Effects of electrode configuration on threshold functions for electrical stimulation of the cochlea

Evaluating Multipulse Integration as a Neural-Health Correlate in Human Cochlear Implant Users: Effects of Stimulation Mode

Previous psychophysical studies have shown that a steep detection-threshold-versus-stimulation-rate function (multipulse integration; MPI) is associated with laterally positioned electrodes producing a broad neural excitation pattern. These findings are consistent with steep MPI depending on either a certain width of neural excitation allowing a large population of neurons operating at a low point on their dynamic range to respond to an increase in stimulation rate or a certain slope of excitation pattern that allows recruitment of neurons at the excitation periphery. Results of the current study provide additional support for these mechanisms by demonstrating significantly flattened MPI functions in narrow bipolar than monopolar stimulation. The study further examined the relationship between the steepness of the psychometric functions for detection (d' versus log current level) and MPI. In contrast to findings in monopolar stimulation, current data measured in bipolar stimulation suggest that steepness of the psychometric functions explained a moderate amount of the across-site variance in MPI. Steepness of the psychometric functions, however, cannot explain why MPI flattened in bipolar stimulation, since slopes of the psychometric functions were comparable in the two stimulation modes. Lastly, our results show that across-site mean MPI measured in monopolar and bipolar stimulation correlated with speech recognition in opposite signs, with steeper monopolar MPI being associated with poorer performance but steeper bipolar MPI being associated with better performance. If steeper MPI requires broad stimulation of the cochlea, the correlation between monopolar MPI and speech recognition can be interpreted as the detrimental effect of poor spectral resolution on speech recognition. Assuming bipolar stimulation produces narrow excitation, and MPI measured in bipolar stimulation reflects primarily responses of the on-site neurons, the correlation between bipolar MPI and speech recognition can be understood in light of the importance of neural survival for speech recognition.

A physiological and behavioral system for hearing restoration with cochlear implants

Cochlear implants are neuroprosthetic devices that provide hearing to deaf patients, although outcomes are highly variable even with prolonged training and use. The central auditory system must process cochlear implant signals, but it is unclear how neural circuits adapt-or fail to adapt-to such inputs. The knowledge of these mechanisms is required for development of next-generation neuroprosthetics that interface with existing neural circuits and enable synaptic plasticity to improve perceptual outcomes. Here, we describe a new system for cochlear implant insertion, stimulation, and behavioral training in rats. Animals were first ensured to have significant hearing loss via physiological and behavioral criteria. We developed a surgical approach for multichannel (2- or 8-channel) array insertion, comparable with implantation procedures and depth in humans. Peripheral and cortical responses to stimulation were used to program the implant objectively. Animals fitted with implants learned to use them for an auditory-dependent task that assesses frequency detection and recognition in a background of environmentally and self-generated noise and ceased responding appropriately to sounds when the implant was temporarily inactivated. This physiologically calibrated and behaviorally validated system provides a powerful opportunity to study the neural basis of neuroprosthetic device use and plasticity.

Design and fabrication of multichannel cochlear implants for animal research

The effectiveness of multichannel cochlear implants depends on the activation of perceptually distinct regions of the auditory nerve. Increased information transfer is possible as the number of channels and dynamic range are increased and electrical and neural interaction among channels is reduced. Human and animal studies have demonstrated that specific design features of the intracochlear electrode directly affect these performance factors. These features include the geometry, size, and orientation of the stimulating sites, proximity of the device to spiral ganglion neurons, shape and position of the insulating carrier, and the stimulation mode (monopolar, bipolar, etc.). Animal studies to directly measure the effects of changes in electrode design are currently constrained by the lack of available electrodes that model contemporary clinical devices. This report presents methods to design and fabricate species-specific customizable electrode arrays. We have successfully implanted these arrays in guinea pigs and cats for periods of up to 14 months and have conducted acute electrophysiological experiments in these animals. Modifications enabling long-term intracochlear drug infusion are also described. Studies using these scale model arrays will improve our understanding of how these devices function in human subjects and how we can best optimize future cochlear implants.

Speech perception with mono- and quadrupolar electrode configurations: a crossover study

OBJECTIVE

To study the effect of two multipolar electrode configurations on speech perception, pitch perception, and the intracochlear electrical field.

STUDY DESIGN

Crossover design; within subject.

SETTING

Tertiary referral center.

PATIENTS

Eight experienced adult cochlear implant users.

INTERVENTION

Each subject used each of three experimental processors for 3 weeks. The following processors were compared that differed only in electrode configuration: 1) monopolar; 2) hybrid quadrupolar, in which half of the current returned to the extracochlear reference electrode and half to two electrodes immediately to the left and right of the active electrode; and 3) flat tripolar +2, which directed all the current to four reference electrodes (two on each side), separated from the active electrode by two inactive electrodes. All the processors used the standard Advanced Bionics HiRes speech-processing strategy, 12 channels, 1,220 pulses per second per channel, and with a pulse width of 33 (micros/phase).

RESULTS

The monopolar processors had the largest stimulation efficiency and the smallest dynamic range in linear current units. The reverse was true of flat tripolar +2 processor, whereas the hybrid quadrupolar processor fell in between. Insufficient loudness growth prevented the use of the flat tripolar +2 processor in three subjects. Word recognition did not differ between the clinically used 16-channel monopolar processor and the experimental monopolar processor, regardless of the differences in the number of channels, pulse rate, and duration of experience. Word recognition with the flat tripolar +2 processor was significantly poorer than with the monopolar and hybrid quadrupolar processors; monopolar and quadrupolar processors did not differ. There was no significant interaction between processor type and competing noise type (stationary or fluctuating), but performance at the higher level of fluctuating noise was best with the hybrid quadrupolar processor in almost all the subjects. Pitch scaling showed ceiling performance in five subjects and differed between processors in the two other subjects with imperfect tonotopy. Intracochlear current spread was considerable with the monopolar configuration; it was reduced with the hybrid quadrupolar configuration and virtually absent beyond the active electrodes with the tripolar configuration.

CONCLUSION

More confined configurations reduced the longitudinal width of the electrical field, which was expected to enhance channel separation, but no improvement in word recognition was found. More research is needed to test confined configurations that have enhanced efficiency and to evaluate the fundamental effects of configuration on channel discriminability.

A longitudinal study of electrical stimulation levels and electrode impedance in children using the Clarion cochlear implant

CONCLUSIONS

Electrical stimulation levels and electrode impedance values (EIVs) in children using the Clarion cochlear implant (CI) programmed with CIS strategy stabilized after 3 months of implant use. The data presented here may be useful as a general guideline for the programming of infants and young children and may further be of help for the identification of patients who fall outside the "average" range.

OBJECTIVES

The purpose of the present study was to evaluate changes in electrical stimulation levels, i.e. threshold (T) levels, comfortable (M) levels, dynamic range (DR), and EIVs during the first 18 months of implant use, in children using the Clarion CI.

MATERIALS AND METHODS

The maps of 18 pre-lingual children (mean age at implantation 4.2 years; range 1-8), using the Enhanced Bipolar 1.2 or Bipolar standard electrode with the S-Series speech processor programmed with CIS strategy, were examined at five time points: connection, and 3, 6, 12, and 18 months post-initial stimulation. T levels, M levels, DR and EIVs were analyzed according to four cochlear segments: apical, apical-medial, medial-basal, and basal.

RESULTS

During the first 3 months of implant use T levels increased to some extent, whereas M levels and DR increased significantly. From 3 months and through the entire follow-up, T and M levels as well as DR were stable. EIVs of current carrying electrodes decreased significantly from connection to the 3-month visit; thereafter a stabilization of values was evident. Electrical stimulation levels and EIVs did not differ among the cochlear segments during the entire follow-up.

Cochlear implantation in rats: a new surgical approach

The laboratory rat has been used extensively in auditory research but has had limited use in cochlear implant related research due mainly to the surgically restricted access to the scala tympani. We have developed a new surgical method for cochlear implantation in rats. The key to this protocol was cauterizing the stapedial artery (SA) and making a small cochleostomy near the round window in order to enlarge the surgical access to the scala tympani. Five normal hearing Hooded Wistar rats were used to investigate the effect of cauterizing the SA on hearing and auditory nerve survival. Results showed that cauterizing the SA was surgically feasible, afforded excellent exposure of the round window niche for cochleostomy, and did not adversely affect acoustic thresholds measured electrophysiologically. Moreover, there was no difference in spiral ganglion cell densities for any cochlear turn when compared with the contralateral control ears. Three deafened rats were subsequently implanted with a scala tympani electrode array using this new surgical approach. Electrically evoked auditory brainstem responses using bipolar stimulation, and subsequent cochlear histopathology demonstrated that cochlear implantation using a custom-made rat electrode array was safe and effective. The surgical approach presented in this paper presents a safe and effective procedure for acute or chronic cochlear implantation in the rat model.

Electrophysiologic channel interaction, electrode pitch ranking, and behavioral threshold in straight versus perimodiolar cochlear implant electrode arrays

The primary goal of this study was to examine electrophysiologic measures of channel interaction, electrode pitch discrimination ability using a pitch-ranking task, and behavioral threshold levels in individuals implanted with a straight electrode array versus a perimodiolar array. It was hypothesized that perimodiolar arrays should yield lower thresholds, less channel interaction as measured with the electrically evoked compound action potential (ECAP), and better electrode pitch-ranking ability. Results from ten adult Nucleus 24 recipients (N=5 straight array, N=5 perimodiolar Contour array) showed no significant difference in threshold between the two electrode designs; however, there was significantly better electrode pitch-ranking ability and less channel interaction as measured with the ECAP for perimodiolar electrodes. Additionally, there was a significant positive correlation between behavioral threshold and width of the ECAP interaction function for Contour group data. There was no significant correlation between behavioral threshold and electrode pitch-ranking ability. These data suggest that electrode design and/or perimodiolar position may reduce physiologic channel interaction in the cochlea and improve electrode pitch discrimination ability; however, this positive finding did not translate into significantly better speech perception ability for Contour subjects.

Effects of waveform shape on human sensitivity to electrical stimulation of the inner ear

Psychophysical measures of the electrically stimulated human auditory system were obtained for different types of symmetric and asymmetric charge-balanced waveforms. Absolute detection thresholds of biphasic, pseudomonophasic, and 'alternating monophasic' current waveforms delivered by a bipolar intra-cochlear electrode pair were determined for four subjects implanted with the LAURA device. Thresholds for alternating monophasic stimuli, in which anodic and cathodic phases alternated every 5 ms, were 5-8 dB lower than for the biphasic waveforms for all four subjects. For two of the four subjects, thresholds for the pseudomonophasic waveforms were also significantly lower than for the biphasic waveforms. These pseudomonophasic thresholds were greatly affected neither by a 500-micros gap inserted between the two phases, nor by whether the shorter phase preceded or followed the longer one. Loudness balancing measures performed at the most comfortable levels also showed that, for equal loudness, alternating monophasic stimuli required a lower level than biphasic and pseudomonophasic waveforms. For three of the four subjects, the dynamic ranges of the pseudomonophasic (but not alternating monophasic) waveforms were greater than those of the biphasic waveforms. The results demonstrate that thresholds and dynamic ranges of human cochlear implant users can be controlled by manipulating the way in which the charge produced by the initial phase of an electrical pulse is recovered.

Effects of cochlear-implant pulse rate and inter-channel timing on channel interactions and thresholds

Interactions among the multiple channels of a cochlear prosthesis limit the number of channels of information that can be transmitted to the brain. This study explored the influence on channel interactions of electrical pulse rates and temporal offsets between channels. Anesthetized guinea pigs were implanted with 2-channel scala-tympani electrode arrays, and spike activity was recorded from the auditory cortex. Channel interactions were quantified as the reduction of the threshold for pulse-train stimulation of the apical channel by sub-threshold stimulation of the basal channel. Pulse rates were 254 or 4069 pulses per second (pps) per channel. Maximum threshold reductions averaged 9.6 dB when channels were stimulated simultaneously. Among nonsimultaneous conditions, threshold reductions at the 254-pps rate were entirely eliminated by a 1966-micros inter-channel offset. When offsets were only 41 to 123 micros, however, maximum threshold shifts averaged 3.1 dB, which was comparable to the dynamic ranges of cortical neurons in this experimental preparation. Threshold reductions at 4069 pps averaged up to 1.3 dB greater than at 254 pps, which raises some concern in regard to high-pulse-rate speech processors. Thresholds for various paired-pulse stimuli, pulse rates, and pulse-train durations were measured to test possible mechanisms of temporal integration.

Trends in cochlear implants

More than 60,000 people worldwide use cochlear implants as a means to restore functional hearing. Although individual performance variability is still high, an average implant user can talk on the phone in a quiet environment. Cochlear-implant research has also matured as a field, as evidenced by the exponential growth in both the patient population and scientific publication. The present report examines current issues related to audiologic, clinical, engineering, anatomic, and physiologic aspects of cochlear implants, focusing on their psychophysical, speech, music, and cognitive performance. This report also forecasts clinical and research trends related to presurgical evaluation, fitting protocols, signal processing, and postsurgical rehabilitation in cochlear implants. Finally, a future landscape in amplification is presented that requires a unique, yet complementary, contribution from hearing aids, middle ear implants, and cochlear implants to achieve a total solution to the entire spectrum of hearing loss treatment and management.

Electrical field interactions in different cochlear implant systems

The goal of this study was to evaluate electrical field interactions produced by the stimulation of different types of intracochlear electrodes in 12 adult subjects (three Ineraid, four Clarion S-Series, three S-Series with the electrode positioning system-EPS and two Clarion HiFocus-I with the EPS). Psychophysical measurements were conducted with biphasic stimuli (813 pulse per second, 153.8 micros/phase). "Perturbation" signals (300 ms) were applied to one electrode chosen at the middle of the array and their effects on detection thresholds of "probe" signals (30 ms) were measured on the neighbor basal electrode. Perturbation levels were set below the detection threshold of the perturbation electrode (-2 dB re threshold). Measurements were first conducted for simultaneous stimulation of the probe and of the perturbation electrodes, for monopolar for all subjects and for bipolar stimulus configurations for both Clarion HiFocus-I subjects. The tested Clarion electrodes did not present lower monopolar interactions than the Ineraid electrodes. Nevertheless, considering the shorter distance between electrodes for the Clarion than for the Ineraid, the tested Clarion electrodes might be more selective than the Ineraid. We did not find any significant monopolar electrical field-interaction differences between subjects who received the S-Series array with and without the EPS. We did not find lower interactions for both subjects who received the HiFocus-I array than for subjects who received the S-Series. Electrical field interactions were lower for bipolar than for monopolar configurations for both HiFocus-I subjects. A second set of measurements was conducted for nonsimultaneous stimulation similar to the one used in continuous interleaved sampling sound strategy. These measurements showed that interactions evaluated for simultaneous biphasic stimuli were larger than for nonsimultaneous stimulation.

Auditory cortical images of cochlear-implant stimuli: dependence on electrode configuration

This study examines patterns of auditory cortical activity elicited by single-pulse cochlear implant stimuli that vary in electrode configuration, cochlear place of stimulation, and stimulus level. Recordings were made from the primary auditory cortex (area A1) of ketamine-anesthetized guinea pigs. The spatiotemporal pattern of neural spike activity was measured simultaneously across 16 cortical locations spanning approximately 2-3 octaves of the tonotopic axis. Such a pattern, averaged over 40 presentations of any particular stimulus, was defined as the "cortical image" of that stimulus. Acutely deafened guinea pigs were implanted with a 6-electrode animal version of the 22-electrode Nucleus banded electrode array (Cochlear). Cochlear electrode configurations consisted of monopolar (MP), bipolar (BP + N) with N inactive electrodes between the active and return electrodes (0 < or = N < or = 4), tripolar (TP) with one active electrode and two flanking return electrodes, and common ground (CG) with one active electrode and as many as five return electrodes. Cortical images typically showed a focus of maximum spike probability and minimum latency. Spike probabilities tended to decrease, and latencies tended to increase, with increasing cortical distance from that focus. Cortical images of TP stimuli were the most spatially compact, followed by BP + N images, and then MP images, which were the broadest. Images of CG stimuli were rather variable across animals and stimulus channels. The locations of cortical images shifted systematically from caudal to rostral as the cochlear place of stimulation changed from basal to apical. At the most sensitive cortical site for each condition, the dynamic ranges over which spike rates increased with increased current level were restricted to about 1-2 dB, regardless of configuration. Dynamic ranges tended to increase with increasing cortical distance from the most sensitive site. Electrode configurations that produced compact cortical images (e.g., TP and BP + 0) showed the greatest range of thresholds within each cortical image and the largest dynamic range at cortical sites removed from the most sensitive site.

Loudness summation for pulsatile electrical stimulation of the cochlea: effects of rate, electrode separation, level, and mode of stimulation

The aim of these two experiments was to gain systematic data on the amount of loudness summation measured for dual-electrode stimuli with varying temporal and spatial separation of current pulses. Loudness summation is important in the implementation of speech processing strategies for implantees. However, the loudness mapping functions used in current speech processors utilize psychophysical data (thresholds and comfortable loudness levels) derived using single-electrode stimuli, and do not take into account the temporal and spatial patterns of the speech processor output. In the first experiment, the current reduction required to equalize the loudness of a dual-electrode stimulus to that of its component (and equally loud) single-electrode stimuli was measured for three electrode separations (0.75, 2.25, and 7.5 mm), three repetition rates (250, 500, and 1000 Hz), and two loudness levels (comfortably loud, and mid-dynamic range). It was found that electrode separation had little effect on loudness summation, except for interactions with level and rate effects at the smallest separation. More current adjustment (in dB) was required for higher rates and lower levels of stimulation. The second experiment investigated the effects of mode (monopolar versus bipolar) and pulse duration on loudness summation. More current adjustment was required in bipolar mode than in monopolar mode at the lower level only. The main effects in both experiments, and their interactions, are consistent with a loudness model in which the neural excitation density is first obtained by temporal integration of excitation at each cochlear place, then converted to specific loudness via a nonlinear relationship, and finally integrated over cochlear place to obtain the loudness. The two important features which affect the loudness relationships in dual-electrode stimulation in this model are the shape of the excitation density function and the amount by which the neural spike probability per pulse is reduced in areas of overlapping excitation due to refractory effects.

The effect of electrode configuration and duration of deafness on threshold and selectivity of responses to intracochlear electrical stimulation

This report examines the effects of intracochlear electrode configuration and mode of stimulation (bipolar or monopolar) on neural threshold and spatial selectivity in the inferior colliculus (IC) of the cat. Single and multiunit IC recordings were made in three groups of animals; acutely deafened adults (controls), neonatally deafened animals studied at 6 to 18 months of age and neonatally deafened cats studied at 2.5 to 6.5 years. Response thresholds were plotted versus IC depth to measure the spatial distribution of responses. The response selectivity for each stimulating configuration was defined as the width of the resulting spatial tuning curve (STC) measured at 6 dB above threshold. Spiral ganglion cell (SG) survival was examined histologically in all neonatally deafened animals and correlated with physiological results. Animals studied at less than 1.5 years had SG densities of 23.5%-64.4% of normal (mean=42.7%) while animals studied at greater than 2.5 years had densities of 5.1%-18.3% of normal (mean=9.9%). Electrophysiological results include the following. (1) Monopolar thresholds were 7-8 dB lower than bipolar thresholds in the same animals. (2) Varying the configuration of bipolar contacts (measured as radial, offset radial and longitudinal pairs) did not systematically affect IC threshold in either controls or short-term neonatally deafened animals. In contrast, the long-term neonatally deafened animals showed a difference in threshold with each configuration. (3) The spatial distributions (Q(6 dB)) of responses to bipolar stimulation were approximately 40% more restricted than those for monopolar stimulation. (4) The spatial selectivity of neonatally deafened animals studied at ages up to 1.5 years was equal to that of control animals with normal auditory experience. However, selectivity was degraded in the older animals. (5) Selectivity was decreased in some animals with the longitudinal bipolar configuration and multiple response peaks were seen in several cases using this stimulus configuration.

Cochlear implant thresholds: comparison of middle latency responses with psychophysical and cortical-spike-activity thresholds

The electrically evoked middle latency response (EMLR) is a potentially useful measure of activation of the auditory system by a cochlear prosthesis. The present study compared cochlear prosthesis thresholds determined using EMLR with thresholds determined for psychophysical detection and for spike activity in cortical neurons. In systemically deafened guinea pigs, the difference between EMLR and psychophysical threshold level varied, with differences ranging from -4.6 dB (EMLR threshold more sensitive) to +10.7 dB (psychophysical threshold more sensitive) across animals and phase durations. Threshold differences between EMLR and auditory cortex neural spike responses were similar in magnitude and range (-6 to +15 dB) to those seen for EMLR vs. psychophysical thresholds. These ranges are comparable to the behavioral operating range for a given condition. In 3 of 12 subjects, the EMLR was absent for some or all electrode configurations tested, even at levels well above the threshold for psychophysical detection or cortical neuronal response. These results suggest that neither the EMLR thresholds nor cortical neuronal spike thresholds are an adequate substitute for psychophysical measures of threshold. While not sufficient for use in place of psychophysical measures, EMLR threshold level is strongly correlated with psychophysical threshold level across subjects (R(2)=0.82). Interestingly, plots of thresholds vs. phase duration were roughly parallel for psychophysical and EMLR thresholds, in contrast to the divergence of psychophysical and more peripheral (e.g., electrically evoked auditory brainstem response) evoked neural threshold vs. phase duration functions.

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.

Changes in cochlear electrical stimulation induced Fos expression in the rat inferior colliculus following deafness

Fos immunoreactive (IR) staining was used to examine changes in excitatory neuronal activity in the rat inferior colliculus (IC) between normal hearing and 21 day deaf rats evoked by basal or apical monopolar cochlear electrical stimulation. The location of evoked Fos IR neurons was consistent with expected tonotopic areas. The number of Fos IR cells increased as stimulation intensity increased in both normal and 21 day deaf animals. Stimulation at 1. 5x threshold evoked fewer Fos IR cells in 21 day deafened animals compared to normal hearing animals. At 5x and above, however, significantly increased numbers of Fos IR neurons (in a larger grouping) were evoked in 21 day deafened animals compared to normal hearing animals. Another group of animals had 7 days of deafness followed by 14 days of chronic basal cochlear electrical stimulation. In this group basal monopolar stimulation at 5x evoked not only a greater number of Fos IR neurons, compared to normal hearing animals, but the location of their grouping was slightly shifted to a more dorso-lateral region in the contralateral IC, compared to the normal hearing and 21 day deaf groups. These observations indicate that both deafness and chronic electrical stimulation may alter central auditory processing.

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.

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

Several species have been, and continue to be, used as subjects in studies of electrical stimulation of the cochlea. Few attempts, however, have been made to determine if data obtained from different species are quantitatively or qualitatively similar. The present work compares psychophysical absolute detection threshold vs. frequency functions for sinusoidal stimuli obtained from humans, nonhuman primates, cats, and guinea pigs. Threshold data for monopolar and bipolar electrode configurations from both previously published and unpublished studies are compared. In general, within all four species, significant intersubject variation in detection threshold level was found, but slopes of threshold vs. frequency functions were relatively well conserved within a species, under the conditions studied. With one exception (cat bipolar stimulation), threshold functions reached a minimum at or near 100 Hz across species and electrode configurations. In all cases, thresholds were significantly lower for monopolar, as compared with bipolar, configurations. Statistically, there were no significant differences in absolute threshold level across species. Threshold levels increased with frequency above 100 Hz at a rate of 3.0-7.9 dB/octave, depending on both electrode configuration and species. Slopes were steeper for monopolar than for bipolar configurations. When slopes were averaged between 200 and 2000 Hz, no statistically significant differences in overall slopes were found, nor was there a significant interaction between electrode configuration and species. There were, however, consistent species differences within more restricted regions of the function. Human functions for both monopolar and bipolar stimulation were steeper than all animal functions in the range of 100-300 Hz. Within this range, the differences between slopes for human and nonhuman subjects were statistically significant. In addition, differences were noted in the frequency at which slope decreased, with slopes for nonhuman subjects showing the decrease at higher frequencies than did those for human subjects. These differences may be true species differences, or may reflect the influence of confounding variables associated with each experimental-subject model.

Encoding loudness by electric stimulation of the auditory nerve

Electric charge has long been hypothesized to be the effective stimulus variable that determines loudness evoked by directly stimulating the auditory nerve. This 'equal-charge, equal-loudness' hypothesis predicts that stimulus amplitude and duration can be traded linearly to produce equal loudness. Loudness sensations from threshold to maximum loudness were measured systematically as a function of stimulus amplitude and duration in cochlear implant listeners. The measured data do not support the equal-charge, equal-loudness hypothesis: an increment in stimulus amplitude produces a significantly louder sensation than the same change in stimulus duration. Instead of the linear equal-charge model, a power-function model successfully predicts the measured data and should be used to encode loudness in electric hearing.

Effects of stimulus configuration on psychophysical operating levels and on speech recognition with cochlear implants

Effects of electrode configuration and pulse duration on operating levels and on speech recognition performance were studied in a group of 14 adult postlingually deaf human subjects with Nucleus cochlear implants. The operating levels (based on detection threshold and maximum comfortable loudness levels) for narrowly spaced bipolar (BP) stimulation were found to be about 11 dB higher on average than those for widely spaced bipolar (BP+6) or monopolar (MP1) stimulation. Operating levels for common ground (CG) stimulation fell between those for BP and BP+6; the difference between BP and CG detection thresholds depended on pulse duration. Variation in detection thresholds and maximum comfortable loudness levels across the electrode array (electrodes 1-15) was larger for BP and CG stimulation than for BP+6 or MP1 stimulation, suggesting narrower spread of activation for the BP and CG configurations despite the higher current levels. Speech recognition performance was tested using experimental processor configurations. Among the experimental electrode configurations tested (BP, CG, and BP+6), the highest speech recognition scores were obtained with the BP+6 configuration in many subjects. Effects of pulse duration on speech recognition were less consistent and usually smaller than the effects of electrode configuration. The results indicate that electrode configuration is an important variable determining speech recognition performance and suggest that restriction of the size of neural population activated by individual channels of the prosthesis is not necessarily advantageous.

Effects of electrode configuration on psychophysical strength-duration functions for single biphasic electrical stimuli in cats

The interface between electrode and neural target tissue is thought to influence certain characteristics of neural and behavioral responses to electrical stimulation of the auditory system. At present, the biophysical properties of this interface are not well understood. Here the effects of biphasic phase duration and electrode configuration on psychophysical threshold in response to electrical stimulation in cats are described. Five cats were trained to respond to acoustic stimuli using food as a reward in an operant reinforcement paradigm. After training, the animals were unilaterally deafened and implanted with a multicontact intracochlear electrode array. Thresholds for single presentations of biphasic current pulses were measured as a function of phase duration and electrode arrangement. Statistical analyses of the data indicated that strength-duration function slopes between 200 and 1600 microseconds/phase were significantly different for the different electrode configurations and, overall, were unrelated to the absolute level of the strength-duration function (i.e., were independent of absolute threshold). For all subjects, the slope of this function for intermediate pulse durations was dependent on electrode configuration and most shallow for radial-bipolar configurations (-3.4 dB/doubling), was steepest for monopolar arrangements (-5.9 dB/doubling), and was intermediate for longitudinal-bipolar pairings. (-4.4 dB/doubling). Slopes for both shorter and longer phase duration stimuli were not significantly different. The underlying mechanisms for these effects may include, or be a combination of altered electrical field patterns, integrated activity across multiple fibers, and stochastic behavior of individual auditory neurons to electrical stimulation.

Functional responses from guinea pigs with cochlear implants. I. Electrophysiological and psychophysical measures

We examined electrophysiological and psychophysical measures of the electrically stimulated auditory system of guinea pigs implanted with chronic intracochlear electrodes. Guinea pigs were trained to detect low-level acoustic stimuli and then unilaterally deafened and implanted with one extracochlear and two intracochlear electrodes. Electrically evoked auditory brainstem responses (EABRs) and psychophysical detection thresholds were obtained from the same animals using pulsatile stimuli. Supplementary EABR data were obtained from additional, untrained, animals. Thresholds were obtained as a function of stimulus phase duration and monopolar and longitudinal-bipolar electrode configurations. The slopes of the EABR and psychophysical functions for bipolar stimulation, averaged across subjects within 1 month after implantation, were -5.25 and -6.18 dB per doubling of pulse duration, respectively. These slopes were obtained with pulse durations ranging from 20 to 400 microseconds/phase; slope was reduced at longer pulse durations. Strength-duration slope also varied as a function of electrode configuration: monopolar stimulation produced steeper functions than did bipolar stimulation. Differences between EABR and psychophysical strength-duration measures suggest the existence of central mechanisms of stimulus integration in addition to that occurring at the level of the auditory nerve. Differences observed with variation of stimulus parameters (e.g., monopolar vs. bipolar stimulation modes) suggest that the specific mode of intracochlear electrical stimulation can influence stimulus integration. Such observations may be useful in the design of prosthetic devices and furthering our understanding of electrical excitation of the auditory system.