Effects of level on nonspectral frequency difference limens for electrical and acoustic stimuli

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.

Psychophysical assessment of stimulation sites in auditory prosthesis electrode arrays

Auditory prostheses use implanted electrode arrays that permit stimulation at many sites along the tonotopic axis of auditory neurons. Psychophysical studies demonstrate that measures of implant function, such as detection and discrimination thresholds, vary considerably across these sites, that the across-site patterns of these measures differ across subjects, and that the likely mechanisms underlying this variability differ across measures. Psychophysical and speech recognition studies suggest that not all stimulation sites contribute equally to perception with the prosthesis and that some sites might have negative effects on perception. Studies that reduce the number of active stimulation sites indicate that most cochlear implant users can effectively utilize a maximum of only about seven sites in their processors. These findings support a strategy for improving implant performance by selecting only the best stimulation sites for the processor map. Another approach is to revise stimulation parameters for ineffective sites in an effort to improve acuity at those sites. In this paper, we discuss data supporting these approaches and some potential pitfalls.

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

This study examined current-level discrimination ability in listeners with cochlear implants using bipolar and monopolar electrode configurations. Current-level discrimination ability was measured as a function of electrode configuration (monopolar and bipolar), stimulation site (8 and 16) and level (5%, 15%, 25%, 50% and 80% of the dynamic range). Weber fractions usually decreased with increasing level. Differences in Weber fractions between monopolar and bipolar configurations were observed for listeners with short durations of deafness (<5 years). For these listeners, in the bipolar condition at the more-apical site 16, Weber fractions remained constant with increasing level, and the Weber fractions at low levels were smaller than in other conditions. We suggest that nerve density was better and the nerve-to-site-of-action-potential distance was smaller in these cases such that more fibers could be recruited with a unit increase in current level, leading to better current-level sensitivity.

Intensity discrimination and increment detection in cochlear-implant users

Intensity difference limens (DLs) were measured in users of the Nucleus 22 and Clarion v1.2 cochlear implants and in normal-hearing listeners to better understand mechanisms of intensity discrimination in electric and acoustic hearing and to evaluate the possible role of neural adaptation. Intensity DLs were measured for three modes of presentation: gated (intensity increments gated synchronously with the pedestal), fringe (intensity increments delayed 250 or 650 ms relative to the onset of the pedestal), and continuous (intensity increments occur in the presence of a pedestal that is played throughout the experimental run). Stimuli for cochlear-implant listeners were trains of biphasic pulses; stimuli for normal-hearing listeners were a 1-kHz tone and a wideband noise. Clarion cochlear-implant listeners showed level-dependent effects of presentation mode. At low pedestal levels, gated thresholds were generally similar to thresholds obtained in the fringe and continuous conditions. At higher pedestal levels, however, the fringe and continuous conditions produced smaller intensity DLs than the gated condition, similar to the gated-continuous difference in intensity DLs observed in acoustic hearing. Nucleus cochlear-implant listeners did not show consistent threshold differences for the gated and fringe conditions, and were not tested in the continuous condition. It is not clear why a difference between gated and fringe thresholds occurred for the Clarion but not the Nucleus subjects. Normal-hearing listeners showed improved thresholds for the continuous condition relative to the gated condition, but the effect was larger for the 1-kHz tonal carrier than for the noise carrier. Findings suggest that adaptation occurring central to the inner hair cell synapse mediates the gated-continuous difference observed in Clarion cochlear-implant listeners and may also contribute to the gated-continuous difference in acoustic hearing.

Auditory cortical images of cochlear-implant stimuli: coding of stimulus channel and current level

This study quantified the accuracy with which populations of neurons in the auditory cortex can represent aspects of electrical cochlear stimuli presented through a cochlear implant. We tested the accuracy of coding of the place of stimulation (i.e., identification of the active stimulation channel) and of the stimulus current level. Physiological data came from the companion study, which recorded spike activity of neurons simultaneously from 16 sites along the tonotopic axis of the guinea pig's auditory cortex. In that study, cochlear electrical stimuli were presented to acutely deafened animals through 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 = 3), 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. In the present analysis, an artificial neural network was trained to recognize spatiotemporal patterns of cortical activity in response to single presentations of particular stimuli and, thereby, to identify those stimuli. The accuracy of pair-wise discrimination of stimulation channels or of current levels was represented by the discrimination index, d', where d' = 1 was taken as threshold. In many cases, the threshold for discrimination of place of cochlear stimulation was < 0.75 mm, and the threshold for discrimination of current levels was < 1 dB. Cochlear electrode configurations varied in the accuracy with which they signaled to the auditory cortex the place of cochlear stimulation. The BP + N and TP configurations provided considerably greater sensitivity to place of stimulation than did the MP configuration. The TP configuration maintained accurate signaling of place of stimulation up to the highest current levels, whereas sensitivity was degraded at high current levels in BP + N configurations. Electrode configurations also varied in the dynamic range over which they signaled stimulus current level. Dynamic ranges were widest for the BP + 0 configuration and narrowest for the TP configuration. That is, the configuration that showed the most accurate signaling of cochlear place of stimulation (TP) showed the most restricted dynamic range for signaling of current level. These results suggest that the choice of the optimal electrode configuration for use by human cochlear-prosthesis users would depend on the particular demands of the speech-processing strategy that is to be employed.

Auditory detection and discrimination in deaf cats: psychophysical and neural thresholds for intracochlear electrical signals

More than 30,000 hearing-impaired human subjects have learned to use cochlear implants for speech perception and speech discrimination. To understand the basic mechanisms underlying the successful application of contemporary speech processing strategies, it is important to investigate how complex electrical stimuli delivered to the cochlea are processed and represented in the central auditory system. A deaf animal model has been developed that allows direct comparison of psychophysical thresholds with central auditory neuronal thresholds to temporally modulated intracochlear electrical signals in the same animals. Behavioral detection thresholds were estimated in neonatally deafened cats for unmodulated pulse trains (e.g., 30 pulses/s or pps) and sinusoidal amplitude-modulated (SAM) pulse trains (e.g., 300 pps, SAM at 30 Hz; 300/30 AM). Animals were trained subsequently in a discrimination task to respond to changes in the modulation frequency of successive SAM signals (e.g., 300/8 AM vs. 300/30 AM). During acute physiological experiments, neural thresholds to pulse trains were estimated in the inferior colliculus (IC) and the primary auditory cortex (A1) of the anesthetized animals. Psychophysical detection thresholds for unmodulated and SAM pulse trains were virtually identical. Single IC neuron thresholds for SAM pulse trains showed a small but significant increase in threshold (0.4 dB or 15.5 microA) when compared with thresholds for unmodulated pulse trains. The mean difference between psychophysical and minimum neural thresholds within animals was not significant (mean = 0.3 dB). Importantly, cats also successfully discriminated changes in the modulation frequencies of the SAM signals. Performance on the discrimination task was not affected by carrier rate (100, 300, 500, 1,000, or 1,500 pps). These findings indicate that 1) behavioral and neural response thresholds are based on detection of the peak pulse amplitudes of the modulated and unmodulated signals, and 2) discrimination of successive SAM pulse trains is based on temporal resolution of the envelope frequencies. Overall, our animal model provides a robust framework for future studies of behavioral discrimination and central neural temporal processing of electrical signals applied to the deaf cochlea by a cochlear implant.

Effect of current level on electrode discrimination in electrical stimulation

The effect of the stimulation intensity (current amplitude) on the ability to discriminate electrodes was tested in an experiment with four adult users of the Nucleus-22 cochlear implant. A total of 12 adjacent pairs of electrodes were used in the four-interval forced-choice discrimination task with random current variation. Tests were carried out at three average stimulation levels: 40 and 70% of the dynamic range and close to maximum comfortable loudness. Analysis of variance revealed a significant (P<0.0001) deterioration in electrode discrimination with a decreasing level. However, the overall effect was very small, representing a deterioration in the discrimination score of only 18% correct from the highest to lowest levels tested. The reason for the small deterioration in discriminability with a decreasing level is difficult to determine from this experiment, however, the results are consistent with the hypothesis that changes in the 'peak' or 'edge' of the excitation pattern are more important for discrimination tasks than the relative amount of non-overlap of the excitation areas from the two electrodes.

Effects of stimulus level on electrode-place discrimination in human subjects with cochlear implants

Effects of stimulus level on discrimination of one stimulation site from another were examined in 15 human subjects with Nucleus-22 cochlear implant systems. Bipolar stimulation was used in all cases with electrodes in the bipolar pair separated by 1.5 mm (center to center). Subjects were first tested at a medium loudness level, using an adaptive tracking procedure, to determine the regions of the electrode array where electrode-place discrimination was best and the regions where it was poorest. Electrode-place discrimination was then tested at three regions distributed throughout the array, which included the regions of best and poorest discrimination. At each region, electrode-place discrimination was tested at three levels: 25%, 50%, and 75% of the dynamic range. For each of these nine conditions (3 sites x 3 levels), the test-electrode pairs were loudness balanced with the reference-electrode pairs. A two-interval forced-choice same-different procedure was then used to determine discriminability of the reference-electrode pair from the nearest, apical, test-electrode pair. If P(C)max was <0.707 at all three levels, additional testing was done using the next, more apical, electrode pair as the test-electrode pair. A tendency toward better discrimination at more apical regions of the array was observed. Electrode pairs with poor discrimination typically had smaller dynamic ranges than those with good discrimination. There was a weak tendency toward better discrimination at higher levels of stimulation. However, effects of level on electrode-place discrimination were less pronounced and less consistent than previously observed effects of level on temporal discriminations. These results suggest interactions between current spread and the condition of the implanted cochlea as underlying mechanisms.

The perceptual effects of current pulse duration in electrical stimulation of the auditory nerve

In cochlear implants employing pulsatile stimulation, loudness is controlled by current amplitude and/or pulse duration. Five experiments were conducted with cochlear implantees to investigate the hypothesis that perceptual effects other than loudness result from changes in pulse duration for durations from 50 to 266 microseconds. In experiment 1, five subjects' ability to discriminate equally loud pulse trains employing differing pulse durations was measured at four electrode positions. In 11 of the 20 cases, subjects could significantly discriminate these stimuli. In experiments 2 and 3, discrimination was measured of dual-electrode stimuli which differed in overall temporal pattern but had an equal temporal pattern on each of the individual electrodes (separated by 0 to 9 mm). Discrimination was compared for stimuli employing short or long pulse durations and, in experiment 3, employing different pulse durations on each electrode. When the pulse duration was longer, six out of seven subjects could either combine temporal information across electrode positions at wider electrode separations (experiment 2) or had better discrimination at the same electrode separation (experiment 3). This result was consistent with the hypothesis that longer pulse durations result in a greater spread of excitation than equally loud stimuli using shorter pulse durations. In experiment 4, pulse rate discrimination was compared for stimuli with differing pulse durations, and in four out of five subjects, there was no effect of pulse duration. Finally, the dB change in current per doubling of pulse duration for threshold and equally loud stimuli was calculated for nine subjects (52 electrodes). Values ranged from -5.9 to -2.0 dB/doubling, and were significantly correlated with the absolute intensity of the stimulus. This result was hypothesized to be due to a relationship between the neural charging characteristics and the distance of the excited neural elements from the electrode.

Fos-like immunoreactivity in the auditory brainstem evoked by bipolar intracochlear electrical stimulation: effects of current level and pulse duration

Fos-like immunoreactivity was used to compare the auditory brain stem excitation elicited by bipolar electrical stimulation of the cochlea at various current levels relative to the electrically evoked auditory brain stem response threshold for a 50-micros/phase monophasic pulse. Fos-like immunoreactive cells were labeled in primary auditory brain stem regions. The distribution of labeled cells was restricted to regions known to be cochleotopically related to the stimulated region of the scala tympani. Some labeled cells were observed at 2x electrically evoked auditory brain stem response threshold. The number, density and spatial distribution of labeled cells were quantified in the dorsal cochlear nucleus and inferior colliculus, and found to increase with increasing level of stimulation. For 50-micros pulses, the location of labeled neurons remained reasonably restricted to narrow bands within each region until the 1Ox level of stimulation (20 dB above electrically evoked auditory brain stem response threshold) was reached. While a monotonic increase in Fos-like immunoreactivity with increasing stimulus level was observed in most nuclei, for cells of the superficial layer of the dorsal cochlear nucleus, a non-monotonic change with increasing stimulus level was seen. This dorsal cochlear nucleus non-monotonicity may indicate that, at higher levels of stimulation, a secondary indirect inhibitory input, probably associated with activation of deep layer dorsal cochlear nucleus cells, reduces excitatory responses at the superficial layer of the dorsal cochlear nucleus. Electrically evoked auditory brain stem response and Fos expression showed parallel changes as a function of stimulus level and pulse duration. The data indicate that discrete activation of cell populations within the central auditory pathways can occur with bipolar electrical stimulation to the highest levels of stimulation typically useful in humans. The data also indicate a close, but not identical, quantitative relationship between Fos-like immunoreactivity and electrophysiological response amplitude. These findings support the view that a study of Fos-like immunoreactivity can provide a powerful and quantitative tool for study of the dynamic response characteristics of cells of the central auditory system to electrical stimulation at suprathreshold levels. The data suggest that there is a monotonic increase in the number of neurons responsive to intracochlear electrical stimulation as a function of stimulus level, at least through the upper half of the dynamic range, but that this increase does not result in a complete loss of spatial selectivity. Coupled with previous psychophysical studies, these results suggest that the increase in the number of activated neurons is functionally beneficial, resulting in improved discrimination of changes in the electrical signals.

Effects of chronic high-rate electrical stimulation on the cochlea and eighth nerve in the deafened guinea pig

This study was undertaken to examine the effects of chronic high-rate stimulation on the eighth nerve and cochlea. Fifty-four male pigmented guinea pigs were deafened and implanted with single ball electrodes in scala tympani. Four groups of animals received chronic electrical stimulation at a level of 5 microCol/cm2/ph for 1000 h as follows: Group A: 1000 Hz, 100 microseconds/ph duration, 100 microA peak; Group B: 250 Hz, 100 microseconds/ph duration, 100 microA peak; Group C: 2750 Hz, 36 microseconds/ph duration, 250 microA peak; Group D: 250 Hz, 400 microseconds/ph duration, 25 microA peak. Also, two control groups received 20 min stimulation during weekly electrically evoked auditory brainstem response (eABR) measurement (Group E) and about 5 s stimulation (Group F) during a brief eABR 3 day postimplantation and at perfusion. On Day 50, animals were perfused, midmodiolar sections cut and a quantitative assessment of spiral ganglion cells (SGC) performed. All stimulated subjects showed a similar decrease in eABR thresholds and dynamic range over time. No stimulation conditions induced pathology. All stimulation conditions enhanced survival of SGCs compared to unimplanted ears and implanted non-stimulated ears (Group F). There were no statistically significant differences in SGC survival between any stimulated groups, including Group E stimulated once a week. In conclusion, high-rate stimulation, under the conditions of this study, provides no additional risks and the same benefits to SGC survival as low-rate stimulation.

Effects of stimulus level on nonspectral frequency discrimination by human subjects

Frequency difference limens were determined as a function of reference-stimulus level for pulsatile electrical stimuli in 5 postlingually deaf human subjects with Nucleus-22 cochlear implants and for sinusoidally amplitude-modulated acoustic white noise stimuli in 4 normal-hearing humans. Subjects were tested at levels throughout the dynamic range and extending to the lowest detectable levels. Response stability was measured over the course of 10 sessions. For electrical stimulation in the deaf ears, difference limens decreased as a function of level throughout much or all of the dynamic range of hearing. This result contrasts with the case for nonspectral acoustic stimulation of normal-hearing subjects, where nonspectral frequency difference limens were strongly affected by level only near the detection threshold. These data suggest differences in the acoustic and electrical response spaces that must be considered in the design of auditory prosthesis processors.

Electrical stimulation of the auditory nerve: effects of pulse width on frequency discrimination

Effects of pulse width on discrimination of simultaneous changes in frequency and level of electrical pulse trains were studied in a monkey subject with a cochlear implant. At test-stimulus levels where performance was minimum, frequency difference limens were larger for longer-duration pulses than that for shorter-duration pulses. Several factors may have contributed to these differences.