Christopher W. Turner

Use of temporal envelope cues in speech recognition by normal and hearing‐impaired listeners

The temporal acuity of listeners with sensorineural hearing loss is currently a matter of some controversy. In this study, the ability of normal-hearing and hearing-impaired listeners to utilize temporal cues of speech was measured directly. In addition to natural (unprocessed) nonsense syllables, several processed-speech conditions were employed. Nonsense syllables were digitally processed to remove the original spectral information, resulting in a time-varying speech envelope amplitude modulating a noise carrier. The processed-speech conditions were the envelope of a broadband speech signal modulating a broadband noise, a low-pass speech signal modulating a low-pass noise, a high-pass speech signal modulating a high-pass noise, and a two-channel signal comprised of the low- and high-pass modulated signals combined. Recognition of the envelope stimuli in quiet and also in modulated and steady noise backgrounds was tested. Listeners were tested at presentation levels yielding their maximum performance on a syllable recognition task. The hearing-impaired listeners performed more poorly on a recognition task than the normal-hearing listeners for unprocessed speech signals. However, for listeners with hearing losses of either flat or sloping configuration, there was no significant deficit in their ability to use temporal cues in speech, even in frequency regions of hearing loss up to 70 dB HL. These results demonstrate that moderate to severe sensorineural hearing loss does not impair the temporal (nonspectral) acuity of listeners in terms of speech recognition, when audibility of the stimuli is compensated for.

Recovery from prior stimulation II: Effects upon intensity discrimination

We obtained just-noticeable differences (jnds) for the intensity of pure tones following a forward masker. The masker was a 100-ms burst of narrow-band noise centered at 1000 Hz presented at 90 dB SPL; the pure-tone signal was at 1000 Hz and was 25 ms in duration. The masker-signal delay was 100 ms. Under these conditions, there is no threshold shift for the detection of the pure-tone signal following the forward masker. In contrast with the absence of a forward-masker effect upon detection thresholds, unusually large midlevel (40-60 dB SPL) jnds were observed. These large midlevel jnds were measured as a function of signal delay, revealing that they are not completely recovered to the normal (unmasked) values by 400 ms. We interpret these data as a consequence of the slower recovery of low-spontaneous rate, high-threshold neurons following prior stimulation (Relkin and Doucet, 1990). These experiments may therefore provide psychophysical evidence that the low-spontaneous rate, high-threshold neurons are a necessary physiological component in the coding of the large dynamic range for intensity. In addition, the present data provide evidence that the assumption that the effect of forward masking is limited to 100-200 ms is inappropriate, as this recovery time does not necessarily apply to suprathreshold tasks.

Binaural loudness matches in unilaterally impaired listeners

Binaural loudness matching data using a 2IFC adaptive procedure were obtained in high-frequency, unilateral cochlear-impaired listeners. The matches were obtained at frequencies where both ears had similarly normal thresholds, and also at other frequencies where the impaired ear had various degrees of hearing loss. In these listeners, one presumed difference between the ears is the limited or altered spread of excitation in the impaired ear. In agreement with previous studies using other approaches (Hellman, 1974, 1978; Hellman & Meiselman, 1986; Moore, Glasberg, Hess & Birchall, 1985; Schneider & Parker, 1987), the results of the present study suggest that both the range and the slope of loudness growth function are not dependent on the spread of excitation, but instead are related primarily to the degree of threshold elevation at the test frequency. Following this suggestion, a spread-of-excitation-independent model, based upon a group of neurons with the same characteristic frequency (CF) but different thresholds, is proposed to account for loudness growth in both normal and recruitment cases. In particular, it is shown quantitatively that a compressed distribution of thresholds due to threshold elevation may be responsible for loudness recruitment in sensorineural hearing loss.

Growth of masking in sensorineural hearing loss

The purpose of the present study was to measure the growth of masking in both normal-hearing and sensorineural hearing-impaired subjects. The masker was a narrow band of noise centered at 1,000 Hz, and masked thresholds for signal frequencies both above and below the masker frequency were obtained for a range of masker levels. For signal frequencies above the masker frequency, the slopes of the growth of masking functions were greater than 1 dB/dB for the normal-hearing group, while for the hearing-impaired subjects the slopes were less than those of the normal subjects, and in many cases slopes were less than 1. The slope of masking was inversely related to the threshold at the signal frequency. These data support the concept that a loss of nonlinearity at the signal place is responsible for the slower growth of masking in hearing-impaired subjects for signal frequencies greater than the masker. In addition, the slower-than-normal growth of masking of the hearing-impaired subjects suggests that some hearing aid signal-processing strategies which provide greater amounts of high-frequency emphasis at high input levels may not be appropriate.

Effects of Noise and Hearing Loss upon Frequency Discrimination

Frequency discrimination in quiet and in the presence of several levels of low-frequency masking noise was measured at 500, 1,000 and 2,000 Hz in both normal-hearing and hearing-impaired subjects. The test signals were presented at 80 dB SPL; all test signals were at least 10 dB above each subjects masked threshold. As a group, the hearing impaired subjects had larger frequency difference limens than the normal subjects, especially in frequency regions of greatest sensitivity loss. The masking noise produced a similar increase in frequency difference limens in both groups. Although several previous speech recognition studies have demonstrated an interactive effect between the presence of hearing loss and masking noise, the results of the present study demonstrate that frequency discrimination performance for normal-hearing and hearing-impaired subjects is not differentially affected by the addition of masking noise.

Identification and Discrimination of Stop Consonants: Formants Versus Spectral Peaks

Identification and discrimination of three-formant, stop-consonant approximations were measured for several types of stimuli. Stimuli were synthesized using: 1) pure tones, 2) formant filters swept across a harmonic voicing-source spectrum, and 3) formant filters swept across a frication (noise) source to approximate whispered speech. Despite the fact that the pure tones represented the formant frequencies with the greatest spectral precision, discrimination and identification of the stimuli was more accurate for the broader spectrum “source and formant” stimuli.