Todd A. Ricketts

Horizontal-Plane Localization of Noise and Speech Signals by Postlingually Deafened Adults Fitted With Bilateral Cochlear Implants *

Objectives: The main purpose of the study was to assess the ability of adults with bilateral cochlear implants to localize noise and speech signals in the horizontal plane. A second objective was to measure the change in localization performance in these adults between approximately 5 and 15 mo after activation. A third objective was to evaluate the relative roles of interaural level difference (ILD) and interaural temporal difference (ITD) cues in localization by these subjects. Design: Twenty-two adults, all postlingually deafened and all bilaterally fitted with MED-EL COMBI 40+ cochlear implants, were tested in a modified source identification task. Subjects were tested individually in an anechoic chamber, which contained an array of 43 numbered loudspeakers extending from –90° to +90° azimuth. On each trial, a 200-msec signal (either a noise burst or a speech sample) was presented from one of 17 active loudspeakers (span: ±80°), and the subject had to identify which source from the 43 loudspeakers in the array produced the signal. Subjects were tested in three conditions: left device only active, right device only active, and both devices active. Twelve of the 22 subjects were retested approximately 10 mo after their first test. In Experiment 2, the spectral content and rise-decay time of the noise stimulus were manipulated. Results: The relationship between source azimuth and response azimuth was characterized in terms of the adjusted constant error (Ĉ). (1) With both devices active, Ĉ for the noise stimulus varied from 8.1° to 43.4° (mean: 24.1°). By comparison, Ĉ for a group of listeners with normal hearing ranged from 3.5° to 7.8° (mean: 5.6°). When subjects listened in unilateral mode (with one device turned off), Ĉ was at or near chance (50.5°) in all cases. However, when considering unilateral performance on each subjects better side, average Ĉ for the speech stimulus was 47.9°, which was significantly (but only slightly) better than chance. (2) When listening bilaterally, error score was significantly lower for the speech stimulus (mean Ĉ = 21.5°) than for the noise stimulus (mean Ĉ = 24.1°). (3) As a group, the 12 subjects who were retested 10 mo after their first visit showed no significant improvement in localization performance during the intervening time. However, two subjects who performed very poorly during their first visit showed dramatic improvement (error scores were halved) over the intervening time. In Experiment 2, removing the high-frequency content of noise signals resulted in significantly poorer performance, but removing the low-frequency content or increasing the rise-decay time did not have an effect. Conclusions: In agreement with previously reported data, subjects with bilateral cochlear implants localized sounds in the horizontal plane remarkably well when using both of their devices, but they generally could not localize sounds when either device was deactivated. They could localize the speech signal with slightly, but significantly better accuracy than the noise, possibly due to spectral differences in the signals, to the availability of envelope ITD cues with the speech but not the noise signal, or to more central factors related to the social salience of speech signals. For most subjects the remarkable ability to localize sounds has stabilized by 5 mo after activation. However, for some subjects who perform poorly initially, there can be substantial improvement past 5 mo. Results from Experiment 2 suggest that ILD cues underlie localization ability for noise signals, and that ITD cues do not contribute.

Directional hearing AIDS.

A miniature hearing aid unit to be fitted to and carried on a persons head; there being a directional microphone in the housing which has front and rear sound admitting openings to respectively supply sounds to the fore and aft ports of the directional microphone; the rear opening in the hearing aid housing being located significantly closer to the inner side of the housing than the forward opening so that the alignment between front and rear openings is at an oblique angle of approximately 20° relative to the frontal direction.

Interaural time and level difference thresholds for acoustically presented signals in post-lingually deafened adults fitted with bilateral cochlear implants using CIS+ processing.

Objectives: The main purpose of the study was to measure thresholds for interaural time differences (ITDs) and interaural level differences (ILDs) for acoustically presented noise signals in adults with bilateral cochlear implants (CIs). A secondary purpose was to assess the correlation between the ILD and ITD thresholds and error scores in a horizontal-plane localization task, to test the hypothesis that localization by individuals with bilateral implants is mediated by the processing of ILD cues. Design: Eleven adults, all postlingually deafened and all bilaterally fitted with MED-EL COMBI 40+ CIs, were tested in ITD and ILD discrimination tasks in which signals were presented acoustically through headphones that fit over their two devices. The stimulus was a 200-msec burst of Gaussian noise bandpass filtered from 100 to 4000 Hz. A two-interval forced-choice adaptive procedure was used in which the subject had to respond on each trial whether the lateral positions of the two sound images (with the interaural difference favoring the left and right sides in the two intervals) moved from left-to-right or right-to-left. Results: In agreement with previously reported data, ITD thresholds for the subjects with bilateral implants were poor. The best threshold was ∼400 &mgr;sec, and only five of 11 subjects tested achieved thresholds <1000 &mgr;sec. In contrast, ILD thresholds were relatively good; mean threshold was 3.8 dB with the initial compression circuit on the implant devices activated and 1.9 dB with the compression deactivated. The ILD and ITD thresholds were higher than previously reported thresholds obtained with direct electrical stimulation (generally, <1.0 dB and 100 to 200 &mgr;sec, respectively). When the data from two outlying subjects were omitted, ILD thresholds were highly correlated with total error score in a horizontal-plane localization task, computed for sources near midline (r = 0.87, p < 0.01). Conclusions: The higher ILD and ITD thresholds obtained in this study with acoustically presented signals (when compared with prior data with direct electrical stimulation) can be attributed—at least partially—to the signal processing carried out by the CI in the former case. The processing strategy effectively leaves only envelope information as a basis for ITD discrimination, which, for the acoustically presented noise stimuli, is mainly coded in the onset information. The operation of the compression circuit reduces the ILDs in the signal, leading to elevated ILD thresholds for the acoustically presented signals in this condition. The large magnitude of the ITD thresholds indicates that ITDs could not have contributed to the performance in the horizontal-plane localization task. Overall, the results suggest that for subjects using bilateral implants, localization of noise signals is mediated entirely by ILD cues, with little or no contribution from ITD information.

Impact of noise source configuration on directional hearing aid benefit and performance.

Objective: To evaluate the impact of the position of noise source(s) and reverberation on the directional benefit and performance of three commercially available directional hearing aids. Design: Directional benefit and performance were measured for four different configurations of competing noise source(s) in two different reverberant rooms. Three pairs of hearing aids representing three commercial models were selected based on electroacoustic evaluation of directivity. Directional benefit and performance of 25 subjects with symmetrical, sloping, sensorineural hearing loss were measured in all test environments using a modified version of the Hearing in Noise Test. Results: Both reverberation and configuration of the competing noise source(s) significantly affected directional benefit and performance. There was no significant correlation between directional benefit and directional performance. The order of benefit and performance across hearing aid brands (from best to worst) varied depending on the noise source configuration. Conclusions: Data revealed increasing reverberation significantly decreased directional benefit and performance. The absolute and relative (rank ordering) directional benefit and performance varied across hearing aid brand, with noise source configuration. These results suggest that data collected in traditional test environments (e.g., a single competing noise placed at 180° azimuth) cannot be used to accurately predict directional benefit or performance in the majority of other test and real‐world environments. The impact of reverberation and noise source configuration on directional benefit/performance can be explained fairly well by the interaction between the spatial properties of the noise source(s) and the polar directivity patterns of the hearing aids.

Directivity quantification in hearing aids: fitting and measurement effects.

Objective: To evaluate the impact of venting, microphone port orientation, and compression on the electroacoustically measured directivity of directional and omnidirectional behind‐the‐ear hearing aids. In addition, the average directivity provided across three brands of directional and omnidirectional behind‐the‐ear hearing aids was compared with that provided by the open ear. Design: Three groups of hearing aids (four instruments in each group) representing three commercial models (a total of 12) were selected for electroacoustic evaluation of directivity. Polar directivity patterns were measured and directivity index was calculated across four different venting configurations, and for five different microphone port angles. All measurements were made for instruments in directional and omnidirectional modes. Single source traditional, and two‐source modified front‐to‐back ratios were also measured with the hearing aids in linear and compression modes. Results: The directivity provided by the open (Knowles Electronics Manikin for Acoustic Research) ear was superior to that of the omnidirectional hearing aids in this study. Although the directivity measured for directional hearing aids was significantly better than that of omnidirectional models, significant variability was measured both within and across the tested models both on average and at specific test frequencies. Both venting and microphone port orientation affected the measured directivity. Although compression reduced the magnitude of traditionally measured front‐toback ratios, no difference from linear amplification was noted using a modified methodology. Conclusions: The variation in the measured directivity both within and across the directional microphone hearing aid brands suggests that manufacturers specification of directivity may not provide an accurate index of the actual performance of all individual instruments. The significant impact of venting and microphone port orientation on directivity indicate that these variables must be addressed when fitting directional hearing aids on hearing‐impaired listeners. Modified front‐to‐back ratio results suggest that compression does not affect the directivity of hearing aids, if it is assumed that the signal of interest from one azimuth, and the competing signal from a different azimuth, occur at the same time.

Evaluation of an adaptive, directional-microphone hearing aid.

The effectiveness of adaptive directional processing for improvement of speech recognition in comparison to non-adaptive directional and omnidirectional processing was examined across four listening environments intended to simulate those found in the real world. The test environment was a single, moderately reverberant room with four loudspeaker configurations: three with fixed discrete noise source positions and one with a single panning noise source. Sentence materials from the Hearing in Noise Test (HINT) and Connected Speech Test (CST) were selected as test materials. Speech recognition across all listening conditions was evaluated for 20 listeners fitted binaurally with Phonak Claro behind-the-ear (BTE) style hearing aids. Results indicated improved speech recognition performance with adaptive and non-adaptive directional processing over that measured with the omnidirectional processing across all four listening conditions. While the magnitudes of directional benefit provided to subjects listening in adaptive and fixed directional modes were similar in some listening environments, a significant speech recognition advantage was measured for the adaptive mode in specific conditions. The advantage for adaptive over fixed directional processing was most prominent when a competing noise was presented from the listeners sides (both fixed and panning noise conditions), and was partially predictable from electroacoustically measured directional pattern data. Sumario La efectividad del procesamiento direccional adaptable para mejorar el reconocimiento del lenguaje en comparación con el procesamiento omnidireccional o el direccional no adaptable, fue examinada en cuatro ambientes diferentes, tratando de simular aquellos encontrados en el mundo real. El ambiente de evaluación fue una habitación moderadamente reverberante con una configuratión de cuatro parlantes: tres con posiciones discretas y fijas de la fuente de ruido y uno con una fuente única de ruido variable. Se seleccionó material de oraciones de la Prueba de Auditión en Ruido (HINT) y de la Prueba de Lenguaje Conectado (CST) como material de evaluatión. El reconocimiento del lenguaje en todas las diferentes condiciones auditivas fue evaluado en 20 sujetos adaptados binauralmente con audifonos retroauriculares (BTE) del tipo Phonak Claro. Los resultados indicaron mejoria en el rendimiento de la función de reconocimiento del lenguaje con procesamiento direccional adaptable o no adaptable, comparado con el medido a través de procesamiento omnidireccional en los cuatro ambientes. En tanto la magnitud del beneficio direccional resultanle fue similar en algunos ambientes de audition para sujetos que escuchan en modo direccional adaptable y fijo, una ventaja significativa en el reconocimiento del lenguaje fue obtenida para el modo adaptable, en condiciones especificas. La ventaja del procesamiento direccional adaptable sobre el fijo fue más prominente cuando se presentaba un ruido de competencia a los lados del sujeto (tanto en condiciones de ruido fijo y variable), y fue parcialmente predecible con base en la información sobre el patrón direccional electroacústicamente medido.

Speech Recognition for Unilateral and Bilateral Cochlear Implant Modes in the Presence of Uncorrelated Noise Sources

Objective: The purpose of the current investigation was to compare speech recognition in noise for bilateral and unilateral modes within postlingually deafened, adult bilateral cochlear implant recipients. In addition, it was of interest to evaluate the time course of the bilateral speech-recognition advantage and the effect of changing signal-to-noise ratio (SNR) on the magnitude of the bilateral advantage. Design: In the first experiment, 16 postlingually deafened adults who were bilaterally implanted with the MED-EL C40+ cochlear device were evaluated in unilateral left, unilateral right, and bilateral conditions 4 to 7 mo after activation. Speech recognition in the presence of five spatially separated, uncorrelated noise sources was evaluated using both a single fixed SNR of +10 dB and an adaptive-SNR method. In a follow-up study, a subset of 10 participants was re-evaluated using an identical fixed-SNR method 12 to 17 mo after activation to examine the time course of speech-recognition performance in both unilateral and bilateral modes at a single SNR. A third study was performed with a subset of six participants to examine performance over a range of SNRs. In this study, speech recognition was measured 12 to 17 mo after activation in quiet and at +5, +10, +15, and +20 dB SNRs using the same five uncorrelated noise sources. Results: The speech-recognition data revealed a significant bilateral advantage of 3.3 dB using the adaptive-SNR method. A significant bilateral advantage of 9% was also measured using a fixed +10 dB SNR. Results from the second study revealed that experience resulted in a significant (11 to 20%) increase in speech-recognition-in-noise performance for both unilateral and bilateral modes; however, the magnitude of the bilateral advantage was not affected by experience. Results from the third study revealed the largest bilateral advantage at the poorest SNR evaluated. In addition, performance in quiet was significantly better than that measured in the presence of noise, even at the +20 dB SNR. Conclusions: The results of these experiments support a small but significant bilateral speech-recognition-in-noise advantage for cochlear implant recipients in an environment with multiple noise sources. This advantage is presumed to be attributable to the combined effects of binaural squelch and diotic summation. Although experience generally improved speech-recognition-in-noise performance in both unilateral and bilateral modes, a consistent bilateral advantage (approximately 10%) was measured at 4 to 7 mo and at 12 to 17 mo postactivation.

Full time directional versus user selectable microphone modes in hearing aids.

Objective The purpose of this experiment was to systematically examine hearing aid benefit as measured by speech recognition and self-assessment methods across omnidirectional and directional hearing aid modes. These data were used to compare directional benefit as measured by speech recognition in the laboratory to hearing aid wearers perceptions of benefit in everyday environments across full-time directional, full-time omnidirectional, and user selectable directional fittings. Identification of possible listening situations that resulted in different self reported hearing aid benefit as a function of microphone type was a secondary objective of this experiment. Design Fifteen adults with symmetrical, sloping sensorineural hearing loss were fitted bilaterally with in-the-ear (ITE) directional hearing aids. Measures of hearing aid benefit included the Profile of Hearing Aid Benefit (PHAB), the Connected Sentence Test (CST), the Hearing in Noise Test (HINT), and a daily use log. Additionally, two new subscales were developed for administration with the PHAB. These subscales were developed to specifically address situations in which directional hearing aids may provide different degrees of benefit than omnidirectional hearing aids. Participants completed these measures in three conditions: omnidirectional only (O), directional only with low-frequency gain compensation (D), and user-selectable directional/omnidirectional (DO). Results Results from the speech intelligibility in noise testing indicated significantly more hearing aid benefit in directional modes than omnidirectional. PHAB results indicated more benefit on the background noise subscale (BN) in the DO condition than in the O condition; however, this directional advantage was not present for the D condition. Although the reliability of the newly proposed subscales is as yet unknown, the data were interpreted as revealing a directional advantage in situations where the signal of interest was in front of the participant and a directional disadvantage in situations where the signal of interest was behind the listener or localization was required. Conclusions Laboratory directional benefit is reflected in self-assessment measures that focus on listening in noise when the sound source of interest is in front of the listener. The use of a directional hearing aid mode; however, may have either a positive, a neutral, or a negative impact on hearing aid benefit measured in noisy situations, depending on the specific listening situation.

Distance and reverberation effects on directional benefit.

Objective Understanding the potential benefits and limitations of directional hearing aids across a wide range of listening environments is important when counseling persons with hearing loss regarding realistic expectations for these devices. The purpose of this study was to examine the impact of speaker-to-listener distance on directional benefit in two reverberant environments, in which the dominate noise sources were placed close to the hearing aid wearer. In addition, speech transmission index (STI) measures made in the test environments were compared to measured sentence recognition to determine if performance was predictable across changes in distance, reverberation and microphone mode. Design The aided sentence recognition, in noise, for fourteen adult participants with symmetrical sensorineural hearing impairment was measured in six environmental conditions in both directional and omnidirectional modes. A single room, containing four uncorrelated noise sources served as the test environment. The room was modified to exhibit either low (RT60 = 0.3 sec) or moderate (RT60 = 0.9 sec) levels of reverberation. Sentence recognition was measured in both reverberant environments at three different speech loudspeaker-to-listener distances (1.2 m, 2.4 m, and 4.8 m). STI measures also were made in each of the 12 listening conditions (2 microphone modes × 3 distances × 2 reverberation environments). Results A decrease in directional benefit was measured with increasing distance in the moderate reverberation condition. Although reduced, directional benefit was still present in the moderately reverberant environment at the farthest speech speaker-to-listener distance tested in this experiment. A similar decrease with increasing speaker-to-listener distance was not measured in the low reverberation condition. The pattern of average sentence recognition results across varying distances and two different reverberation times agreed with the pattern of STI values measured under the same conditions. Conclusions Although these data support increased directional benefit in noise for reduced speaker-to-listener distance, some benefit was still obtained by listeners when listening beyond “effective” critical distance under conditions of low (300 msec) to moderate (900 msec) reverberation. It is assumed that the directional benefit was due to the reduction of the direct sound energy from the noise sources near the listener. The use of aided STI values for the prediction of average word recognition across listening conditions that differ in reverberation, microphone directivity, and speaker-to-listener distance also was supported.

How hearing aids, background noise, and visual cues influence objective listening effort.

Objectives: The purpose of this article was to evaluate factors that influence the listening effort experienced when processing speech for people with hearing loss. Specifically, the change in listening effort resulting from introducing hearing aids, visual cues, and background noise was evaluated. An additional exploratory aim was to investigate the possible relationships between the magnitude of listening effort change and individual listeners’ working memory capacity, verbal processing speed, or lipreading skill. Design: Twenty-seven participants with bilateral sensorineural hearing loss were fitted with linear behind-the-ear hearing aids and tested using a dual-task paradigm designed to evaluate listening effort. The primary task was monosyllable word recognition and the secondary task was a visual reaction time task. The test conditions varied by hearing aids (unaided, aided), visual cues (auditory-only, auditory-visual), and background noise (present, absent). For all participants, the signal to noise ratio was set individually so that speech recognition performance in noise was approximately 60% in both the auditory-only and auditory-visual conditions. In addition to measures of listening effort, working memory capacity, verbal processing speed, and lipreading ability were measured using the Automated Operational Span Task, a Lexical Decision Task, and the Revised Shortened Utley Lipreading Test, respectively. Results: In general, the effects measured using the objective measure of listening effort were small (~10 msec). Results indicated that background noise increased listening effort, and hearing aids reduced listening effort, while visual cues did not influence listening effort. With regard to the individual variables, verbal processing speed was negatively correlated with hearing aid benefit for listening effort; faster processors were less likely to derive benefit. Working memory capacity, verbal processing speed, and lipreading ability were related to benefit from visual cues. No variables were related to changes in listening effort resulting from the addition of background noise. Conclusions: The results of this study suggest that, on the average, hearing aids can reduce objectively measured listening effort. Furthermore, people who are slow verbal processors are more likely to derive hearing aid benefit for listening effort, perhaps because hearing aids improve the auditory input. Although background noise increased objective listening effort, no listener characteristic predicted susceptibility to noise. With regard to visual cues, while there was no effect on average of providing visual cues, there were some listener characteristics that were related to changes in listening effort with vision. Although these relationships are exploratory, they do suggest that these inherent listener characteristics like working memory capacity, verbal processing speed, and lipreading ability may influence susceptibility to changes in listening effort and thus warrant further study.