Brett A. Swanson

Adaptive dynamic range optimization for cochlear implants: a preliminary study.

Objective The present study investigated the acceptability and the effect of Adaptive dynamic range optimization (ADRO) on speech perception for cochlear implant subjects. ADRO is a preprocessing scheme that continuously adjusts the gain in each frequency band to optimize the signal in the output dynamic range. Design Speech processor programs were created with and without ADRO processing. Nine subjects were tested in the laboratory and encouraged to use both programs in everyday listening situations. Take-home experience was assessed with preference questionnaires. Speech perception performance was compared for the standard and ADRO programs using City University of New York (CUNY) sentences, consonant-nucleus-consonant (CNC) words, and closed set spondees presented in quiet. A range of presentation levels were used; from 70 to 40 dB sound pressure level (unweighted RMS). CUNY sentences were also presented in multi-talker babble with +15 dB and +10 dB signal to noise ratios. Results There was a significant improvement in speech perception scores with the ADRO programs compared with the standard. At 50 dB, the mean open set sentence scores in quiet improved by 16% (p < 0.001). There was an improvement in mean word score for CNC words presented at 60 dB of 9.5% (p < 0.001) and a 20% improvement in mean score for spondees presented at 40 dB. There was no significant difference in sentence scores between the ADRO and the standard program for sentences presented in either noise condition. ADRO was the preferred program in 59% of listening situations, with five out of nine subjects indicating a strong overall preference and three subjects indicating a slight preference for ADRO. Conclusions Continual adjustment of channel gains using ADRO provided improved sound quality and improved speech perception performance. Therefore, ADRO is a viable alternative to fixed channel gain and offers a means for cochlear implantees to gain more benefit from their devices.

An adaptive Australian Sentence Test in Noise (AuSTIN).

Objectives: The aim of this research is to describe the development of an adaptive Australian Sentence Test in Noise and to validate the test in terms of test–retest reliability and efficiency using data obtained from its clinical application. Design: The relative intelligibility of 1264 Bamford-Kowal-Bench (BKB)–like sentences in the presence of competing four-talker babble was assessed with cochlear implant recipients. Intensity adjustments to the babble segments were made to reduce intersentence variability. Computer software was developed to administer an adaptive speech reception threshold (SRT) test using these adjusted sentence/babble pairs and test–retest SRT data from a separate group of 23 cochlear implant recipients was analyzed, comparing different SRT calculation and test stopping rules. Results: The adjusted sentence/babble pairs were used in clinical studies to obtain an SRT by presenting 32 sentences. Analysis of test–retest pairs of SRT data from 23 recipients indicated that a psychometric fit SRT calculation rule provided better reliability than did the Hearing in Noise Test (HINT) calculation rule, or rules based on mean turns. This rule, using the morpheme correct scores for each sentence, gave a standard deviation for a single SRT of 0.76 dB. Further analyses revealed that the test could be shortened to 20 sentences with an increase of 0.19 dB in variability, while reducing the median test time by approximately 2 min. Conclusions: This article reports validation data for a new Australian Sentence Test In Noise. When 20 BKB–like sentences are used with a psychometric fit calculation rule, a standard deviation of approximately 1 dB is obtained in approximately 3 min 36 sec.

Perceptual Interactions Between Electrodes Using Focused and Monopolar Cochlear Stimulation

In today’s cochlear implant (CI) systems, the monopolar (MP) electrode configuration is the most commonly used stimulation mode, requiring only a single current source. However, with an implant that will allow simultaneous activation of multiple independent current sources, it is possible to implement an all-polar (AP) stimulation mode designed to create a focused electrical field. The goal of this experiment was to study the potential benefits of this all-polar mode for reducing uncontrolled electrode interactions compared with the monopolar mode. The five participants who took part in the study were implanted with a research device that was connected via a percutaneous connector to a benchtop stimulator providing 22 independent current sources. The perceptual effects of the AP mode were tested in three experiments. In Experiment 1, the current level difference between loudness-matched sequential and simultaneous stimuli composed of 2 spatially separated pulse trains was measured as function of the electrode separation. Results indicated a strong current-summation interaction for simultaneous stimuli in the MP mode for separations up to at least 4.8 mm. No significant interaction was found in the AP mode beyond a separation of 2.4 mm. In Experiment 2, a forward-masking paradigm was used with fixed equally loud probes in AP and MP modes, and AP maskers presented on different electrode positions. Results indicated a similar spatial masking pattern between modes. In Experiment 3, subjects were asked to discriminate between across-electrode temporal delays. It was hypothesized that discrimination would decrease with electrode separation faster in AP compared to MP modes. However, results showed no difference between the two modes. Overall, the results indicated that the AP mode produced less current spread than MP mode but did not lead to a significant advantage in terms of spread of neuronal excitation at equally loud levels.

Investigating cochlear implant place‐pitch perception with the Modified Melodies test

Abstract There has been speculation that cochlear implant place-of-excitation cues could be more akin to the brightness attribute of timbre (the spectral profile) than to melodic pitch. As brightness can be ordered on a low-to-high scale, it would allow high scores on pitch-ranking tests. In contrast, the Modified Melodies test measures pitch perception in a melodic context. In each trial, a familiar melody was presented twice. In one presentation, randomly selected, the pitch was deliberately modified. The subjects task was to select the un-modified melody. Six Nucleus implant recipients were tested with melodies presented as pure tones in the frequency range C5–C6 (523–1046 Hz) through the ACE strategy on the Freedom processor. All subjects were able to identify incorrect melodic contours and three subjects were able to recognize errors in musical intervals. These results are consistent with the hypothesis that cochlear implant place cues alone are sufficient to convey a melody. Copyright

The Effect of Automatic Gain Control Structure and Release Time on Cochlear Implant Speech Intelligibility

Nucleus cochlear implant systems incorporate a fast-acting front-end automatic gain control (AGC), sometimes called a compression limiter. The objective of the present study was to determine the effect of replacing the front-end compression limiter with a newly proposed envelope profile limiter. A secondary objective was to investigate the effect of AGC speed on cochlear implant speech intelligibility. The envelope profile limiter was located after the filter bank and reduced the gain when the largest of the filter bank envelopes exceeded the compression threshold. The compression threshold was set equal to the saturation level of the loudness growth function (i.e. the envelope level that mapped to the maximum comfortable current level), ensuring that no envelope clipping occurred. To preserve the spectral profile, the same gain was applied to all channels. Experiment 1 compared sentence recognition with the front-end limiter and with the envelope profile limiter, each with two release times (75 and 625 ms). Six implant recipients were tested in quiet and in four-talker babble noise, at a high presentation level of 89 dB SPL. Overall, release time had a larger effect than the AGC type. With both AGC types, speech intelligibility was lower for the 75 ms release time than for the 625 ms release time. With the shorter release time, the envelope profile limiter provided higher group mean scores than the front-end limiter in quiet, but there was no significant difference in noise. Experiment 2 measured sentence recognition in noise as a function of presentation level, from 55 to 89 dB SPL. The envelope profile limiter with 625 ms release time yielded better scores than the front-end limiter with 75 ms release time. A take-home study showed no clear pattern of preferences. It is concluded that the envelope profile limiter is a feasible alternative to a front-end compression limiter.

A Statistical Method for the Analysis of Speech Intelligibility Tests.

Speech intelligibility tests are conducted on hearing-impaired people for the purpose of evaluating the performance of a hearing device under varying listening conditions and device settings or algorithms. The speech reception threshold (SRT) is typically defined as the signal-to-noise ratio (SNR) at which a subject scores 50% correct on a speech intelligibility test. An SRT is conventionally measured with an adaptive procedure, in which the SNR of successive sentences is adjusted based on the subjects scores on previous sentences. The SRT can be estimated as the mean of a subset of the SNR levels, or by fitting a psychometric function. A set of SRT results is typically analyzed with a repeated measures analysis of variance. We propose an alternative approach for analysis, a zero-and-one inflated beta regression model, in which an observation is a single sentence score rather than an SRT. A parametrization of the model is defined that allows efficient maximum likelihood estimation of the parameters. Fitted values from this model, when plotted against SNR, are analogous to a mean psychometric function in the traditional approach. Confidence intervals for the fitted value curves are obtained by parametric bootstrap. The proposed approach was applied retrospectively to data from two studies that assessed the speech perception of cochlear implant recipients using different sound processing algorithms under different listening conditions. The proposed approach yielded mean SRTs for each condition that were consistent with the traditional approach, but were more informative. It provided the mean psychometric curve of each condition, revealing differences in slope, i.e. differential performance at different parts of the SNR spectrum. Another advantage of the new method of analysis is that results are stated in terms of differences in percent correct scores, which is more interpretable than results from the traditional analysis.

Cochlear Implant Rate Pitch and Melody Perception as a Function of Place and Number of Electrodes

Six Nucleus cochlear implant recipients participated in a study investigating the effect of place of stimulation on melody perception using rate-pitch cues. Each stimulus was a pulse train delivered on either a single electrode or multiple electrodes sequentially. Four spatial stimulation patterns were used: a single apical electrode, a single mid electrode, a pair of electrodes (apical and mid), and 11 electrodes (from apical to mid). Within one block of trials, all stimuli had the same spatial stimulation pattern, with pulse rate varying from 131 to 262 pps. An additional pulse rate range of 262 to 523 pps was tested with the single-electrode stimuli. Two experimental procedures were used: note ranking; and a modified melodies test with backwards and warp modification. In each trial of the modified melodies test, a familiar melody and a version with modified pitch were presented (in random order), and the subject’s task was to select the unmodified melody. There were no significant differences in performance for stimulation on 1, 2, or 11 electrodes, implying that recipients were unable to combine temporal information from different places in the cochlea to give a stronger pitch cue. No advantage of apical electrodes was found: at the lower pulse rates, there were no significant differences between electrodes; and at the higher pulse rates, scores on the apical electrode dropped more than those on the mid electrode.

Effect of fast AGC on cochlear implant speech intelligibility

This study investigated the effect of fast-acting Automatic Gain Control (AGC) on the speech intelligibility of cochlear implant users as a function of presentation level. Both low and high signal-to-noise ratio conditions were investigated. The AGC substantially reduced the amount of clipping, but did not give consistent improvements in intelligibility. With no AGC, and high signal-to-noise ratio, speech scores were not significantly degraded until more than 25% of stimulation pulses were affected by clipping.

A simulation analysis of the variability of the roving level hearing test

In the study of auditory prostheses, the Speech Recognition Threshold (SRT) is the Signal to Noise Ratio (SNR) at which 50% of words are correctly identified. SRT is typically measured using an adaptive procedure wherein speech is presented at a fixed sound pressure level (SPL) and the noise level is varied according to the subjects responses. A roving level SRT test has been used by researchers with the goal of including the effectiveness of Automatic Gain Control (AGC) systems in SRT measurements. The roving method presents speech at three different SPLs with the level for each sentence chosen pseudo-randomly, while adaptively varying the SNR. This study used simulations to compare roving and fixed level SRT tests. It was found that roving level tests have significantly increased variability when there are level-dependent differences in subject scores. The interleaved level test is recommended as an alternative as it provides clear visibility of level-dependent performance and a better understanding of overall subject performance.