Across-species comparisons of psychophysical detection thresholds for electrical stimulation of the cochlea: I. Sinusoidal stimuli

Insights Into Electrophysiological Metrics of Cochlear Health in Cochlear Implant Users Using a Computational Model

PURPOSE

The hearing outcomes of cochlear implant users depend on the functional status of the electrode-neuron interface inside the cochlea. This can be assessed by measuring electrically evoked compound action potentials (eCAPs). Variations in cochlear neural health and survival are reflected in eCAP-based metrics. The difficulty in translating promising results from animal studies into clinical use has raised questions about to what degree eCAP-based metrics are influenced by non-neural factors. Here, we addressed these questions using a computational model.

METHODS

A 2-D computational model was designed to simulate how electrical signals from the stimulating electrode reach the auditory nerve fibers distributed along the cochlea, evoking action potentials that can be recorded as compound responses at the recording electrodes. Effects of physiologically relevant variations in neural survival and in electrode-neuron and stimulating-recording electrode distances on eCAP amplitude growth functions (AGFs) were investigated.

RESULTS

In line with existing literature, the predicted eCAP AGF slopes and the inter-phase gap (IPG) effects depended on the neural survival, but only when the IPG effect was calculated as the difference between the slopes of the two AGFs expressed in linear input-output scale. As expected, shallower eCAP AGF slopes were obtained for increased stimulating-recording electrode distance and larger eCAP thresholds for greater electrode-neuron distance. These non-neural factors had also minor interference on the predicted IPG effect.

CONCLUSIONS

The model predictions demonstrate previously found dependencies of eCAP metrics on neural survival and non-neural aspects. The present findings confirm data from animal studies and provide insights into applying described metrics in clinical practice.

Comparison of response properties of the electrically stimulated auditory nerve reported in human listeners and in animal models

Cochlear implants (CIs) provide acoustic information to implanted patients by electrically stimulating nearby auditory nerve fibers (ANFs) which then transmit the information to higher-level neural structures for further processing and interpretation. Computational models that simulate ANF responses to CI stimuli enable the exploration of the mechanisms underlying CI performance beyond the capacity of in vivo experimentation alone. However, all ANF models developed to date utilize to some extent anatomical/morphometric data, biophysical properties and/or physiological data measured in non-human animal models. This review compares response properties of the electrically stimulated auditory nerve (AN) in human listeners and different mammalian models. Properties of AN responses to single pulse stimulation, paired-pulse stimulation, and pulse-train stimulation are presented. While some AN response properties are similar between human listeners and animal models (e.g., increased AN sensitivity to single pulse stimuli with long interphase gaps), there are some significant differences. For example, the AN of most animal models is typically more sensitive to cathodic stimulation while the AN of human listeners is generally more sensitive to anodic stimulation. Additionally, there are substantial differences in the speed of recovery from neural adaptation between animal models and human listeners. Therefore, results from animal models cannot be simply translated to human listeners. Recognizing the differences in responses of the AN to electrical stimulation between humans and other mammals is an important step for creating ANF models that are more applicable to various human CI patient populations.

Cochlear Implant Research and Development in the Twenty-first Century: A Critical Update

Cochlear implants (CIs) are the world's most successful sensory prosthesis and have been the subject of intense research and development in recent decades. We critically review the progress in CI research, and its success in improving patient outcomes, from the turn of the century to the present day. The review focuses on the processing, stimulation, and audiological methods that have been used to try to improve speech perception by human CI listeners, and on fundamental new insights in the response of the auditory system to electrical stimulation. The introduction of directional microphones and of new noise reduction and pre-processing algorithms has produced robust and sometimes substantial improvements. Novel speech-processing algorithms, the use of current-focusing methods, and individualised (patient-by-patient) deactivation of subsets of electrodes have produced more modest improvements. We argue that incremental advances have and will continue to be made, that collectively these may substantially improve patient outcomes, but that the modest size of each individual advance will require greater attention to experimental design and power. We also briefly discuss the potential and limitations of promising technologies that are currently being developed in animal models, and suggest strategies for researchers to collectively maximise the potential of CIs to improve hearing in a wide range of listening situations.

Ramped pulse shapes are more efficient for cochlear implant stimulation in an animal model

In all commercial cochlear implant (CI) devices, the electric stimulation is performed with a rectangular pulse that generally has two phases of opposite polarity. To date, developing new stimulation strategies has relied on the efficacy of this shape. Here, we investigate the potential of a novel stimulation paradigm that uses biophysically-inspired electrical ramped pulses. Using electrically-evoked auditory brainstem response (eABR) recordings in mice, we found that less charge, but higher current level amplitude, is needed to evoke responses with ramped shapes that are similar in amplitude to responses obtained with rectangular shapes. The most charge-efficient pulse shape had a rising ramp over both phases, supporting findings from previous in vitro studies. This was also true for longer phase durations. Our study presents the first physiological data on CI-stimulation with ramped pulse shapes. By reducing charge consumption ramped pulses have the potential to produce more battery-efficient CIs and may open new perspectives for designing other efficient neural implants in the future.

Higher sensitivity of human auditory nerve fibers to positive electrical currents

Most contemporary cochlear implants (CIs) stimulate the auditory nerve with trains of amplitude-modulated, symmetric biphasic pulses. Although both polarities of a pulse can depolarize the nerve fibers and generate action potentials, it remains unknown which of the two (positive or negative) phases has the stronger effect. Understanding the effects of pulse polarity will help to optimize the stimulation protocols and to deliver the most relevant information to the implant listeners. Animal experiments have shown that cathodic (negative) current flows are more effective than anodic (positive) ones in eliciting neural responses, and this finding has motivated the development of novel speech-processing algorithms. In this study, we show electrophysiologically and psychophysically that the human auditory system exhibits the opposite pattern, being more sensitive to anodic stimulation. We measured electrically evoked compound action potentials in CI listeners for phase-separated pulses, allowing us to tease out the responses to each of the two opposite-polarity phases. At an equal stimulus level, the anodic phase yielded the larger response. Furthermore, a measure of psychophysical masking patterns revealed that this polarity difference was still present at higher levels of the auditory system and was therefore not solely due to antidromic propagation of the neural response. This finding may relate to a particular orientation of the nerve fibers relative to the electrode or to a substantial degeneration and demyelination of the peripheral processes. Potential applications to improve CI speech-processing strategies are discussed.

A dual-process integrator-resonator model of the electrically stimulated human auditory nerve

A phenomenological dual-process model of the electrically stimulated human auditory nerve is presented and compared to threshold and loudness data from cochlear implant users. The auditory nerve is modeled as two parallel processes derived from linearized equations of conductance-based models. The first process is an integrator, which dominates stimulation for short-phase duration biphasic pulses and high-frequency sinusoidal stimuli. It has a relatively short time constant (0.094 ms) arising from the passive properties of the membrane. The second process is a resonator, which induces nonmonotonic functions of threshold vs frequency with minima around 80 Hz. The ion channel responsible for this trend has a relatively large relaxation time constant of about 1 ms. Membrane noise is modeled as a Gaussian noise, and loudness sensation is assumed to relate to the probability of firing of a neuron during a 20-ms rectangular window. Experimental psychophysical results obtained in seven previously published studies can be interpreted with this model. The model also provides a physiologically based account of the nonmonotonic threshold vs frequency functions observed in biphasic and sinusoidal stimulation, the large threshold decrease obtained with biphasic pulses having a relatively long inter-phase gap and the effects of asymmetric pulses.

Effects of pulse rate on thresholds and loudness of biphasic and alternating monophasic pulse trains in electrical hearing

Detection thresholds and most comfortable loudnesses (MCLs) were determined as a function of pulse rate for standard biphasic pulse trains (BP) and for anodic and cathodic monophasic phases alternating at fixed intervals (ALT-m). Three different phase durations were examined. With a 100-micros phase duration, thresholds for the ALT-m stimulus were substantially (up to 12 dB) lower than for the BP stimuli at relatively low rates (200 pps), but were similar to the BP thresholds at high rates (1000 pps). Thresholds for BP pulse trains decreased monotonically with increasing rate, whereas the function for ALT-m waveforms was non-monotonic with a maximum between 400 and 1000 pps. These trends occurred for three different cochlear implant devices, different electrode configurations, and, generally, for different phase durations (10.8, 25, and 100 micros/phase). However, at the shorter phase durations, thresholds remained lower for the ALT-m stimulus, even at 5000 pps, the highest rate studied. Dynamic ranges of the BP pulse trains increased with increasing rate, irrespective of the phase duration under test, but for the ALT-m stimuli this was only true at the shorter phase durations tested. At a 100-mus phase duration, dynamic ranges for the ALT-m waveforms did not differ significantly as a function of rate. The results confirm previous reports that delaying charge recovery, in this case by switching from a BP to an ALT-m wave shape, can substantially reduce thresholds [Van Wieringen, A., Carlyon, R.P., Laneau, J., Wouters, J., 2005. Effects of waveform shape on human sensitivity to electrical stimulation of the inner ear. Hear. Res. 200, 73-86; Carlyon, R.P., van Wieringen, A., Deeks, J.M., Long, C.J., Lyzenga, J, Wouters, J., 2005. Effect of inter-phase gap on the sensitivity of cochlear implant users to electrical stimulation. Hear. Res. 205, 210-224]. However, at high pulse rates, this advantage only occurs at short phase durations. In addition, we show that the complex interaction between the effects of pulse shape, rate, and phase duration on thresholds can be captured by the simple linear model described by Carlyon et al.

Effect of inter-phase gap on the sensitivity of cochlear implant users to electrical stimulation

Human behavioral thresholds for trains of biphasic pulses applied to a single channel of Nucleus CI24 and LAURA cochlear implants were measured as a function of inter-phase gap (IPG). Experiment 1 used bipolar stimulation, a 100-pps pulse rate, and a 400-ms stimulus duration. In one condition, the two phases of each pulse had opposite polarity. Thresholds continued to drop by 9-10 dB as IPG was increased from near zero to the longest value tested (2900 micros for CI24, 4900 micros for LAURA). This time course is much longer than reported for single-cell recordings from animals. In a second condition, the two phases of each pulse had the same polarity, which alternated from pulse to pulse. Thresholds were independent of IPG, and similar to those in condition 1 at IPG=4900 micros. Experiment 2 used monopolar stimulation. One condition was similar to condition 1 of experiment 1, and thresholds also dropped up to the longest IPG studied (2900 micros). This also happened when the pulse rate was reduced to 20 pps, and when only a single pulse was presented on each trial. Keeping IPG constant at 8 micros and adding an extra biphasic pulse x ms into each period produced thresholds that were roughly independent of x, indicating that the effect of IPG in the other conditions was not due to a release from refractoriness at sites central to the auditory nerve. Experiment 3 measured thresholds at three IPGs, which were less than, equal to, and more than one half of the interval between successive pulses. Thresholds were lowest at the intermediate IPG. The results of all experiments could be fit by a linear model consisting of a lowpass filter based on the function relating threshold to the frequency of sinusoidal electrical stimulation. The data and model have implications for reducing the power consumption of cochlear implants.

Psychophysical threshold variability in cochlear implant subjects

The dramatic differences observed when comparing auditory neural responses to electrical and acoustic stimulation may illustrate one of the important mechanisms underlying the sometimes poor speech recognition abilities of individuals with cochlear implants. Recent research has suggested that the absence of a stochastic component in neural responses to electrical activation may be an important potential mechanism for this degradation in speech recognition performance. There are few psychophysical data, however, demonstrating that this stochastic behavior can be measured directly in implant subjects. In this study, variability in psychophysical threshold was investigated as a measure of the stochastic nature of the underlying neural response in human and non-human subjects implanted with intracochlear electrode arrays. Threshold data collected in both monopolar and bipolar stimulation modes at several phase durations from cat and human subjects are presented. The nature of the neural input/output curve suggests that threshold variability should increase as the slope of the input/output curve is decreased, i.e. as phase duration is increased. These predictions are confirmed by the pattern of psychophysical results measured experimentally in cat and human subjects. Furthermore, the data may suggest that subjects with higher threshold variability, i.e. a relatively greater stochastic component, are more likely to have higher speech recognition scores.

Neuronal responses in cat primary auditory cortex to electrical cochlear stimulation: IV. Activation pattern for sinusoidal stimulation

Patterns of threshold distributions for single-cycle sinusoidal electrical stimulation and single pulse electrical stimulation were compared in primary auditory cortex of the adult cat. Furthermore, the effects of auditory deprivation on these distributions were evaluated and compared across three groups of adult cats. Threshold distributions for single and multiple unit responses from the middle cortical layers were obtained on the ectosylvian gyrus in an acutely implanted animal; 2 wk after deafening and implantation (short-term group); and neonatally deafened animals implanted following 2-5 yr of deafness (long-term group). For all three cases, we observed similar patterns of circumscribed regions of low response thresholds in the region of primary auditory cortex (AI). A dorsal and a ventral region of low response thresholds were found separated by a narrow, anterior-posterior strip of elevated thresholds. The ventral low-threshold regions in the short-term group were cochleotopically arranged. By contrast, the dorsal region in the short-term animals and both low-threshold regions in long-term deafened animals maintained only weak cochleotopicity. Analysis of the spatial extent of the low-threshold regions revealed that the activated area for sinusoidal stimulation was smaller and more circumscribed than for pulsatile stimulation for both dorsal and ventral AI. The width of the high-threshold ridge that separated the dorsal and ventral low-threshold regions was greater for sinusoidal stimulation. Sinusoidal and pulsatile threshold behavior differed significantly for electrode configurations with low and high minimum thresholds. Differences in threshold behavior and cortical response distributions between the sinusoidal and pulsatile stimulation suggest that stimulus shape plays a significant role in the activation of cortical activity. Differences in the activation pattern for short-term and long-term deafness reflect deafness-induced reorganizational changes based on factors such as differences in excitatory and inhibitory balance that are affected by the stimulation parameters.

Effects of chronic stimulation on auditory nerve survival in ototoxically deafened animals

For almost 10 years, chronic stimulation has been known to affect spiral ganglion cell (SGC) survival in the deaf ear. However, the reported effects of chronic stimulation vary across preparations and studies. In this review, the effects of chronic stimulation on the deafened auditory periphery are examined, and variables that may impact on the efficacy of chronic stimulation are identified. The effects of deafening on the unstimulated peripheral and central auditory system are also described, as the deafened, unstimulated system is the canvas upon which stimulation-mediated effects are imposed. Discrepancies in the effects of chronic stimulation across studies may be attributable in large part to the combined effects of the deafening method and the post-deafening delay prior to chronic stimulation, which vary across studies. Emphasis is placed on the need to consider the natural progression of SGC loss following deafening in the absence of chronic stimulation, as the rate of SGC loss almost certainly affects both the efficacy of stimulation, and the impact of any delay between deafening and initiation of stimulation. The differences across preparations complicate direct comparison of protective efficacy of stimulation. At the same time, these differences can be used to our advantage, aiding characterization of the effects of different factors on the efficacy of chronic stimulation as a neuroprotective intervention.

Across-species comparisons of psychophysical detection thresholds for electrical stimulation of the cochlea: II. Strength-duration functions for single, biphasic pulses

This paper compares psychophysical detection threshold data (new and previously published) for pulsatile electrical stimulation of the deafened inner ear, obtained from different human and nonhuman subjects. Subjects were grouped according to species. Other variables, however, such as the electrode array type and method of deafening, varied within and across species. Detection threshold levels and slopes of threshold versus phase duration functions for presentations of single, biphasic pulsatile stimuli (strength-duration functions) were compared for humans, macaques, cats, and guinea pigs. For bipolar stimulation, statistically significant differences in threshold level were observed between human subjects and all other species. The species difference did not depend on the phase duration tested. For monopolar stimulation, only nonhuman species were tested. Effects of electrode configuration on both the level and slope of psychophysical strength-duration functions were statistically significant across nonhuman species, but there was not a statistically significant interaction between species and electrode configuration. The similarity in function shape and relative paucity of significant differences in psychophysical functions across species support the continued use of multiple species for cochlear implant research.