Normal-hearing and hearing-impaired listeners were tested to determine F0 difference limens for synthetic tokens of 5 steady-state vowels. The same stimuli were then used in a concurrent-vowel labeling task with the F0 difference between concurrent vowels ranging between 0 and 4 semitones. Finally, speech recognition was tested for synthetic sentences in the presence of a competing synthetic voice with the same, a higher, or a lower F0. Normal-hearing listeners and hearing-impaired listeners with small F0-discrimination (deltaF0) thresholds showed improvements in vowel labeling when there were differences in F0 between vowels on the concurrent-vowel task. Impaired listeners with high deltaF0 thresholds did not benefit from F0 differences between vowels. At the group level, normal-hearing listeners benefited more than hearing-impaired listeners from F0 differences between competing signals on both the concurrent-vowel and sentence tasks. However, for individual listeners, deltaF0 thresholds and improvements in concurrent-vowel labeling based on F0 differences were only weakly associated with F0-based improvements in performance on the sentence task. For both the concurrent-vowel and sentence tasks, there was evidence that the ability to benefit from F0 differences between competing signals decreases with age.
Listeners with normal-hearing sensitivity recognize speech more accurately in the presence of fluctuating background sounds, such as a single competing voice, than in unmodulated noise at the same overall level. These performance differences are greatly reduced in listeners with hearing impairment, who generally receive little benefit from fluctuations in masker envelopes. If this lack of benefit is entirely due to elevated quiet thresholds and the resulting inaudibility of low-amplitude portions of signal + masker, then listeners with hearing impairment should derive increasing benefit from masker fluctuations as presentation levels increase. Listeners with normal-hearing (NH) sensitivity and listeners with hearing impairment (HI) were tested for sentence recognition at moderate and high presentation levels in competing speech-shaped noise, in competing speech by a single talker, and in competing time-reversed speech by the same talker. NH listeners showed more accurate recognition at moderate than at high presentation levels and better performance in fluctuating maskers than in unmodulated noise. For these listeners, modulated versus unmodulated performance differences tended to decrease at high presentation levels. Listeners with HI, as a group, showed performance that was more similar across maskers and presentation levels. Considered individually, only 2 out of 6 listeners with HI showed better overall performance and increasing benefit from masker fluctuations as presentation level increased. These results suggest that audibility alone does not completely account for the group differences in performance with fluctuating maskers; suprathreshold processing differences between groups also appear to play an important role. Competing speech frequently provided more effective masking than time-reversed speech containing temporal fluctuations of equal magnitude. This finding is consistent with "informational" masking resulting from competitive processing of words and phrases within the speech masker that would notoccur for time-reversed sentences.
The results suggest that distorted processing of audible speech cues was a primary factor accounting for differences in speech scores across subjects and that reduced ability to use TFS cues may be an important component of this distortion. The influence of TFS cues on speech scores was comparable in steady-state and modulated noise. Speech recognition was not related to audibility, represented by the SII, once high-frequency sensitivity differences across subjects (beginning at 5 kHz) were removed statistically. This might indicate that high-frequency hearing loss is associated with distortions in processing in lower-frequency regions.
Abnormal frequency resolution associated with sensorineural hearing impairment produces a smearing of spectral detail in the internal representation of complex acoustic stimuli. As a result, listeners with hearing loss may have difficulty locating spectral peaks (e.g., vowel formants) within stimuli which cue their identity. This study examined the relationship between frequency separation of peaks in a complex spectrum and the degree of spectral contrast preserved in the internal representations in normal and impaired auditory systems. Hearing-impaired and normal-hearing subjects discriminated a flat-spectrum bandpass stimulus from a stimulus containing a sinusoidal ripple across its frequency range. The peak-to-valley amplitude (in dB) necessary for detection of the ripple was measured for ripple frequencies ranging from 1 to 9 cycles/oct. Auditory filter characteristics were also measured at 1 and 3 kHz in order to examine the internal representations of the stimuli after cochlear processing. There were clear differences between groups in both auditory filter characteristics and spectral contrast detection. However, the amount of contrast in the internal representations predicted from these measurements was nearly the same for all subjects, suggesting that the reduced frequency resolution of the hearing-impaired group was largely responsible for differences in required peak-to-valley amplitude in the input spectra. Further, for all subjects, there was a trade-off between the absolute level of internal contrast necessary for ripple detection and the number of samples of this contrast available to the listener.
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