Relative contribution of target and masker temporal fine structure to the unmasking of consonants in noise.
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ABSTRACT:
SUBMITTER: Apoux F
PROVIDER: S-EPMC3253603 | biostudies-other | 2011 Dec
REPOSITORIES: biostudies-other
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Project description:Normal-hearing listeners receive less benefit from momentary dips in the level of a fluctuating masker for speech processed to degrade spectral detail or temporal fine structure (TFS) than for unprocessed speech. This has been interpreted as evidence that the magnitude of the fluctuating-masker benefit (FMB) reflects the ability to resolve spectral detail and TFS. However, the FMB for degraded speech is typically measured at a higher signal-to-noise ratio (SNR) to yield performance similar to normal speech for the baseline (stationary-noise) condition. Because the FMB decreases with increasing SNR, this SNR difference might account for the reduction in FMB for degraded speech. In this study, the FMB for unprocessed and processed (TFS-removed or spectrally smeared) speech was measured in a paradigm that adjusts word-set size, rather than SNR, to equate stationary-noise performance across processing conditions. Compared at the same SNR and percent-correct level (but with different set sizes), processed and unprocessed stimuli yielded a similar FMB for four different fluctuating maskers (speech-modulated noise, one opposite-gender interfering talker, two same-gender interfering talkers, and 16-Hz interrupted noise). These results suggest that, for these maskers, spectral or TFS distortions do not directly impair the ability to benefit from momentary dips in masker level.
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Project description:The obstruent consonants (e.g., stops) are more susceptible to noise than vowels, raising the question whether the degradation of speech intelligibility in noise can be attributed, at least partially, to the loss of information carried by obstruent consonants. Experiment 1 assesses the contribution of obstruent consonants to speech recognition in noise by presenting sentences containing clean obstruent consonants but noise-corrupted voiced sounds (e.g., vowels). Results indicated substantial (threefold) improvement in speech recognition, particularly at low signal-to-noise ratio levels (-5 dB). Experiment 2 assessed the importance of providing partial information, within a frequency region, of the obstruent-consonant spectra while leaving the remaining spectral region unaltered (i.e., noise corrupted). Access to the low-frequency (0-1000 Hz) region of the clean obstruent-consonant spectra was found to be sufficient to realize significant improvements in performance and that was attributed to improvement in transmission of voicing information. The outcomes from the two experiments suggest that much of the improvement in performance must be due to the enhanced access to acoustic landmarks, evident in spectral discontinuities signaling the onsets of obstruent consonants. These landmarks, often blurred in noisy conditions, are critically important for understanding speech in noise for better determination of the syllable structure and word boundaries.
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Project description:Background:Ecological communities of interacting species analyzed as complex networks have shown that species dependence on their counterparts is more complex than expected at random. As for other potentially mutualistic interactions, ant-plant networks mediated by extrafloral nectar show a nested (asymmetric) structure with a core of generalist species dominating the interaction pattern. Proposed factors structuring ecological networks include encounter probability (e.g., species abundances and habitat heterogeneity), behavior, phylogeny, and body size. While the importance of underlying factors that influence the structure of ant-plant networks have been separately explored, the simultaneous contribution of several biological and ecological attributes inherent to the species, guild or habitat level has not been addressed. Methods:For a tropical seasonal site we recorded (in 48 censuses) the frequency of pairwise ant-plant interactions mediated by extrafloral nectaries (EFN) on different habitats and studied the resultant network structure. We addressed for the first time the role of mechanistic versus neutral determinants at the 'fine-grain' structure (pairwise interactions) of ant-plant networks. We explore the simultaneous contribution of several attributes of plant and ant species (i.e., EFN abundance and distribution, ant head length, behavioral dominance and invasive status), and habitat attributes (i.e., vegetation structure) in prevailing interactions as well as in overall network topology (community). Results:Our studied network was highly-nested and non-modular, with core species having high species strengths (higher strength values for ants than plants) and low specialization. Plants had higher dependences on ants than vice versa. We found that habitat heterogeneity in vegetation structure (open vs. shaded habitats) was the main factor explaining network and fine-grain structure, with no evidence of neutral (abundance) effects. Discussion:Core ant species are relevant to most plants species at the network showing adaptations to nectar consumption and deterrent behavior. Thus larger ants interact with more plant species which, together with higher dependence of plants on ants, suggests potential biotic defense at a community scale. In our study site, heterogeneity in the ant-plant interactions among habitats is so prevalent that it emerges at community-level structural properties. High frequency of morphologically diverse and temporarily-active EFNs in all habitats suggests the relevance and seasonality of plant biotic defense provided by ants. The robust survey of ecological interactions and their biological/ecological correlates that we addressed provides insight of the interplay between adaptive-value traits and neutral effects in ecological networks.
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Project description:This study evaluated the role of temporal fine structure in the lateralization and understanding of speech in six normal-hearing listeners. Interaural time differences (ITDs) were introduced to invoke lateralization. Speech reception thresholds (SRTs) were evaluated in backgrounds of two-talker babble and speech-shaped noise. Two-syllable words with ITDs of 0 and 700 micros were used as targets. A vocoder technique, which systematically randomized fine structure, was used to evaluate the effects of fine structure on these tasks. Randomization of temporal fine structure was found to significantly reduce the ability of normal-hearing listeners to lateralize words, although for many listeners, good lateralization performance was achieved with as much as 80% fine-structure randomization. Most listeners demonstrated some rudimentary ability to lateralize with 100% fine-structure randomization. When ITDs were 0 micros, randomization of fine structure had a much greater effect on SRT in two-talker babble than in speech-shaped noise. Binaural advantages were also observed. In steady noise, the difference in SRT between words with 0- vs 700-micros ITDs was, on average, 6 dB with no fine-structure randomization and 2 dB with 100% fine-structure randomization. In two-talker babble this difference was 1.9 dB and, for most listeners, showed little effect of the degree of fine-structure randomization. These results suggest that (1) improved delivery of temporal fine structure would improve speech understanding in noise for implant recipients, (2) bilateral implant recipients might benefit from temporal envelope ITDs, and (3) improved delivery of temporal information could improve binaural benefits.
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Project description:ObjectivesPrevious studies investigating speech perception in noise have typically been conducted with static masker positions. The aim of this study was to investigate the effect of spatial separation of source and masker (spatial release from masking, SRM) in a moving masker setup and to evaluate the impact of adaptive beamforming in comparison with fixed directional microphones in cochlear implant (CI) users.DesignSpeech reception thresholds (SRT) were measured in S0N0 and in a moving masker setup (S0Nmove) in 12 normal hearing participants and 14 CI users (7 subjects bilateral, 7 bimodal with a hearing aid in the contralateral ear). Speech processor settings were a moderately directional microphone, a fixed beamformer, or an adaptive beamformer. The moving noise source was generated by means of wave field synthesis and was smoothly moved in a shape of a half-circle from one ear to the contralateral ear. Noise was presented in either of two conditions: continuous or modulated.ResultsSRTs in the S0Nmove setup were significantly improved compared to the S0N0 setup for both the normal hearing control group and the bilateral group in continuous noise, and for the control group in modulated noise. There was no effect of subject group. A significant effect of directional sensitivity was found in the S0Nmove setup. In the bilateral group, the adaptive beamformer achieved lower SRTs than the fixed beamformer setting. Adaptive beamforming improved SRT in both CI user groups substantially by about 3 dB (bimodal group) and 8 dB (bilateral group) depending on masker type.ConclusionsCI users showed SRM that was comparable to normal hearing subjects. In listening situations of everyday life with spatial separation of source and masker, directional microphones significantly improved speech perception with individual improvements of up to 15 dB SNR. Users of bilateral speech processors with both directional microphones obtained the highest benefit.
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Project description:Cochlear implant (CI) users are unable to receive masking release and the reasons are unclear. The present study examines the hypothesis that when listening to speech in fluctuating maskers, CI users cannot fuse the pieces of the message over temporal gaps because they are not able to perceive reliably the information carried by obstruent consonants (e.g., stops). To test this hypothesis, CI users were presented with sentences containing clean obstruent segments, but corrupted sonorant segments (e.g., vowels). Results indicated that CI users received masking release at low signal-to-noise ratio levels. Experiment 2 assessed the contribution of acoustic landmarks alone by presenting to CI users noise-corrupted stimuli which had clearly marked vowel/consonant boundaries, but lacking clean obstruent consonant information. These stimuli were created using noise-corrupted envelopes processed using logarithmic compression during sonorant segments and a weakly-compressive mapping function during obstruent segments. Results indicated that the use of segment-dependent compression yielded significant improvements in intelligibility, but no masking release. The results from these experiments suggest that in order for CI users to receive masking release, it is necessary to perceive reliably not only the presence and location of acoustic landmarks (i.e., vowel/consonant boundaries) but also the information carried by obstruent consonants.
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