Current-level discrimination using bipolar and monopolar electrode configurations in cochlear implants

Comparison of Intensity Discrimination between Children Using Cochlear Implants and Typically Developing Children

OBJECTIVES

Differential sensitivity of intensity is known to be important for the perception of the relative distance of sounds in the environment, emotions of speakers, and localize sounds. However, a few features in listening devices, such as cochlear implants, used by individuals with hearing loss alter the output intensity heard by them. This makes soft sounds loud and loud sounds soft. Hence, the aim of the present study was to compare the intensity discrimination of children using cochlear implants with that of typically developing children.

MATERIALS AND METHODS

Intensity discrimination of 30 children (15 using cochlear implants and 15 typically developing children) was obtained for three warble tones (500 Hz, 1000 Hz, and 4000 Hz) and three vowels (/a/, /i/, and /u/). The responses of the two participant groups, obtained using a 3-alternative forced-choice technique, were compared.

RESULTS

Children using cochlear implants performed significantly poorer than typically developing children for the 4000 Hz warble tone and for the vowels /a/ and /u/. However, there was no significant difference for the remaining stimuli.

CONCLUSION

The study indicated that the intensity discrimination threshold varies as a function of the frequency of the signals in children using cochlear implants. Intensity discrimination for high-frequency tones was significantly poorer for typically developing children, but not for low-frequency tones. In contrast, children using cochlear implants performed similarly to typically developing children for the high-frequency vowel but not for the mid- and low-frequency vowel.

Comparisons between detection threshold and loudness perception for individual cochlear implant channels

OBJECTIVE

The objective of this study was to examine how the level of current required for cochlear implant listeners to detect single-channel electrical pulse trains relates to loudness perception on the same channel. The working hypothesis was that channels with relatively high thresholds, when measured with a focused current pattern, interface poorly to the auditory nerve. For such channels, a smaller dynamic range between perceptual threshold and the most comfortable loudness would result, in part, from a greater sensitivity to changes in electrical field spread compared to low-threshold channels. The narrower range of comfortable listening levels may have important implications for speech perception.

DESIGN

Data were collected from eight, adult cochlear implant listeners implanted with the HiRes90k cochlear implant (Advanced Bionics Corp.). The partial tripolar (pTP) electrode configuration, consisting of one intracochlear active electrode, two flanking electrodes carrying a fraction (σ) of the return current, and an extracochlear ground, was used for stimulation. Single-channel detection thresholds and most comfortable listening levels were acquired using the most focused pTP configuration possible (σ ≥ 0.8) to identify three channels for further testing-those with the highest, median, and lowest thresholds-for each subject. Threshold, equal-loudness contours (at 50% of the monopolar dynamic range), and loudness growth functions were measured for each of these three test channels using various pTP fractions.

RESULTS

For all test channels, thresholds increased as the electrode configuration became more focused. The rate of increase with the focusing parameter σ was greatest for the high-threshold channel compared to the median- and low-threshold channels. The 50% equal-loudness contours exhibited similar rates of increase in level across test channels and subjects. Additionally, test channels with the highest thresholds had the narrowest dynamic ranges (for σ ≥ 0.5) and steepest growth of loudness functions for all electrode configurations.

CONCLUSIONS

Together with previous studies using focused stimulation, the results suggest that auditory responses to electrical stimuli at both threshold and suprathreshold current levels are not uniform across the electrode array of individual cochlear implant listeners. Specifically, the steeper growth of loudness and thus smaller dynamic ranges observed for high-threshold channels are consistent with a degraded electrode-neuron interface, which could stem from lower numbers of functioning auditory neurons or a relatively large distance between the neurons and electrodes. These findings may have potential implications for how stimulation levels are set during the clinical mapping procedure, particularly for speech-processing strategies that use focused electrical fields.

Intensity coding in electric hearing: effects of electrode configurations and stimulation waveforms

OBJECTIVES

Current cochlear implants typically stimulate the auditory nerve with biphasic pulses and monopolar electrode configurations. Tripolar stimulation can increase spatial selectivity and potentially improve place pitch related perception but requires higher current levels to elicit the same loudness as monopolar stimulation. The present study combined delayed pseudomonophonasic pulses, which produce lower thresholds, with tripolar stimulation in an attempt to solve the power-performance tradeoff problem.

DESIGN

The present study systematically measured thresholds, dynamic range, loudness growth, and intensity discrimination using either biphasic or delayed pseudomonophonasic pulses under both monopolar and tripolar stimulation. Participants were five Clarion cochlear implant users. For each subject, data from apical, middle, and basal electrode positions were collected when possible.

RESULTS

Compared with biphasic pulses, delayed pseudomonophonasic pulses increased the dynamic range by lowering thresholds while maintaining comparable maximum allowable levels under both electrode configurations. However, delayed pseudomonophonasic pulses did not change the shape of loudness growth function and actually increased intensity discrimination limens, especially at lower current levels.

CONCLUSIONS

The present results indicate that delayed pseudomonophonasic pulses coupled with tripolar stimulation cannot provide significant power savings nor can it increase the functional dynamic range. Whether this combined stimulation could improve functional spectral resolution remains to be seen.

Extending the limits of place and temporal pitch perception in cochlear implant users

A series of experiments investigated the effects of asymmetric current waveforms on the perception of place and temporal pitch cues. The asymmetric waveforms were trains of pseudomonophasic (PS) pulses consisting of a short, high-amplitude phase followed by a longer (and lower amplitude) opposite-polarity phase. When such pulses were presented in a narrow bipolar ("BP+1") mode and with the first phase anodic relative to the most apical electrode (so-called PSA pulses), pitch was lower than when the first phase was anodic re the more basal electrode. For a pulse rate of 12 pulses per second (pps), pitch was also lower than with standard symmetric biphasic pulses in either monopolar or bipolar mode. This suggests that PSA pulses can extend the range of place-pitch percepts available to cochlear implant listeners by focusing the spread of excitation in a more apical region than common stimulation techniques. Temporal pitch was studied by requiring subjects to pitch-rank single-channel pulse trains with rates ranging from 105 to 1,156 pps; this task was repeated at several intra-cochlear stimulation sites and using both symmetric and pseudomonophasic pulses. For PSA pulses presented to apical electrodes, the upper limit of temporal pitch was significantly higher than that for all the other conditions, averaging 713 pps. Measures of discriminability obtained using the method of constant stimuli indicated that this pitch percept was probably weak. However, a multidimensional scaling study showed that the percept associated with a rate change, even at high rates, was orthogonal to that of a place change and therefore reflected a genuine change in the temporal pattern of neural activity.

Current-level discrimination and spectral profile analysis in multi-channel electrical stimulation

In experiment 1, six cochlear-implant (CI) listeners discriminated a stimulation pattern eliciting equal loudness for each electrode from a stimulation pattern in which the stimulation at one or more electrodes was increased (peak) or decreased (notch). Three cochlear locations and three bandwidths were tested, without and with level roving. Listeners could always detect peaks but not always notches. Increasing the bandwidth beyond two electrodes produced no improvement in just-noticeable differences (JNDs). JNDs for the basal location were higher than for the apical and middle locations, although listeners had highly individual tendencies. In experiment 2, listeners discriminated changes in the peak heights and notch depths. JNDs for higher peaks were better while JNDs for deeper notches were worse than for experiment 1. In experiment 3, listeners discriminated the electrode position of peaks or notches. JNDs were approximately one electrode. In experiment 4, the first three experiments were repeated with large amounts of level roving. There was no evidence that CI listeners performed an across-channel comparison in these tasks.

Microelectronic retinal prosthesis: I. A neurostimulator for the concurrent activation of multiple electrodes

An application specific integrated circuit (ASIC) capable of delivering charge to multiple electrodes in unison has been developed. The ASIC is designed to function as part of an epi-retinal prosthesis driving an array of electrodes in a hexagonal mosaic. This unique organization of electrodes and the use of current sources and sinks is implemented to reduce the electrical cross-talk that occurs when many electrodes are activated in unison. Due to the large numbers of electrodes needed to provide useful vision to implantees, the interleaving strategies used in cochlear implants will not suffice for a vision prosthesis, where the "frame rate" is important for acceptable perceptual outcomes. This paper describes the design, and architectural approaches and performance testing of the developed ASIC.

Spatially distinct functional output regions within the central nucleus of the inferior colliculus: implications for an auditory midbrain implant

The inferior colliculus central nucleus (ICC) has potential as a new site for an auditory prosthesis [i.e., auditory midbrain implant (AMI)] for deaf patients who cannot benefit from cochlear implants (CIs). We have previously shown that ICC stimulation achieves lower thresholds, greater dynamic ranges, and more localized, frequency-specific primary auditory cortex (A1) activation than CI stimulation. However, we also observed that stimulation location along the caudorostral (isofrequency) dimension of the ICC affects thresholds and frequency specificity in A1, suggesting possible differences in functional (output) organization within the ICC. In this study, we electrically stimulated different regions along the isofrequency laminas of the ICC and recorded the corresponding A1 activity in ketamine-anesthetized guinea pigs using multisite probes to systematically assess ICC stimulation location effects. Our results indicate that stimulation of more rostral and somewhat ventral regions within an ICC lamina achieves lower thresholds, smaller discriminable level steps, and larger evoked potentials in A1. We also observed longer first spike latencies, which correlated with reduced spiking precision, when stimulating in more caudal and dorsal ICC regions. These findings suggest that at least two spatially distinct functional output regions exist along an ICC lamina: a caudal-dorsal region and a rostral-ventral region. The AMI will be implanted along the tonotopic axis of the ICC to achieve frequency-specific activation. However, stimulation location along the ICC laminas affects response properties that have shown to be important for speech perception performance, and needs to be considered when implanting future AMI patients.

Predictions of Psychophysical Measurements for Sinusoidal Amplitude Modulated (SAM) Pulse-Train Stimuli From a Stochastic Model

Two approaches have been proposed to reduce the synchrony of the neural response to electrical stimuli in cochlear implants. One approach involves adding noise to the pulse-train stimulus, and the other is based on using a high-rate pulse-train carrier. Hypotheses regarding the efficacy of the two approaches can be tested using computational models of neural responsiveness prior to time-intensive psychophysical studies. In our previous work, we have used such models to examine the effects of noise on several psychophysical measures important to speech recognition. However, to date there has been no parallel analytic solution investigating the neural response to the high-rate pulse-train stimuli and their effect on psychophysical measures. This work investigates the properties of the neural response to high-rate pulse-train stimuli with amplitude modulated envelopes using a stochastic auditory nerve model. The statistics governing the neural response to each pulse are derived using a recursive method. The agreement between the theoretical predictions and model simulations is demonstrated for sinusoidal amplitude modulated (SAM) high rate pulse-train stimuli. With our approach, predicting the neural response in modern implant devices becomes tractable. Psychophysical measurements are also predicted using the stochastic auditory nerve model for SAM high-rate pulse-train stimuli. Changes in dynamic range (DR) and intensity discrimination are compared with that observed for noise-modulated pulse-train stimuli. Modulation frequency discrimination is also studied as a function of stimulus level and pulse rate. Results suggest that high rate carriers may positively impact such psychophysical measures.

Threshold and channel interaction in cochlear implant users: evaluation of the tripolar electrode configuration

The efficacy of cochlear implants is limited by spatial and temporal interactions among channels. This study explores the spatially restricted tripolar electrode configuration and compares it to bipolar and monopolar stimulation. Measures of threshold and channel interaction were obtained from nine subjects implanted with the Clarion HiFocus-I electrode array. Stimuli were biphasic pulses delivered at 1020 pulses/s. Threshold increased from monopolar to bipolar to tripolar stimulation and was most variable across channels with the tripolar configuration. Channel interaction, quantified by the shift in threshold between single- and two-channel stimulation, occurred for all three configurations but was largest for the monopolar and simultaneous conditions. The threshold shifts with simultaneous tripolar stimulation were slightly smaller than with bipolar and were not as strongly affected by the timing of the two channel stimulation as was monopolar. The subjects' performances on clinical speech tests were correlated with channel-to-channel variability in tripolar threshold, such that greater variability was related to poorer performance. The data suggest that tripolar channels with high thresholds may reveal cochlear regions of low neuron survival or poor electrode placement.

Current-level discrimination in the context of interleaved, multichannel stimulation in cochlear implants: effects of number of stimulated electrodes, pulse rate, and electrode separation

The ability of cochlear implantees to detect an increment in current level at one of many stimulated electrodes was investigated. Such changes in the electric profile provide information for cochlear implantees to discriminate numerous sounds, especially vowels. In Experiment 1, sensitivity to increases in current level at one stimulation site in the electric profile decreased as the number of stimulated electrodes increased. This outcome was most likely a result of decreased stimulus levels at individual electrodes that were required to retain a comfortable loudness when the number of active electrodes was increased. Experiment 2 investigated the effects of pulse rate and separation between stimulation sites when the levels in percent of dynamic range and number of stimulated electrodes were held constant. The effect of pulse rate and electrode separation varied among listeners. The sensitivity of 6 of 9 listeners was best at the pulse rate that they used clinically. This might have been the result of adaptation to the clinical pulse rate, or listeners might have chosen their inherently best pulse rate during the clinical fitting.

Auditory cortical responses to electrical stimulation of the inferior colliculus: implications for an auditory midbrain implant

The success and limitations of cochlear implants (CIs) along with recent advances in deep brain stimulation and neural engineering have motivated the development of a central auditory prosthesis. In this study, we investigated the effects of electrical stimulation of the inferior colliculus central nucleus (ICC) on primary auditory cortex (A1) activity to determine the potential benefits of an auditory midbrain implant (AMI). We recorded multiunit activity in A1 of ketamine-anesthetized guinea pigs in response to single-pulse (200 micros/phase) monopolar stimulation of the ICC using multisite silicon-substrate probes. We then compared measures of threshold, dynamic range, and tonotopic spread of activation for ICC stimulation with that of published data for CI stimulation. Our results showed that compared with cochlear stimulation, ICC stimulation achieved: 1) thresholds about 8 dB lower; 2) dynamic ranges > or = 4 dB greater; and 3) more localized, frequency-specific activation, even though frequency specificity was partially lost at higher stimulus levels for low-frequency ICC regions. Our results also showed that stimulation of rostral ICC regions elicited lower thresholds but with greater activation spread along the tonotopic gradient of A1 than did stimulation of more caudal regions. These results suggest that an AMI may improve frequency and level coding with lower energy requirements compared with CIs. However, a trade-off between lower perceptual thresholds and better frequency discrimination may exist that depends on location of stimulation along the caudorostral dimension of the ICC. Overall, this study provides the foundation for future AMI research and development.

Sensitivity to isolated and concurrent intensity and fundamental frequency increments by cochlear implant users under natural listening conditions

Sensitivity to acoustic cues in cochlear implant (CI) listening under natural conditions is a potentially complex interaction between a number of simultaneous factors, and may be difficult to predict. In the present study, sensitivity was measured under conditions that approximate those of natural listening. Synthesized words having increases in intensity or fundamental frequency (F0) in a middle stressed syllable were presented in soundfield to normal-hearing listeners and to CI listeners using their everyday speech processors and programming. In contrast to the extremely fine sensitivity to electrical current observed when direct stimulation of single electrodes is employed, difference limens (DLs) for intensity were larger for the CI listeners by a factor of 2.4. In accord with previous work, F0 DLs were larger by almost one order of magnitude. In a second experiment, it was found that the presence of concurrent intensity and F0 increments reduced the mean DL to half that of either cue alone for both groups of subjects, indicating that both groups combine concurrent cues with equal success. Although sensitivity to either cue in isolation was not related to word recognition in CI users, the listeners having lower combined-cue thresholds produced better word recognition scores.