Adam Svec

Neural coding of formant-exaggerated speech in the infant brain

Speech scientists have long proposed that formant exaggeration in infant-directed speech plays an important role in language acquisition. This event-related potential (ERP) study investigated neural coding of formant-exaggerated speech in 6-12-month-old infants. Two synthetic /i/ vowels were presented in alternating blocks to test the effects of formant exaggeration. ERP waveform analysis showed significantly enhanced N250 for formant exaggeration, which was more prominent in the right hemisphere than the left. Time-frequency analysis indicated increased neural synchronization for processing formant-exaggerated speech in the delta band at frontal-central-parietal electrode sites as well as in the theta band at frontal-central sites. Minimum norm estimates further revealed a bilateral temporal-parietal-frontal neural network in the infant brain sensitive to formant exaggeration. Collectively, these results provide the first evidence that formant expansion in infant-directed speech enhances neural activities for phonetic encoding and language learning.

Inherent envelope fluctuations in forward maskers: Effects of masker-probe delay for listeners with normal and impaired hearing.

Forward-masked thresholds increase as the magnitude of inherent masker envelope fluctuations increase for both normal-hearing (NH) and hearing-impaired (HI) adults for a short masker-probe delay (25 ms). The slope of the recovery from forward masking is shallower for HI than for NH listeners due to reduced cochlear nonlinearities. However, effects of hearing loss on additional masking due to inherent envelope fluctuations across masker-probe delays remain unknown. The current study assessed effects of hearing loss on the slope and amount of recovery from forward maskers that varied in inherent envelope fluctuations. Forward-masked thresholds were measured at 2000 and 4000 Hz, for masker-probe delays of 25, 50, and 75 ms, for NH and HI adults. Four maskers at each center frequency varied in inherent envelope fluctuations: Gaussian noise (GN) or low-fluctuation noise (LFN), with 1 or 1/3 equivalent rectangular bandwidths (ERBs). Results suggested that slopes of recovery from forward masking were shallower for HI than for NH listeners regardless of masker fluctuations. Additional masking due to inherent envelope fluctuations was greater for HI than for NH listeners at longer masker-probe delays, suggesting that inherent envelope fluctuations are more disruptive for HI than for NH listeners for a longer time course.

Effects of inherent envelope fluctuations in forward maskers for listeners with normal and impaired hearing

Gaussian noise simultaneous maskers yield higher masked thresholds for pure tones than low-fluctuation noise simultaneous maskers for listeners with normal hearing. This increased masking effectiveness is thought to be due to inherent fluctuations in the temporal envelope of Gaussian noise, but effects of fluctuating forward maskers are unknown. Because differences in forward masking due to age and hearing loss are known, the current study assessed effects of masker envelope fluctuations for forward maskers in younger and older adults with normal hearing and older adults with hearing loss. Detection thresholds were measured in these three participant groups for a pure-tone probe in quiet and in Gaussian and low-fluctuation noise forward maskers with either 1 or 1/3 equivalent rectangular bandwidths. Higher masked thresholds were obtained for forward maskers with greater inherent envelope fluctuations for younger adults with normal hearing. This increased effectiveness of highly fluctuating forward maskers was similar for older adults with normal and impaired hearing. Because differences in recovery from forward masking between listeners with normal and impaired hearing may relate to differences in cochlear nonlinearities, these results suggest that mechanisms other than cochlear nonlinearities may be responsible for recovery from rapid masker envelope fluctuations.

Perceptual effects of fluctuating envelopes

Signal/masker envelope fluctuations have important effects on detection and discrimination. Narrowband Gaussian noise (GN) forward maskers yield higher masked thresholds for detecting pure tones than do low-fluctuation noise (LFN) forward maskers. The increased residual masking is due to inherent fluctuations in the temporal envelope of GN producing listener uncertainty. This uncertainty persists for longer durations for hearing-impaired (HI) than for normal-hearing (NH) listeners. In addition to listener uncertainty, amplitude-modulation (AM) forward masking may contribute to masking that occurs in complex listening tasks. In a recent study of AM forward masking, an unmodulated GN masker yielded more masking than an unmodulated LFN, suggesting that inherent envelope fluctuations were responsible for the amount of AM forward masking measured across listener groups. Contrary to predictions, there were no differences in AM forward masking between NH and HI listeners, revealing little effect of hearing loss ...

Modulation masking attributes of narrowband and low-noise noise forward maskers in normal-hearing and hearing-impaired listeners

Savel and Bacon (2003) measured detection thresholds for a 4000 Hz pure-tone signal in the presence of a narrowband noise (NBN) or a low-noise noise (LNN) simultaneous masker. The authors asserted that fluctuations in the envelope of the NBN were likely responsible for its increased masking effectiveness. Because modulation detection interference (MDI) is larger for hearing-impaired (HI) than normal-hearing (NH) listeners using modulated simultaneous maskers (e.g., Lorenzi et al., 1997) and forward maskers (e.g., Koopman et al., 2008), we measured detection thresholds for NH and HI listeners for pure tones in the presence of either NBN or LNN forward maskers that were either 1-ERB wide or 1/3-ERB wide. Based on previous MDI findings (Koopman et al., 2008), we predicted larger differences in masked thresholds for the NBN and LNN conditions for HI than NH listeners. These results for detection thresholds for pure-tone signals have implications for interpreting differences in modulated forward masking for NH...

Effects of hearing loss and linguistic context for phoneme and word recognition in noise.

This project measured whole-word and phoneme recognition in normally hearing (NH) and hearing-impaired (HI) participants listening in steady-state noise. Stimuli were modeled after those used in Olsen et al. [EarHear 1997] including 10-word lists of consonant-vowel-consonant (CVC) monosyllables containing 10 vowels and 20 consonants. Words were presented in isolation and in contextually correct sentences. Speech-shaped noise was presented at a range of signal-to-noise ratios, from −9 to + 3 dB SNR for NH listeners, and from −6 to + 6 dB SNR for HI listeners. Results suggested that HI listeners needed an increased proportion of components (pp) to recognize a whole word (pw) relative to their NH counterparts (e.g., a higher j value) when stimuli were presented at unfavorable SNRs. For -3 and 0 dB SNR conditions, j factors for NH listeners (0 dB SNR: j = 2.17) were consistently lower than those for HI listeners (0 dB SNR: j = 2.52). However, for the 3 dB SNR conditions, j factors were higher for NH listeners (j = 3.06) than those for HI listeners (j = 2.58). This suggests an unexpected interaction between hearing acuity, background noise, and the recognition of phonemes versus whole words. [Research supported by DC008306 to Peggy Nelson.]

Nonadditivity of forward and simultaneous masking.

The current study measured the additional masking obtained for combinations of forward and simultaneous maskers as a function of forward masker bandwidth, signal delay, and simultaneous masker level. The effects of the two individual maskers were equated in all conditions. Additional masking increased with increasing masker level, increasing signal delay, and decreasing masker bandwidth. The portion of the simultaneous masker that made the greater contribution to additional masking was the part that overlapped with the signal, not with the forward masker. The changes in additional masking observed as a function of forward masker bandwidth and the interaction between the effects of forward and simultaneous maskers call into question the use of additional masking as a measure of basilar membrane compression and present problems for the use of simultaneous noise to simulate hearing loss.

The effects of temporal envelope confusion on release of masking

Widened auditory filters in hearing impaired (HI) listeners may force them to rely more on temporal envelope (TE) cues when listening to speech. We propose that reduced masking release in HI listeners may be partially due to the confusion of the TEs of the masker and target. The current study investigates HI listeners comprehension of low- or high-pass vocoded spondees in the presence of fluctuating and stationary background noise. The spectral relationship of the target and masker was systematically varied from greater to no spectral overlap; the TEs of the masker and target were varied in similarity along two aspects — amplitude-modulation rate and shape. Preliminary data have shown the TE confusion in some HI impaired listeners results in speech understanding scores that are poorer in the presence of fluctuating noise (at a rate of 4Hz) than when the stationary noise is present. On the other hand, another group of HI listeners has demonstrated masking release. The effect of TE confusion of speech-en...

Variability in speech understanding in noise by listeners with hearing loss

Listeners with sensorineural hearing loss (SNHL) report significant difficulties when listening to speech in the presence of background noise and are highly variable in their tolerance to such noise. In our studies of speech perception, audibility predicts understanding of speech in quiet for most young listeners with SNHL. In background noise, however, the speech recognition performance of some young listeners with SNHL deviates significantly from audibility predictions. We hypothesize that vulnerability to background noise may be related to listeners’ broader auditory filters, to a loss of discrimination ability for rapid spectral changes, or to a disruption of the speech temporal envelopes by the addition of noise. Measures of spectral resolution, spectral change detection, and envelope confusion will be presented for listeners with SNHL. Relationships between those estimates and speech recognition in noise will be described. Results may suggest a range of custom strategies for improving tolerance for ...

Investigating the effect of speech stimuli on temporal masking release

The effect of masking release is still the source of numerous active investigations. However, differences in findings are sometimes noted among studies, especially related to potential gate frequency effects. The present study investigated the effect of masking release on listeners using a variety of speech materials . The main goal of this study was to investigate the effect of speech material on measures of masking release for listeners with normal hearing and hearing loss. The test stimuli were IEEE sentences and modified spondee words. To eliminate confounds of audibility and duration, the test stimuli were equated for duration and audibility. Listeners were tested across a wide range of audibility and gate frequencies. Performance and masking release results will be presented for these speech stimuli. [Work supported by NIDCD 008306.]