steady masker and that the magnitude of this electrophysiological masking release is of the same order as that of the behavioral masking release for the same stimuli. DESIGN: Cortical potentials evoked by an 80-ms /ba/ stimulus were measured in two steady maskers (30 and 65 dB SPL), and in a masker that modulated between these two levels at a rate of 25 Hz. In each masker, a level series was undertaken to determine electrophysiological threshold. Behavioral detection thresholds were determined in the same maskers using an adaptive tracking procedure. Masking release was defined as the difference between signal thresholds measured in the steady 65-dB SPL masker and the modulated masker. A total of 23 normal-hearing adults participated. RESULTS: Electrophysiological thresholds were uniformly elevated relative to behavioral thresholds by about 6.5 dB. However, the magnitude of masking release was about 13.5 dB for both measurement domains. CONCLUSIONS: Electrophysiological measures of masking release using speech-evoked cortical auditory evoked potentials correspond closely to behavioral estimates for the same stimuli. This suggests that objective measures based on electrophysiological techniques can be used to reliably gauge aspects of temporal processing ability.

Speech recognition in one- and two-talker maskers in school-age children and adults: Development of perceptual masking and glimpsing.

Children perform more poorly than adults on a wide range of masked speech perception paradigms, but this effect is particularly pronounced when the masker itself is also composed of speech. The present study evaluated two factors that might contribute to this effect: the ability to perceptually isolate the target from masker speech, and the ability to recognize target speech based on sparse cues (glimpsing). Speech reception thresholds (SRTs) were estimated for closed-set, disyllabic word recognition in children (5-16 years) and adults in a one- or two-talker masker. Speech maskers were 60 dB sound pressure level (SPL), and they were either presented alone or in combination with a 50-dB-SPL speech-shaped noise masker. There was an age effect overall, but performance was adult-like at a younger age for the one-talker than the two-talker masker. Noise tended to elevate SRTs, particularly for older children and adults, and when summed with the one-talker masker. Removing time-frequency epochs associated with a poor target-to-masker ratio markedly improved SRTs, with larger effects for younger listeners; the age effect was not eliminated, however. Results were interpreted as indicating that development of speech-in-speech recognition is likely impacted by development of both perceptual masking and the ability recognize speech based on sparse cues.

Factors affecting the development of speech recognition in steady and modulated noise.

This study used a checkerboard-masking paradigm to investigate the development of the speech reception threshold (SRT) for monosyllabic words in synchronously and asynchronously modulated noise. In asynchronous modulation, masker frequencies below 1300 Hz were gated off when frequencies above 1300 Hz were gated on, and vice versa. The goals of the study were to examine development of the ability to use asynchronous spectro-temporal cues for speech recognition and to assess factors related to speech frequency region and audible speech bandwidth. A speech-shaped noise masker was steady or was modulated synchronously or asynchronously across frequency. Target words were presented to 5-7 year old children or to adults. Overall, children showed higher SRTs and smaller masking release than adults. Consideration of the present results along with previous findings supports the idea that children can have particularly poor masked SRTs when the speech and masker spectra differ substantially, and that this may arise due to children requiring a wider speech bandwidth than adults for speech recognition. The results were also consistent with the idea that children are relatively poor in integrating speech cues when the frequency regions with the best signal-to-noise ratios vary across frequency as a function of time.

The effect of noise fluctuation and spectral bandwidth on gap detection.

Experiment 1 investigated gap detection for random and low-fluctuation noise (LFN) markers as a function of bandwidth (25-1600 Hz), level [40 or 75 dB sound pressure level (SPL)], and center frequency (500-4000 Hz). Gap thresholds for random noise improved as bandwidth increased from 25 to 1600 Hz, but there were only minor effects related to center frequency and level. For narrow bandwidths, thresholds were lower for LFN than random markers; this difference extended to higher bandwidths at the higher center frequencies and was particularly large at high stimulus level. Effects of frequency and level were broadly consistent with the idea that peripheral filtering can increase fluctuation in the encoded LFN stimulus. Experiment 2 tested gap detection for 200-Hz-wide noise bands centered on 2000 Hz, using high-pass maskers to examine spread of excitation effects. Such effects were absent or minor for random noise markers and the 40-dB-SPL LFN markers. In contrast, some high-pass maskers substantially worsened performance for the 75-dB-SPL LFN markers. These results were consistent with an interpretation that relatively acute gap detection for the high-level LFN gap markers resulted from spread of excitation to higher-frequency auditory filters where the magnitude and phase characteristics of the LFN stimuli are better preserved.

Speech-evoked ABR: Effects of age and simulated neural temporal jitter.

The speech-evoked auditory brainstem response (sABR) provides a measure of encoding complex stimuli in the brainstem, and this study employed the sABR to better understand the role of neural temporal jitter in the response patterns from older adults. In experiment 1, sABR recordings were used to investigate age-related differences in periodicity encoding of the temporal envelope and fine structure components of the response to a /da/speech token. A group of younger and a group of older adults (n = 22 per group) participated. The results demonstrated reduced amplitude of the fundamental frequency and harmonic components in the spectral domain of the recorded response of the older listeners. In experiment 2, a model of neural temporal jitter was employed to simulate in a group of young adults (n = 22) the response patterns measured from older adults. A small group of older adults (n = 7) were also tested under the jitter simulation conditions. In the young adults, the results showed a systematic reduction in the response amplitude of the most robust response components as the degree of applied jitter increased. In contrast, the older adults did not demonstrate significant response reduction when tested under jitter conditions. The overall pattern of results suggests that older adults have reduced neural synchrony for encoding periodic, complex signals at the level of the brainstem, and that this reduced synchrony can be modeled by simulating neural jitter via disruption of the temporal waveform of the stimulus.

Development of speech glimpsing in synchronously and asynchronously modulated noise.

This study investigated development of the ability to integrate glimpses of speech in modulated noise. Noise was modulated synchronously across frequency or asynchronously such that when noise below 1300 Hz was "off," noise above 1300 Hz was "on," and vice versa. Asynchronous masking was used to examine the ability of listeners to integrate speech glimpses separated across time and frequency. The study used the Word Intelligibility by Picture Identification (WIPI) test and included adults, older children (age 8-10 yr) and younger children (5-7 yr). Results showed poorer masking release for the children than the adults for synchronous modulation but not for asynchronous modulation. It is possible that children can integrate cues relatively well when all intervals provide at least partial speech information (asynchronous modulation) but less well when some intervals provide little or no information (synchronous modulation). Control conditions indicated that children appeared to derive less benefit than adults from speech cues below 1300 Hz. This frequency effect was supported by supplementary conditions where the noise was unmodulated and the speech was low- or high-pass filtered. Possible sources of the developmental frequency effect include differences in frequency weighting, effective speech bandwidth, and the signal-to-noise ratio in the unmodulated noise condition.

Across-frequency envelope correlation discrimination and masked signal detection.

This study compared the dependence of comodulation masking release (CMR) and monaural envelope correlation perception (MECP) on the degree of envelope correlation for the same narrowband noise stimuli. Envelope correlation across noise bands was systematically varied by mixing independent bands with a base set of comodulated bands. The magnitude of CMR fell monotonically with reductions in envelope correlation, and CMR varied over a range of envelope correlations that were not discriminable from each other in the MECP paradigm. For complexes of 100-Hz-wide noise bands, discrimination thresholds in the MECP task were similar whether the standard was a comodulated set of noise bands or a completely independent set of noise bands. This was not the case for 25-Hz-wide noise bands. Although the data demonstrate that CMR and MECP exhibit different dependencies on the degree of envelope correlation, some commonality across the two phenomena was observed. Specifically, for 25-Hz-wide bands of noise, there was a robust relationship between individual listeners' sensitivity to decorrelation from an otherwise comodulated set of noise bands and the magnitude of CMR measured for those same comodulated noise bands.

Wideband monaural envelope correlation perception.

This study investigated monaural envelope correlation perception (Richards 1987) for noise bandwidths ranging from 25 to 1,600 Hz. The high-frequency side of the low band was fixed at 3,000 Hz and the low-frequency side of the high band was fixed at 3,500 Hz. When comodulated, the magnitude spectra of the pair of noise bands were either identical or reflected around the midpoint. Six listeners with normal hearing participated. Listeners showed similar performance for identical and reflected-spectrum conditions, with best performance usually occurring for bandwidths between 200 and 800 Hz. Results were considered in terms of envelope comparisons of waveforms at the outputs of multiple peripheral filters or envelope comparisons of waveforms at the outputs of central filters set to the bandwidths of the noise stimuli. Some aspects of the results were incompatible with the account based on multiple peripheral filters. However, the results of a supplementary condition involving the gating of band subregions indicated that this incompatibility could be accounted for by nonoptimal weighting of peripheral filter outputs.

The monaural temporal window based on masking period pattern data in school-aged children and adults.

Several lines of evidence indicate that auditory temporal resolution improves over childhood, whereas other data implicate the development of processing efficiency. The present study used the masking period pattern paradigm to examine the maturation of temporal processing in normal-hearing children (4.8 to 10.7 yrs) compared to adults. Thresholds for a brief tone were measured at 6 temporal positions relative to the period of a 5-Hz quasi-square-wave masker envelope, with a 20-dB modulation depth, as well as in 2 steady maskers. The signal was a pure tone at either 1000 or 6500 Hz, and the masker was a band of noise, either spectrally wide or narrow (21.3 and 1.4 equivalent rectangular bandwidths, respectively). Masker modulation improved thresholds more for wide than narrow bandwidths, and adults tended to receive more benefit from modulation than young children. Fits to data for the wide maskers indicated a change in window symmetry with development, reflecting relatively greater backward masking for the youngest listeners. Data for children >6.5 yrs of age appeared more adult-like for the 6500- than the 1000-Hz signal. Differences in temporal window asymmetry with listener age cannot be entirely explained as a consequence of a higher criterion for detection in children, a form of inefficiency.

Monaural envelope correlation perception for bands narrower or wider than a critical band.

Monaural envelope correlation perception concerns the ability of listeners to discriminate stimuli based on the degree of correlation between the temporal envelopes of two or more frequency-separated bands of noise [Richards, J. Acoust. Soc. Am. 82, 1621-1630 (1987)]. Previous work has examined this ability for relatively narrow bandwidths, generally 100 Hz or less. The present experiment explored a wide range of bandwidths, from 25 to 1600 Hz, which included bands narrower and wider than a critical bandwidth. Stimuli were pairs of noise bands separated by a 500-Hz-wide spectral gap centered on 2250 Hz. The magnitude spectra of the pair of comodulated bands were either identical or reflected around the midpoint of the band, and performance was assessed with and without a low-pass noise masker. Although discrimination was best for intermediate bandwidths, mean performance was above chance for all bandwidths tested. Data were similar for stimuli with identical and reflected magnitude spectra, and for stimuli with and without the low-pass masker. The one exception was particularly good performance for intermediate-bandwidth stimuli with identical spectra, for which some listeners reported hearing a tonal cue. Results indicate that listeners are flexible in selecting spectral regions upon which to base across-frequency comparisons.

Factors affecting the processing of intensity in school-aged children.

PURPOSE: Thresholds of school-aged children are elevated relative to those of adults for intensity discrimination and amplitude modulation (AM) detection. It is unclear how these findings are related or what role stimulus gating and dynamic envelope cues play in these results. Two experiments assessed the development of sensitivity to intensity increments in different stimulus contexts. METHOD: Thresholds for detecting an increment in level were estimated for normal-hearing children (5- to 10-year-olds) and adults. Experiment 1 compared intensity discrimination for gated and continuous presentation of a 1-kHz tone, with a 65-dB-SPL standard level. Experiment 2 compared increment detection and 16-Hz AM detection introduced into a continuous 1-kHz tone, with either 35- or 75-dB-SPL standard levels. RESULTS: Children had higher thresholds than adults overall. All listeners were more sensitive to increments in the continuous than the gated stimulus and performed better at the 75- than at the 35-dB-SPL standard level. Both effects were comparable for children and adults. There was some evidence that children's AM detection was more adultlike than increment detection. CONCLUSION: These results imply that memory for loudness across gated intervals is not responsible for children's poor performance but that multiple dynamic envelope cues may benefit children more than adults.

Effects of masker envelope irregularities on tone detection in narrowband and broadband noise maskers.

Introducing coherent masker envelope modulation to frequency regions neighboring the signal frequency can reduce detection thresholds for a pure-tone signal. Verhey and Ernst (2009) reported that irregular masker modulation conferred greater benefit than regular modulation when the masker was broadband, but that there was no difference when the masker was narrowband. The present study evaluated two possible explanations for this result: one based on modulation adaptation and the other based on the introduction of relatively long-duration modulation minima in the irregular masker modulation condition. The first experiment replicated the results of Verhey and Ernst (2009), but also included conditions in which a 12.5-ms signal was presented in a 12.5-ms modulation minimum, which was exempted from envelope jitter. The second experiment used a continuous masker and suspended jitter during epochs associated with either a 12.5- or 87.5-ms signal. No benefit of masker envelope irregularity before or after the signal was observed in either experiment. These findings are inconsistent with an explanation based on modulation adaptation, implicating instead the introduction of relatively long-duration modulation minima in the large masking release obtained for a long-duration signal in an irregularly modulated masker.

Frequency modulation detection as a measure of temporal processing: age-related monaural and binaural effects.

The detection of low-rate frequency modulation (FM) carried by a low-frequency tone has been employed as a means of assessing the fidelity of temporal fine structure coding. Detection of low-rate FM can be made more acute, relative to the monaural case, by the addition of a pure tone to the contralateral ear. This study examined whether FM detection in the 500-Hz region could be further improved by using a binaural stimulation mode where the modulator was antiphasic across the two ears. The study also sought to determine whether these dichotic FM conditions were beneficial in identifying the emergence of a temporal fine structure processing deficiency relatively early in the aging process. Young, mid-aged, and older listeners (n = 12 per group) were tested. The results demonstrated better FM acuity in the dichotic task irrespective of listener age. Dichotic FM detection also differentiated between age groups more definitively than diotic detection, especially in terms of distinguishing mid-aged from older listeners. In the group of older listeners, dichotic FM detection was weakly associated with absolute sensitivity to the carrier. In addition, this group failed to show a dichotic benefit in the presence of a marked asymmetry in sensation level across ears. The overall pattern of results suggests that dichotic FM measurements have advantages over monaural measurements for the purposes of assessing age-related temporal processing effects, although a marked asymmetry in absolute thresholds across ears could undermine these advantages.

Binaural beat salience.

Previous studies of binaural beats have noted individual variability and response lability, but little attention has been paid to the salience of the binaural beat percept. The purpose of this study was to gauge the strength of the binaural beat percept by matching its salience to that of sinusoidal amplitude modulation (SAM), and to then compare rate discrimination for the two types of fluctuation. Rate discrimination was measured for standard rates of 4, 8, 16, and 32 Hz - all in the 500-Hz carrier region. Twelve normal-hearing adults participated in this study. The results indicated that discrimination acuity for binaural beats is similar to that for SAM tones whose depths of modulation have been adjusted to provide equivalent modulation salience. The matched-salience SAM tones had relatively shallow depths of modulation, suggesting that the perceptual strength of binaural beats is relatively weak, although all listeners perceived them. The Weber fraction for detection of an increase in binaural beat rate is roughly constant across beat rates, at least for rates above 4 Hz, as is rate discrimination for SAM tones.

Electrophysiological measurement of binaural beats: effects of primary tone frequency and observer age.

OBJECTIVE: The purpose of this study was to determine the reliability of the electrophysiological binaural beat steady state response as a gauge of temporal fine structure coding, particularly as it relates to the aging auditory system. The hypothesis was that the response would be more robust in a lower, than in a higher, frequency region and in younger, than in older, adults. DESIGN: Two experiments were undertaken. The first measured the 40 Hz binaural beat steady state response elicited by tone pairs in two frequency regions: lower (390 and 430 Hz tone pair) and higher (810 and 850 Hz tone pair). Frequency following responses (FFRs) evoked by the tones were also recorded. Ten young adults with normal hearing participated. The second experiment measured the binaural beat and FFRs in older adults but only in the lower frequency region. Fourteen older adults with relatively normal hearing participated. Response metrics in both experiments included response component signal-to-noise ratio (F statistic) and magnitude-squared coherence. RESULTS: Experiment 1 showed that FFRs were elicited in both frequency regions but were more robust in the lower frequency region. Binaural beat responses elicited by the lower frequency pair of tones showed greater amplitude fluctuation within a participant than the respective FFRs. Experiment 2 showed that older adults exhibited similar FFRs to younger adults, but proportionally fewer older participants showed binaural beat responses. Age differences in onset responses were also observed. CONCLUSIONS: The lower prevalence of the binaural beat response in older adults, despite the presence of FFRs, provides tentative support for the sensitivity of this measure to age-related deficits in temporal processing. However, the lability of the binaural beat response advocates caution in its use as an objective measure of fine structure coding.

Exploring the additivity of binaural and monaural masking release.

Experiment 1 examined comodulation masking release (CMR) for a 700-Hz tonal signal under conditions of N(o)S(o) (noise and signal interaurally in phase) and N(o)S(π) (noise in phase, signal out of phase) stimulation. The baseline stimulus for CMR was either a single 24-Hz wide narrowband noise centered on the signal frequency [on-signal band (OSB)] or the OSB plus, a set of flanking noise bands having random envelopes. Masking noise was either gated or continuous. The CMR, defined with respect to either the OSB or the random noise baseline, was smaller for N(o)S(π) than N(o)S(o) stimulation, particularly when the masker was continuous. Experiment 2 examined whether the same pattern of results would be obtained for a 2000-Hz signal frequency; the number of flanking bands was also manipulated (two versus eight). Results again showed smaller CMR for N(o)S(π) than N(o)S(o) stimulation for both continuous and gated masking noise. The CMR was larger with eight than with two flanking bands, and this difference was greater for N(o)S(o) than N(o)S(π). The results of this study are compatible with serial mechanisms of binaural and monaural masking release, but they indicate that the combined masking release (binaural masking-level difference and CMR) falls short of being additive.

Masked detection and discrimination of tone sequences under conditions of monaural and binaural masking release.

Experiment 1 examined detection and discrimination of monaural four-tone sequences composed of 400-, 500-, and 625-Hz sinusoids. In the baseline conditions, the masker was monaural composed of 25-Hz-wide bands of random noise centered on 320, 400, 500, 625, and 781 Hz. In the binaural masking release conditions, the noise was presented diotically. In the monaural masking release conditions, the noise was presented to the same ear as the signal, but it was comodulated. Tones had half-amplitude durations of 30, 60, or 150 ms. There was no delay between successive tones, so the rate of frequency change depended on tone duration. Listeners discriminated between sequences composed of 500-400-625-500 Hz and 500-625-400-500 Hz. Discrimination results were poor for rapid sequences in both monaural and binaural masking release conditions relative to baseline conditions. Results from experiment 2 indicated that poor discrimination for rapid sequences could also occur in the baseline conditions, provided that the frequency separation among tonal components was small. Sluggish processing in the present paradigm was not restricted to conditions relying on binaural cues. It is argued that sluggishness may reflect a long temporal window in monaural and binaural masking release conditions or an interaction between poor cue quality and task difficulty.

Processing of temporal fine structure as a function of age.

OBJECTIVES: The purpose of this study was to determine whether the processing of temporal fine structure diminishes with age, even in the presence of relatively normal audiometric hearing. Temporal fine structure processing was assessed by measuring the discrimination of interaural phase differences (IPDs). The hypothesis was that IPD discrimination is more acute in middle-aged observers than in older observers but that acuity in middle-aged observers is nevertheless poorer than in young adults. DESIGN: Two experiments were undertaken. The first measured discrimination of 0- and π-radian interaural phases as a function of carrier frequency. The stimulus was a 5-Hz sinusoidally amplitude-modulated tone in which, in the signal waveform, the interaural phase of the carrier was inverted during alternate modulation periods. The second experiment measured IPD discrimination at fixed frequencies. The stimulus was a pair of tone pulses in which, in the signal, the trailing pulse contained an IPD. A total of 39 adults with normal audiograms ≤2000 Hz participated in this study, of which 15 were younger, 12 middle aged, and 12 older. RESULTS: Experiment 1 showed that the highest carrier frequency at which a π-radian IPD could be discriminated from the diotic, 0-radian standard was significantly lower in middle-aged listeners than young adults, and still lower in older listeners. Experiment 2 indicated that middle-aged listeners were less sensitive to IPDs than young adults at all but the lowest frequencies tested. Older listeners, as a group, had the poorest thresholds. CONCLUSIONS: These results suggest that deficits in temporal fine structure processing are evident in the presenescent auditory system. This adds to the accumulating evidence that deficiencies in some aspects of auditory temporal processing emerge relatively early in the aging process. It is possible that early-emerging temporal processing deficits manifest themselves in challenging speech in noise environments.

Features of across-frequency envelope coherence critical for comodulation masking release.

The masking release associated with coherent amplitude modulation of the masker is dependent on the degree of envelope coherence across frequency, with the largest masking release for stimuli with perfectly comodulated envelopes. Experiments described here tested the hypothesis that the effects of reducing envelope coherence depend on the unique envelope features of the on-signal masker as compared to the flanking maskers. Maskers were amplitude-modulated tones (Experiments 1 and 3) or amplitude-modulated bands of noise (Experiment 2), and the signal was a tone; across-frequency masker coherence was manipulated to assess the effects of introducing additional modulation minima in either the on-signal or flanking masker envelopes of otherwise coherently modulated maskers. In all three experiments, the detrimental effect of disrupted modulation coherence was more severe when additional modulation minima were introduced in the flanking as compared to on-signal masker envelopes. This was the case for both ipsilateral and contralateral flanking masker presentations, indicating that within-channel cues were not responsible for this finding. Results are consistent with the interpretation that the cue underlying comodulation masking release is based on dynamic spectral features of the stimulus, with transient spectral peaks at the signal frequency reflecting addition of a signal.