Loudness Coding Research Articles

Level coding by phase duration and asymmetric pulse shape reduce channel interactions in cochlear implants

Conventional loudness coding with CIs by pulse current amplitude has a disadvantage: Increasing the stimulation current increases the spread of excitation in the auditory nerve, resulting in stronger channel interactions at high stimulation levels. These limit the number of effective information channels that a CI user can perceive. Stimulus intensity information (loudness) can alternatively be transmitted via pulse phase duration. We hypothesized that loudness coding by phase duration avoids the increase in the spread of the electric field and thus leads to less channel interactions at high stimulation levels. To avoid polarity effects, we combined this coding with pseudomonophasic stimuli. To test whether this affects the spread of excitation, 16 acutely deafened guinea pigs were implanted with CIs and neural activity from the inferior colliculus was recorded while stimulating with either biphasic, amplitude-coded pulses, or pseudomonophasic, duration- or amplitude-coded pulses. Pseudomonophasic stimuli combined with phase duration loudness coding reduced the lowest response thresholds and the spread of excitation. We investigated the channel interactions at suprathreshold levels by computing the phase-locking to a pulse train in the presence of an interacting pulse train on a different electrode on the CI. Pseudomonophasic pulses coupled with phase duration loudness coding reduced the interference by 4-5% compared to biphasic pulses, depending on the place of stimulation. This effect of pseudomonophasic stimuli was achieved with amplitude coding only in the basal cochlea, indicating a distance- or volume dependent effect. Our results show that pseudomonophasic, phase-duration-coded stimuli slightly reduce channel interactions, suggesting a potential benefit for speech understanding in humans.

Dynamic Current Focusing: A Novel Approach to Loudness Coding in Cochlear Implants.

In an attempt to improve spectral resolution and speech intelligibility, several current focusing methods have been proposed to increase spatial selectivity by decreasing intracochlear current spread. For example, tripolar stimulation administers current to a central electrode and uses the two flanking electrodes as the return pathway, creating a narrower intracochlear electrical field and hence increases spectral resolution when compared with monopolar (MP) stimulation. However, more current is required, and in some patients, specifically the ones with high electrode impedances, full loudness growth cannot be supported because of compliance limits. The present study describes and analyses a new loudness encoding approach that uses tripolar stimulation near threshold and gradually broadens the excitation (by decreasing compensation coefficient σ) to increase loudness without the need to increase overall current. It is hypothesized that this dynamic current focusing (DCF) strategy increases spatial selectivity, especially at lower loudness levels, while maintaining maximum selectivity at higher loudness levels, without reaching compliance limits. Eleven adult cochlear implant recipients with postlingual hearing loss, with at least 9 months of experience with their HiRes90K implant, were selected to participate in this study. Baseline performance regarding speech intelligibility in noise (Dutch matrix sentence test), spectral ripple discrimination at 45 and 65 dB, and temporal modulation detection thresholds were assessed using their own clinical program, fitted on a Harmony processor. Subsequently, the DCF strategy was fitted on a research Harmony processor. Threshold levels were determined with σ = 0.8, which means 80% of current is returned to the flanking electrodes and the remaining 20% to the extracochlear ground electrode. Instead of increasing overall pulse magnitude, σ was decreased to determine most comfortable loudness. After 2 to 3 hr of adaptation to the research strategy, the same psychophysical measures were taken. At 45 dB, average spectral ripple scores improved significantly from 2.4 ripples per octave with their clinical program to 3.74 ripples per octave with the DCF strategy (p = 0.016). Eight out of 11 participants had an improved spectral resolution at 65 dB. Nevertheless, no significant difference between DCF and MP was observed at higher presentation levels. Both speech-in-noise and temporal modulation detection thresholds were equal for MP and DCF strategies. Subjectively, 2 participants preferred the DCF strategy over their own clinical program, 2 preferred their own strategy, while the majority of the participants had no preference. Battery life was decreased and ranged from 1.5 to 4 hr. The DCF strategy gives better spectral resolution, at lower loudness levels, but equal performance on speech tests. These outcomes warrant for a longer adaptation period to study long-term outcomes and evaluate if the outcomes in the ripple tests transfer to the speech scores. Further research, for example, with respect to fitting rules and reduction of power consumption, is necessary to make the DCF strategy suitable for routine clinical application.

Application of Real-Time Loudness Models Can Improve Speech Recognition for Cochlear Implant Users

The aim of cochlear implant (CI) stimulation strategies is to appropriately encode the important aspects of sound into a pattern of electrical stimulation. Recent research using numerical models of loudness perception has identified that there are large differences between how loudness is encoded by existing CI sound-processing strategies and how loudness is experienced by normally hearing listeners. In this paper, we present a new CI sound-coding algorithm aimed at addressing these discrepancies. This strategy, named SCORE, uses models of electric and acoustic loudness to modify the output of an existing CI sound-processing scheme in real time, so that the loudness changes are more accurately represented in the patterns of electrical stimulation. Five subjects (six implanted ears) were tested for understanding of speech presented at relatively low levels in quiet conditions. Using SCORE, subjects demonstrated an average 8.8 percentage-point statistically significant improvement in the number of words correctly identified relative to ACE, a commonly used stimulation strategy. These findings show that loudness changes over time are important for speech intelligibility, and that improving loudness coding in existing CI devices may lead to perceptual benefits.

Effects of consonant-vowel intensity ratio on loudness of monosyllabic words

Previous research has suggested that speech loudness is determined primarily by the vowel in consonant-vowel-consonant (CVC) monosyllabic words, and that consonant intensity has a negligible effect. The current study further examines the unique aspects of speech loudness by manipulating consonant-vowel intensity ratios (CVRs), while holding the vowel constant at a comfortable listening level (70 dB), to determine the extent to which vowels and consonants contribute differentially to the loudness of monosyllabic words with voiced and voiceless consonants. The loudness of words edited to have CVRs ranging from -6 to +6 dB was compared to that of standard words with unaltered CVR by 10 normal-hearing listeners in an adaptive procedure. Loudness and overall level as a function of CVR were compared for four CVC word types: both voiceless consonants modified; only initial voiceless consonants modified; both voiced consonants modified; and only initial voiced consonants modified. Results indicate that the loudness of CVC monosyllabic words is not based strictly on the level of the vowel; rather, the overall level of the word and the level of the vowel contribute approximately equally. In addition to furthering the basic understanding of speech perception, the current results may be of value for the coding of loudness by hearing aids and cochlear implants.

Loudness Perception in the Domestic Cat: Reaction Time Estimates of Equal Loudness Contours and Recruitment Effects

The domestic cat is the primary physiological model of loudness coding and recruitment. At present, there are no published descriptions of loudness perception in this species. This study used a reaction time task to characterize loudness perception in six behaviorally trained cats. The psychophysical approach was based on the assumption that sounds of equal loudness elicit responses of equal latency. The resulting equal latency contours reproduced well-known features of human equal loudness contours. At the completion of normal baseline measures, the cats were exposed to intense sound to investigate the behavioral correlates of loudness recruitment, the abnormally rapid growth of loudness that is commonly associated with hearing loss. Observed recruitment effects were similar in magnitude to those that have been reported in hearing-impaired humans. Linear hearing aid amplification is known to improve speech intelligibility but also exacerbate recruitment in impaired listeners. The effects of speech spectra and amplification on recruitment were explored by measuring the growth of loudness for natural and amplified vowels before and after sound exposure. Vowels produced more recruitment than tones, and the effect was exacerbated by the selective amplification of formant structure. These findings support the adequacy of the domestic cat as a model system for future investigations of the auditory processes that underlie loudness perception, recruitment, and hearing aid design.

Alternative pulse shapes in electrical hearing

Cochlear implants (CIs) stimulate the auditory nerve with trains of symmetric biphasic (BI) pulses. We review studies showing that more efficient stimulation can be achieved by modifying these pulses by (1) increasing the inter-phase gap (IPG) between the two phases of each pulse, thereby delaying the recovery of charge, (2) increasing the duration and decreasing the amplitude of one phase - so-called "pseudomonophasic (PS)" waveforms, and (3) combining the pseudomonophasic stimulus with an IPG in a "delayed pseudomonophasic" waveform (PS_IPG). These efficiency gains, measured using changes in threshold and loudness, occur at a wide range of pulse rates, including those commonly used in current CI systems. In monopolar mode, dynamic ranges are larger for PS and for long-IPG pulse shapes than for BI pulses, but this increase in DR is not accompanied by a higher number of discriminable loudness steps, and hence, in a better coding of loudness. Moreover, waveforms with relatively short and long interphase gaps do not yield different patterns of excitation despite the relatively large differences in threshold. Two important findings are that, contrary to data obtained in animal experiments, anodic currents are more effective than cathodic stimulation for human CI patients and that the thresholds decrease with increases in IPG over a much longer time course (more than 3 ms) than for animals. In this review it is discussed how these alternative pulse shapes may be beneficial in terms of reducing power consumption and channel interactions, which issues remain to be addressed, and how models contribute to guiding our research.

Cochlear mechanisms of frequency and intensity coding. II. Dynamic range and the code for loudness

Our preceding paper described SPL-dependent changes in the shape of transfer functions recorded from inner and outer hair cells as well as supporting cells, in the 500–2500 Hz regions of the Mongolian gerbil cochlea. As SPL was increased, large shifts were observed in the peak of the transfer function. A strongly compressive nonlinearity was also observed at CF. This paper examines the data from the perspective of intensity coding in the auditory periphery. Based on the data, we offer a new explanation for the mechanisms underlying the different dynamic ranges of low and high threshold auditory neurons. We also find that, for pure tone stimuli, the growth of excitation at the characteristic place saturates rapidly, and cannot encode the wide dynamic range of loudness. The data are analyzed to explore other excitation pattern candidates for loudness coding. The growth of the peak of the IHC transfer function, as well as the growth of the response area, have been found to be linearly related to loudness growth over most of its dynamic range. Implications of the data for auditory intensity coding are discussed.

Heterophonic auditory adaptation to critical and noncritical bandwidths.

Auditory adaptation effects were measured for narrow bands of noise both within and outside of the critical band for 1 000 Hz. In keeping with previous research, a pure-tone condition resulted in more adaptation than any of the noise band conditions. However, as the noise bands were increased in width, a significant decrease in adaptation was observed, which is not in agreement with earlier findings. Adaptation results converted to theoretical auditory units show a trend which conforms to the Hood-Weiler model for loudness coding.

Further test of a model for loudness coding after adaptation.

The present study attempted to fit Weiler and Hood's (1977) quantitative model for loudness coding to an additional set of simultaneous dichotic loudness balance (SDLB) adaptation data. The error of prediction for test values 10 dB above adapting intensity for 12 normal-hearing listeners was -2.7 to -4.7 dB, an average of -3.5 dB. Since the error of prediction was so nearly constant (S.D. = +/- 737 dB), it seemed reasonable to attribute it do differences in procedural technique, and to conclude that the loudness model does account for coding in the adaptation results.

An improved model for loudness coding during auditory adaptation.

A comparison of loudness adaptation measured at four baseline intensities and at test values 10 and 20 dB above the baseline intensities revealed an orderly decline. The magnitude of the decline in loudness adaptation with the increasing difference between the test values and the baseline was shown to agree with earlier theories. A method for closely estimating the change in loudness adaptation is presented in convenient graphic form and in algebraic equations.

Electrophysiological correlates of loudness recruitment in primates: TTS versus masking

In an earlier study, excellent agreement was observed between reaction time of a discreet behavioral response, as an index of loudness, and the amplitude of the cochlear whole-nerve action potential (N1), during recovery from a noise-induced TTS in primates. In this experiment, in the same behaviorally trained macaque monkeys with chronic inner-ear electrodes, threshold shifts of similar magnitude to those observed during TTS have been produced by 8-kHz octave-band noise masking. The observed patterns of recruitment, as evidenced by reaction times, are distinctly different for TTS and masking, while those evidenced by N1 amplitude are not. The possible significance, in terms of loudness coding, is discussed.

Electrophysiological correlates of behaviorally determined loudness recruitment in primates

Reaction time of a subject's response to an auditory task has been found to be an index of loudness in human experiments, and this technique has been used to generate equal-loudness curves in subhuman primates. In this experiment, macaque monkeys were conditioned to respond to 8-kHz tones and response reaction times were measured. Whole-nerve cochlear action potentials were recorded from chronic inner-ear electrodes. A loudness recruitment phenomenon was experimentally produced by a one-hour exposure to a high-intensity 8-kHz octave band of noise. Excellent agreement was observed between the reaction time function and the action potential input-output function at postexposure testing intervals of 0.5, 12, 24, 48, and 84 hours. These recruitment data are discussed in relation to theories of loudness coding.

Reaction Time as a Measure of Threshold and Suprathreshold Hearing. II. Subjects with Impaired Hearing

Reaction time has been used as a measure to evaluate hearing and hearing changes associated with drugs, lesions, and noise exposure. In this paper, experimentally produced hearing changes in monkeys are compared to hearing changes in man. Reaction time and other psychoacoustic measures are compared in human subjects. Temporary hearing loss was produced in monkey and man by exposure to pure tones one-half octave below a selected test frequency. Exposures produced threshold shifts and changes in the intensity function which were similar for both man and monkeys. These changes are similar to those reported by Moody [Adv. in Otorhinolaryngol. 20, (1973)] for the monkey and are comparable to hearing changes in human subjects with clinically diagnosed sensorineural hearing loss and recruitment. They can be contrasted to the hearing loss seen in man and monkey with a conductive loss. In the humans, the changes in hearing reflected in reaction time performance are similar to those observed in loudness matching studies in the laboratory and in the clinic. The results confirm that the reaction time task is indeed applicable to the experimental study of conductive and cochlear hearing losses and to physiological studies of threshold and loudness coding. [Work supported by PHS Grants NS 08181, RR 00166, NS S0637, and NS 05082.]

Compound action potential of the cochlear nerve evoked electrically. Electrophysiological study of the acoustic nerve (guinea pig).

Nine guinea pigs, anaesthetized and fully curarized, were used in the present experiments. A compound action pptential of the left cochlear nerve was evoked by electrical stimulation of a small section of the homolateral spiral lamina. This compound action potential was recorded for many hours. 1. The auditory nerve when stimulated electrically generates three peaks (N1, N2, N3). The appearance of N2 and N3 depends on the stimulating energy, N1 having the lowest threshold, N2 a higher and N3 the highest threshold. 2. The intensity function of the N1− or N2−- or N3 amplitude is a continuous but non-linear function with only one point of Inflexion. 3. Latencies get shorter as the stimulating energy increases. 4. The time needed for the three peaks to run through the cochlea differs by 0.1 msec. A peripheral coding of loudness generated by the auditory nerve fibres seems to exist.

Auditory Adaptation and the Coding of Loudness

An investigation was performed to test various predictions made by Small, based on data of Hood, Egan, and others. More adaptation occurred with equal test and adapting intensities than with the same test intensities and adapting intensities 10 dB less. No significant decrease occurred with the test and adapting values equal to one another over the case with an equal test intensity and an adapting intensity 10 dB greater. There was, however, significantly less adaptation that when the adapting tone was 30 dB greater. The first two findings are in accord with Small's theory (unmodified); the last is not. Generally with test and adapting tones equally intense, adaptation increased up to an asymptote at approximately 60 dB and then failed to increase significantly through 80 dB. Essentially, this agrees with Jerger and with Palva and Karja. Possible interpretations are discussed.