Abstract:While considerable evidence suggests that bilateral cochlear implant (CI) users' sound localization abilities rely primarily on interaural level difference (ILD) cues, and only secondarily, if at all, on interaural time difference (ITD) cues, this evidence has largely been indirect. This study used head-related transfer functions (HRTFs) to independently manipulate ITD and ILD cues and directly measure their contribution to bilateral CI users' localization abilities. The results revealed a strong reliance on ILD cues, but some CI users also made use of ITD cues. The results also suggest a complex interaction between ITD and ILD cues.

The ability to recognize speech in steady-state noise cannot be predicted from the audiogram. A new classification scheme of hearing impairment based on the audiogram and the speech reception in noise thresholds, as measured with the HINT, may be useful for the characterization of the hearing ability in the global sense. This classification scheme is consistent with Plomp's two aspects of hearing ability (Plomp, 1978).

Binaural abilities are difficult to assess, yet important to understand, in the course of rehabilitating bilateral cochlear implantees. One purpose of this research was to develop a binaural assessment methodology using direct electrical input to the cochlear implant, ''direct connect assessment'', pre-processed by appropriate headrelated transfer functions (HRTFs) to simulate the binaural cues for spatial release from masking and sound localization. A second purpose was to create and evaluate new, modified rules for adaptively measuring the speech reception threshold in noise with the Hearing In Noise Test (HINT) for use with cochlear implant subjects. A third purpose was to develop a new sound localization protocol whose difficulty is adjusted to the sound localization ability of the individual cochlear implant subject. These new protocols were combined with the direct connect method and evaluated in acoustic hearing subjects and cochlear implant subjects. HINT thresholds and sound localization scores measured in the sound field and with the direct connect method did not differ significantly and were highly correlated in both groups. Measures of spatial release from masking, head shadow effects, binaural squelch, and binaural summation derived from the thresholds also were comparable for both methods. The alternative adaptive rules for HINT were found to measure different points on the same performance-intensity function. These rules allowed all cochlear implant subjects in the study to be tested adaptively and their thresholds to be compared to norms. Sound localization was measured successfully in all subjects with a sector localization protocol whose difficulty is to be adjusted to the localization ability of the subject. This protocol reduced by one-third to one-half the time required to complete the assessment of the localization ability.

A psychophysical investigation of timbre was undertaken with the intent of deriving quantitative results that could be useful in musical applications. Recordings of metal objects being struck with percussion mallets were rated by subjects on a unidimensional perceptual scale of perceived mallet hardness. Four acoustical parameters of the attack portion (first 325 ms) were defined and evaluated as predictors of perceived mallet hardness rating. To measure these parameters, a critical-band filter bank was employed. Two curves were extracted from the filter-bank output: (1) spectral level (log of area under spectrum) over time, and (2) spectral centroid over time. For each curve, two parameters were measured: the mean and the slope of the spectral level curve, and the mean and the time-weighted average of the spectral centroid curve. Multiple regression analysis was used to relate the perceptual ratings to these four acoustical parameters, and a good fit was achieved (multiple R-squared = 0.725, F = 1135.8, p less than 0.01). The resulting function is suitable for quantitatively predicting the perceptual dimension of perceived mallet hardness.

Objectives The hypothesis that extending the audible frequency bandwidth beyond the range currently implemented in most hearing aids can improve speech understanding was tested for normal-hearing and hearing-impaired participants using target sentences and spatially separated masking speech. Design The Hearing in Noise Test (HINT) speech corpus was re-recorded and four masking talkers were recorded at a sample rate of 44.1 kHz. All talkers were male native speakers of American English. Reception threshold for Sentences (RTS) were measured in two spatial configurations. In the asymmetric configuration, the target was presented from −45° azimuth and two colocated masking talkers were presented from +45° azimuth. In the diffuse configuration, the target was presented from 0° azimuth and four masking talkers were each presented from a different azimuth: +45°, +135°, −135°, and −45°. The new speech sentences, masking materials and configurations, collectively termed the ‘Hearing in Speech Test (HIST)’, were presented using lowpass filter cutoff frequencies of 4, 6, 8, and 10 kHz. For the normal-hearing participants, stimuli were presented in the sound field using loudspeakers. For the hearing-impaired participants, the spatial configurations were simulated using earphones, and a multi-band wide dynamic range compressor with a modified CAM2 fitting algorithm was used to compensate for each participant’s hearing loss. Results For the normal-hearing participants (N=24, mean age 40 years), the RTS improved significantly by 3.0 dB when the bandwidth was increased from 4 to 10 kHz, and a significant improvement of 1.3 dB was obtained from extending the bandwidth from 6 to 10 kHz, in both spatial configurations. Hearing-impaired participants (N=25, mean age 71 years) also showed a significant improvement in RTS with extended bandwidth, but the effect was smaller than for the normal-hearing participants. The mean decrease in RTS when the bandwidth was increased from 4 to 10 kHz was 1.3 dB for the asymmetric condition and 0.5 dB for the diffuse condition. Conclusions Extending bandwidth from 4 to 10 kHz can improve the ability of normal-hearing and hearing-impaired participants to understand target speech in the presence of spatially separated masking speech. Future studies of the benefits of extended high-frequency amplification should investigate other realistic listening situations, masker types, spatial configurations, and room reverberation conditions, to determine added value in overcoming the technical challenges associated with implementing a device capable of providing extended high-frequency amplification.