Localization in reverberation with cochlear implants: predicting performance from basic psychophysical measures.

Users of bilateral cochlear implants (CIs) experience difficulties localizing sounds in reverberant rooms, even in rooms where normal-hearing listeners would hardly notice the reverberation. We measured the localization ability of seven bilateral CI users listening with their own devices in anechoic space and in a simulated reverberant room. To determine factors affecting performance in reverberant space we measured the sensitivity to interaural time differences (ITDs), interaural level differences (ILDs), and forward masking in the same participants using direct computer control of the electric stimulation in their CIs. Localization performance, quantified by the coefficient of determination r(2) and the root mean squared error, was significantly worse in the reverberant room than in anechoic conditions. Localization performance in the anechoic room, expressed as r(2), was best predicted by subject's sensitivity to ILDs. However, the decrease in localization performance caused by reverberation was better predicted by the sensitivity to envelope ITDs measured on single electrode pairs, with a correlation coefficient of 0.92. The CI users who were highly sensitive to envelope ITDs also better maintained their localization ability in reverberant space. Results in the forward masking task added only marginally to the predictions of localization performance in both environments. The results indicate that envelope ITDs provided by CI processors support localization in reverberant space. Thus, methods that improve perceptual access to envelope ITDs could help improve localization with bilateral CIs in everyday listening situations.

Sound localization in noise by normal-hearing listeners and cochlear implant users.

OBJECTIVES: This study aimed to characterize horizontal plane sound localization in interfering noise at different signal-to-noise ratios (SNRs) and to compare performance across normal-hearing listeners and users of unilateral and bilateral cochlear implants (CIs). CI users report difficulties with listening in noisy environments. Although their difficulties with speech understanding have been investigated in several studies, the ability to localize sounds in background noise has not extensively been examined, despite the benefits of binaural hearing being greatest in noisy situations. Sound localization is a measure of binaural processing and is thus well suited to assessing the benefit of bilateral implantation. The results will inform clinicians and implant manufacturers how to focus their efforts to improve localization with CIs in noisy situations. DESIGN: Six normal-hearing listeners, four unilateral, and 10 bilateral CI users indicated the perceived location of sound sources using a light pointer method. Target sounds were noise pulses played from one of 11 loudspeakers placed between -80 and +80 degrees in the frontal horizontal plane in the free field. Localization was assessed in quiet and in diffuse background noise at SNRs between +10 and -7 dB. Speech reception thresholds were measured and their relation to the localization results examined. RESULTS: Localization performance declined with decreasing SNR: target sounds were perceived closer to the median plane and the standard deviation of responses increased. Localization performance across groups was compared using a measure of "Spatial Resolvability" (SR). This measure gives the angular separation between two sound sources that would enable an ideal observer to correctly distinguish them 69.1% of the time. For all participants SR increased with decreasing SNR, that is, at low SNRs the spatial separation between sound sources remained distinguishable only when it was larger. Normal-hearing participants performed best, with SR between 1.4 and 5.1 degrees in quiet. Bilateral CI users showed SR between 8.3 and 43.6 degrees in quiet, corresponding approximately to the spatial resolution of normal-hearing listeners at an SNR of -5 dB. Most bilateral CI users had lost the ability to correctly determine which side the sound came from at an SNR of -3 dB. Overall, the SNR had to be at least +7 dB to achieve localization performance near to that in quiet for all bilateral CI users. No significant correlation was found between spatial resolution and speech reception thresholds, but the speech processor sensitivity setting did significantly affect performance. Unilateral CI users showed the most severe localization problems, with only two of four participants being able to correctly determine which side sounds came from in quiet. CONCLUSIONS: This study is the first to examine sound localization with CIs at various SNRs and to compare it with normal hearing. The results confirm that localization with CIs is strongly disrupted in noisy situations. Bilateral CIs were shown to be clearly superior over unilateral CIs for localization in quiet and in noisy situations. With bilateral CIs, localization declined at moderately high absolute noise levels (>63 dB SPL), suggesting that an extension of the acoustic-dynamic range to higher levels would be beneficial. The absence of a relation between speech reception thresholds and spatial resolution highlights the need for additional clinical tests to assess the binaural benefit of a second implant.

Algorithmic modeling of the irrelevant sound effect (ISE) by the hearing sensation fluctuation strength.

Background sounds, such as narration, music with prominent staccato passages, and office noise impair verbal short-term memory even when these sounds are irrelevant. This irrelevant sound effect (ISE) is evoked by so-called changing-state sounds that are characterized by a distinct temporal structure with varying successive auditory-perceptive tokens. However, because of the absence of an appropriate psychoacoustically based instrumental measure, the disturbing impact of a given speech or nonspeech sound could not be predicted until now, but necessitated behavioral testing. Our database for parametric modeling of the ISE included approximately 40 background sounds (e.g., speech, music, tone sequences, office noise, traffic noise) and corresponding performance data that was collected from 70 behavioral measurements of verbal short-term memory. The hearing sensation fluctuation strength was chosen to model the ISE and describes the percept of fluctuations when listening to slowly modulated sounds (f(mod) < 20 Hz). On the basis of the fluctuation strength of background sounds, the algorithm estimated behavioral performance data in 63 of 70 cases within the interquartile ranges. In particular, all real-world sounds were modeled adequately, whereas the algorithm overestimated the (non-)disturbance impact of synthetic steady-state sounds that were constituted by a repeated vowel or tone. Implications of the algorithm's strengths and prediction errors are discussed.

A system to simulate and reproduce audio-visual environments for spatial hearing research.

The article reports the experience gained from two implementations of the "Simulated Open-Field Environment" (SOFE), a setup that allows sounds to be played at calibrated levels over a wide frequency range from multiple loudspeakers in an anechoic chamber. Playing sounds from loudspeakers in the free-field has the advantage that each participant listens with their own ears, and individual characteristics of the ears are captured in the sound they hear. This makes an easy and accurate comparison between various listeners with and without hearing devices possible. The SOFE uses custom calibration software to assure individual equalization of each loudspeaker. Room simulation software creates the spatio-temporal reflection pattern of sound sources in rooms which is played via the SOFE loudspeakers. The sound playback system is complemented by a video projection facility which can be used to collect or give feedback or to study auditory-visual interaction. The article discusses acoustical and technical requirements for accurate sound playback against the specific needs in hearing research. An introduction to software concepts is given which allow easy, high-level control of the setup and thus fast experimental development, turning the SOFE into a "Swiss army knife" tool for auditory, spatial hearing and audio-visual research.

Compatibility of a magnetic position tracker with a cochlear implant system.

OBJECTIVES: The aim of this study was to reveal possible interspersion of a magnetic position tracking device into a cochlear implant system, which could lead to harmful signals on the implanted electrode. DESIGN: Signals at the output of the speech processor's compression stage and at the implant's electrode were recorded and analyzed for unwanted distortion or corrupted pulses related to the presence of the magnetic tracking device. RESULTS: No systematic impact of a Polhemus Liberty Latus magnetic tracking system on the output signals of an Advanced Bionics HiRes90k cochlear implant and Platinum Series speech processor was found. CONCLUSIONS: The results suggest no objections for using the Polhemus Liberty Latus magnetic position tracker with the Advanced Bionics Platinum series speech processor and HiRes90k cochlear implant in research, for example, on spatial hearing. This result is likely transferable to other cochlear implants because (i) all manufacturers adhere to equally high electromagnetic interference standards and (ii) electromagnetic signals used by the transmission links of current cochlear implants and trackers differ in frequency by roughly 2 decades, making interference unlikely.

Plasma nitrite concentrations reflect the degree of endothelial dysfunction in humans.

A reduced nitric oxide availability is a hallmark of endothelial dysfunction occurring early in atherosclerosis. Recently, we have shown that plasma nitrite mirrors acute changes in endothelial nitric oxide synthase activity in various mammals, including humans. Here, we examined the hypothesis that plasma nitrite levels are reduced in humans with endothelial dysfunction and the decrease is correlated with increasing numbers of cardiovascular risk factors (RF). Plasma nitrite concentrations were quantified by flow-injection analysis. The coefficient of variation for repeated measurements of plasma nitrite was <8%, and heart rate and blood pressure at the time of blood sampling had no significant effect on nitrite values measured (n=10). Baseline levels of plasma nitrite followed a normal distribution in each group studied and decreased progressively with increasing numbers of cardiovascular risk factors (n=351, p<0.001): 351+/-13 (0 RF), 261+/-10 (1 RF), 253+/-11 (2 RF), 222+/-18 (3 RF), and 171+/-29 nmol/L (4 RF). Intima media thickness (IMT) and flow-mediated dilation (FMD) were determined via ultrasound. Plasma nitrite and FMD levels were lower, whereas IMT was greater in individuals with endothelial dysfunction (n=12) compared to healthy volunteers (n=12). Nitrite correlated significantly with FMD (r=0.56, p<0.001) and inversely with IMT (r= -0.49, p<0.01). Plasma nitrite levels are reliably measurable in humans, indicate endothelial dysfunction, and correlate with cardiovascular risk factors. Future studies are necessary to identify the prognostic relevance of plasma nitrite determination in patients suffering from cardiovascular disease.