Deniz Başkent

Speech recognition in noise as a function of the number of spectral channels : Comparison of acoustic hearing and cochlear implants

Speech recognition was measured as a function of spectral resolution (number of spectral channels) and speech-to-noise ratio in normal-hearing (NH) and cochlear-implant (CI) listeners. Vowel, consonant, word, and sentence recognition were measured in five normal-hearing listeners, ten listeners with the Nucleus-22 cochlear implant, and nine listeners with the Advanced Bionics Clarion cochlear implant. Recognition was measured as a function of the number of spectral channels (noise bands or electrodes) at signal-to-noise ratios of + 15, + 10, +5, 0 dB, and in quiet. Performance with three different speech processing strategies (SPEAK, CIS, and SAS) was similar across all conditions, and improved as the number of electrodes increased (up to seven or eight) for all conditions. For all noise levels, vowel and consonant recognition with the SPEAK speech processor did not improve with more than seven electrodes, while for normal-hearing listeners, performance continued to increase up to at least 20 channels. Speech recognition on more difficult speech materials (word and sentence recognition) showed a marginally significant increase in Nucleus-22 listeners from seven to ten electrodes. The average implant score on all processing strategies was poorer than scores of NH listeners with similar processing. However, the best CI scores were similar to the normal-hearing scores for that condition (up to seven channels). CI listeners with the highest performance level increased in performance as the number of electrodes increased up to seven, while CI listeners with low levels of speech recognition did not increase in performance as the number of electrodes was increased beyond four. These results quantify the effect of number of spectral channels on speech recognition in noise and demonstrate that most CI subjects are not able to fully utilize the spectral information provided by the number of electrodes used in their implant.

Factors Affecting Auditory Performance of Postlinguistically Deaf Adults Using Cochlear Implants: An Update with 2251 Patients

Objective: To update a 15-year-old study of 800 postlinguistically deaf adult patients showing how duration of severe to profound hearing loss, age at cochlear implantation (CI), age at onset of severe to profound hearing loss, etiology and CI experience affected CI outcome. Study Design: Retrospective multicenter study. Methods: Data from 2251 adult patients implanted since 2003 in 15 international centers were collected and speech scores in quiet were converted to percentile ranks to remove differences between centers. Results: The negative effect of long duration of severe to profound hearing loss was less important in the new data than in 1996; the effects of age at CI and age at onset of severe to profound hearing loss were delayed until older ages; etiology had a smaller effect, and the effect of CI experience was greater with a steeper learning curve. Patients with longer durations of severe to profound hearing loss were less likely to improve with CI experience than patients with shorter duration of severe to profound hearing loss. Conclusions: The factors that were relevant in 1996 were still relevant in 2011, although their relative importance had changed. Relaxed patient selection criteria, improved clinical management of hearing loss, modifications of surgical practice, and improved devices may explain the differences.

Pre-, Per- and Postoperative Factors Affecting Performance of Postlinguistically Deaf Adults Using Cochlear Implants: A New Conceptual Model over Time

Objective To test the influence of multiple factors on cochlear implant (CI) speech performance in quiet and in noise for postlinguistically deaf adults, and to design a model of predicted auditory performance with a CI as a function of the significant factors. Study Design Retrospective multi-centre study. Methods Data from 2251 patients implanted since 2003 in 15 international centres were collected. Speech scores in quiet and in noise were converted into percentile ranks to remove differences between centres. The influence of 15 pre-, per- and postoperative factors, such as the duration of moderate hearing loss (mHL), the surgical approach (cochleostomy or round window approach), the angle of insertion, the percentage of active electrodes, and the brand of device were tested. The usual factors, duration of profound HL (pHL), age, etiology, duration of CI experience, that are already known to have an influence, were included in the statistical analyses. Results The significant factors were: the pure tone average threshold of the better ear, the brand of device, the percentage of active electrodes, the use of hearing aids (HAs) during the period of pHL, and the duration of mHL. Conclusions A new model was designed showing a decrease of performance that started during the period of mHL, and became faster during the period of pHL. The use of bilateral HAs slowed down the related central reorganization that is the likely cause of the decreased performance.

Interactions between cochlear implant electrode insertion depth and frequency-place mapping

While new electrode designs allow deeper insertion and wider coverage in the cochlea, there is still considerable variation in the insertion depth of the electrode array among cochlear implant users. The present study measures speech recognition as a function of insertion depth, varying from a deep insertion of 10 electrodes at 28.8 mm to a shallow insertion of a single electrode at 7.2 mm, in four Med-El Combi 40+ users. Short insertion depths were simulated by inactivating apical electrodes. Speech recognition increased with deeper insertion, reaching an asymptotic level at 21.6 or 26.4 mm depending on the frequency-place map used. Başkent and Shannon [J. Acoust. Soc. Am. 116, 3130-3140 (2004)] showed that speech recognition by implant users was best when the acoustic input frequency was matched onto the cochlear location that normally processes that frequency range, minimizing the spectral distortions in the map. However, if an electrode array is not fully inserted into the cochlea, a matched map will result in the loss of considerable low-frequency information. The results show a strong interaction between the optimal frequency-place mapping and electrode insertion depth. Consistent with previous studies, frequency-place matching produced better speech recognition than compressing the full speech range onto the electrode array for full insertion ranges (20 to 25 mm from the round window). For shallower insertions (16.8 and 19.2 mm) a mild amount of frequency-place compression was better than truncating the frequency range to match the basal cochlear location. These results show that patients with shallow electrode insertions might benefit from a map that assigns a narrower frequency range than patients with full insertions.

Speech recognition in normal hearing and sensorineural hearing loss as a function of the number of spectral channels

Speech recognition by normal-hearing listeners improves as a function of the number of spectral channels when tested with a noiseband vocoder simulating cochlear implant signal processing. Speech recognition by the best cochlear implant users, however, saturates around eight channels and does not improve when more electrodes are activated, presumably due to reduced frequency selectivity caused by channel interactions. Listeners with sensorineural hearing loss may also have reduced frequency selectivity due to cochlear damage and the resulting reduction in the nonlinear cochlear mechanisms. The present study investigates whether such a limitation in spectral information transmission would be observed with hearing-impaired listeners, similar to implant users. To test the hypothesis, hearing-impaired subjects were selected from a population of patients with moderate hearing loss of cochlear origin, where the frequency selectivity would be expected to be poorer compared to normal hearing. Hearing-impaired subjects were tested for vowel and consonant recognition in steady-state background noise of varying levels using a noiseband vocoder and as a function of the number of spectral channels. For comparison, normal-hearing subjects were tested with the same stimuli at different presentation levels. In quiet and low background noise, performance by normal-hearing and hearing-impaired subjects was similar. In higher background noise, performance by hearing-impaired subjects saturated around eight channels, while performance by normal-hearing subjects continued to increase up to 12-16 channels with vowels, and 10-12 channels with consonants. A similar trend was observed for most of the presentation levels at which the normal-hearing subjects were tested. Therefore, it is unlikely that the effects observed with hearing-impaired subjects were due to insufficient audibility or high presentation levels. Consequently, the results with hearing-impaired subjects were similar to previous results obtained with implant users, but only for background noise conditions.

Frequency-place compression and expansion in cochlear implant listeners

In multichannel cochlear implants, low frequency information is delivered to apical cochlear locations while high frequency information is delivered to more basal locations, mimicking the normal acoustic tonotopic organization of the auditory nerves. In clinical practice, little attention has been paid to the distribution of acoustic input across the electrodes of an individual patient that might vary in terms of spacing and absolute tonotopic location. In normal-hearing listeners, Başkent and Shannon (J. Acoust. Soc. Am. 113, 2003) simulated implant signal processing conditions in which the frequency range assigned to the array was systematically made wider or narrower than the simulated stimulation range in the cochlea, resulting in frequency-place compression or expansion, respectively. In general, the best speech recognition was obtained when the input acoustic information was delivered to the matching tonotopic place in the cochlea with least frequency-place distortion. The present study measured phoneme and sentence recognition scores with similar frequency-place manipulations in six Med-El Combi 40+ implant subjects. Stimulation locations were estimated using the Greenwood mapping function based on the estimated electrode insertion depth. Results from frequency-place compression and expansion with implants were similar to simulation results, especially for postlingually deafened subjects, despite the uncertainty in the actual stimulation sites of the auditory nerves. The present study shows that frequency-place mapping is an important factor in implant performance and an individual implant patients map could be optimized with functional tests using frequency-place manipulations.

Temporal target integration underlies performance at Lag 1 in the attentional blink

When two targets follow each other directly in rapid serial visual presentation (RSVP), they are often identified correctly but reported in the wrong order. These order reversals are commonly explained in terms of the rate at which the two targets are processed, the idea being that the second target can sometimes overtake the first in the race toward conscious awareness. The present study examined whether some of these order reversals might alternatively be due to a mechanism of temporal integration whereby targets appearing closely in time may be merged into a single representation. To test this integration account, we used an attentional blink task in which the two targets could be combined perceptually in a meaningful way such that the conjunction of the two target elements constituted a possible target stimulus itself. The results showed that when targets appeared at Lag 1, observers frequently reported seeing only a single merged target stimulus, and these reports occurred up to approximately three times as often as (real) order reversals. When the possibility to report the integrated percept was removed, order reversals consequently tripled. These results suggest that integration may actually be the primary cause of order reversals in dual-target RSVP tasks.

Effect of Speech Degradation on Top-Down Repair: Phonemic Restoration with Simulations of Cochlear Implants and Combined Electric–Acoustic Stimulation

The brain, using expectations, linguistic knowledge, and context, can perceptually restore inaudible portions of speech. Such top-down repair is thought to enhance speech intelligibility in noisy environments. Hearing-impaired listeners with cochlear implants commonly complain about not understanding speech in noise. We hypothesized that the degradations in the bottom-up speech signals due to the implant signal processing may have a negative effect on the top-down repair mechanisms, which could partially be responsible for this complaint. To test the hypothesis, phonemic restoration of interrupted sentences was measured with young normal-hearing listeners using a noise-band vocoder simulation of implant processing. Decreasing the spectral resolution (by reducing the number of vocoder processing channels from 32 to 4) systematically degraded the speech stimuli. Supporting the hypothesis, the size of the restoration benefit varied as a function of spectral resolution. A significant benefit was observed only at the highest spectral resolution of 32 channels. With eight channels, which resembles the resolution available to most implant users, there was no significant restoration effect. Combined electric–acoustic hearing has been previously shown to provide better intelligibility of speech in adverse listening environments. In a second configuration, combined electric–acoustic hearing was simulated by adding low-pass-filtered acoustic speech to the vocoder processing. There was a slight improvement in phonemic restoration compared to the first configuration; the restoration benefit was observed at spectral resolutions of both 16 and 32 channels. However, the restoration was not observed at lower spectral resolutions (four or eight channels). Overall, the findings imply that the degradations in the bottom-up signals alone (such as occurs in cochlear implants) may reduce the top-down restoration of speech.

Frequency transposition around dead regions simulated with a noiseband vocoder

In sensorineural hearing loss, damage to inner hair cells or the auditory nerve may result in dead regions in the cochlea, where the information transmission is disrupted. In cochlear implants, similar dead regions might appear if the spiral ganglia do not function. Shannon et al. [J. Assoc. Res. Otolaryngol. 3, 185-199 (2002)] simulated dead regions of varying size and location using a noiseband vocoder. Phoneme recognition by normal-hearing subjects was measured under two frequency-place mapping conditions: the frequency range corresponding to the dead region was (1) removed or (2) reassigned to bands adjacent to the dead region to simulate the off-frequency stimulation of neurons at the edge of a dead region. The present study extends the results of Shannon et al. by including a frequency transposition mapping condition, where the overall acoustic input frequency range was distributed over the entire remaining nondead region. The frequency transposed map provided more acoustic information when compared to the map with the frequency range corresponding to the dead region removed. However, speech perception did not improve for many simulated dead region conditions, possibly due to the spectral distortions in the frequency-place mapping.

Surface Profile Determination from Multiple Sonar Data Using Morphological Processing

This paper presents a novel method for surface profile determination using multiple sensors. Our approach is based on morphological processing techniques to fuse the range data from multiple sensor returns in a manner that directly reveals the target surface profile. The method has the intrinsic ability of suppressing spurious readings due to noise, crosstalk, and higher-order reflections, as well as processing multiple reflections informatively. The approach taken is extremely flexible and robust, in addition to being simple and straightforward. It can deal with arbitrary numbers and configurations of sensors as well as synthetic arrays. The algorithm is verified both by simulations and experiments in the laboratory by processing real sonar data obtained from a mobile robot. The results are compared to those obtained from a more accurate structured-light system, which is, however, more complex and expensive.