Paul J. Abbas

Rate versus level functions for auditory‐nerve fibers in cats: tone‐burst stimuli

Average discharge rate of single auditory‐nerve fibers in cats was measured in response to 400‐msec tone bursts. For each fiber, rate versus stimulus‐level functions were constructed for a number of frequencies. For tones at a fibers characteristic frequency (CF), rate increases rapidly over a range of 20 to 30 dB above threshold. For higher stimulus levels, a range of behaviors is observed. For some fibers, rate saturates completely at higher levels; i.e., there are no further systematic increases in rate when level is increased beyond about 30 dB above threshold. For other units there is a noticeable bend in the rate‐level function at 20 to 30 dB above threshold; however, rate can continue to increase gradually over another 30 to 40 dB. For frequencies above fiber CF, the slope of rate‐level functions measured near the midpoint between maximum and spontaneous rates is a decreasing function of frequency. For frequencies below CF, slope is either approximately constant and equal to the slope at CF or increases to some maximum value as frequency is decreased from the CF. These properties of rate‐level functions are well accounted for by a simple model consisting of a mechanical stage followed by a saturating nonlinearity (transducer stage). The input (pressure) versus output (basilar membrane displacement} functions for the mechanical stage are taken directly from the measurements of Rhode. The input‐output function for the transducer is developed empirically.

Summary of Results Using the Nucleus CI24M implant to record the electrically evoked compound action potential

OBJECTIVE This study outlines a series of experiments using the neural response telemetry (NRT) system of the Nucleus CI24M cochlear implant to measure the electrically evoked compound action potential (EAP). The goal of this investigation was to develop a protocol that allows successful recording of the EAP in a majority of CI24M cochlear implant users. DESIGN Twenty-six postlingually deafened adults participated in this study. A series of experiments were conducted that allowed us to examine how manipulation of stimulation and recording parameters may affect the morphology of the EAP recorded using the Nucleus NRT system. RESULTS Results of this study show consistent responses on at least some electrodes from all subjects. Cross-subject and cross electrode variations in both the growth of the response and the temporal refractory properties of the response were observed. The range of stimulus and recording parameters that can be used to record the EAP with the Nucleus NRT system is described. CONCLUSIONS Using the protocol outlined in this study, it is possible to reliably record EAP responses from most subjects and for most electrodes in Nucleus CI24M cochlear implant users. These responses are robust and recording these responses does not require that the subject sleep or remain still. Based on these results, a specific protocol is proposed for measurement of the EAP using the NRT system of the CI24M cochlear implant. Potential clinical implications of these results are discussed.

The relationship between EAP and EABR thresholds and levels used to program the Nucleus 24 speech processor : Data from adults

Objective: The objective of this study was to determine the relationship between electrically evoked whole nerve action potential (EAP) and electrical auditory brain stem response (EABR) thresholds and MAP threshold (T‐level) and maximum comfort level (C‐level) for subjects who use the Nucleus 24 cochlear implant system. Design: Forty‐four adult Nucleus 24 cochlear implant users participated in this study. EAP thresholds were recorded using the Neural Response Telemetry System developed by Cochlear Corporation. EABR thresholds were measured for a subset of 14 subjects using standard evoked potential techniques. These physiologic thresholds were collected on a set of five electrodes spaced across the cochlea, and were then compared with behavioral measures of T‐level and C‐level used to program the speech processor. Results: EAP thresholds were correlated with MAP T‐ and C‐levels; however, the correlation was not strong. A technique for improving the correlation by combining measures of T‐ and C‐levels made on one electrode with the EAP thresholds was presented. Correlations between predicted and measured T‐ and C‐levels using this technique were 0.83 and 0.77, respectively. Similar results were obtained using the EABR thresholds for a smaller set of subjects. In general, EABR thresholds were recorded at levels that were approximately 4.7 programming units lower than EAP thresholds. Conclusions: Either EAP or EABR thresholds can be used in combination with a limited amount of behavioral information to predict MAP T‐ and C‐levels with reasonable accuracy.

Electrically evoked whole-nerve action potentials : data from human cochlear implant users

This study describes a method for recording the electrically evoked, whole-nerve action potential (EAP) in users of the Ineraid cochlear implant. The method is an adaptation of one originally used by Charlet de Sauvage et al. [J. Acoust. Soc. Am. 73, 615-627 (1983)] in guinea pigs. The response, recorded from 11 subjects, consists of a single negative peak that occurs with a latency of approximately 0.4 ms. EAP input/output functions are steeply sloping and monotonic. Response amplitudes ranging up to 160 micro V have been recorded. Slope of the EAP input/output function correlates modestly (approximately 0.6-0.69) with results of tests measuring word recognition skills. The refractory properties of the auditory nerve were also assessed. Differences across subjects were found in the rate of recovery from the refractory state. These findings imply that there may be difference across subjects in the accuracy with which rapid temporal cues can be coded at the level of the auditory nerve. Reasonably strong correlations (approximately 0.74-0.85) have been found between the magnitude of the slope of these recovery curves and performance on tests of word recognition.

Multivariate Predictors of Audiological Success with Multichannel Cochlear Implants

To predict the audiological outcomes of 2 multichannel cochlear implants, a preoperative battery of historical, audiological, electrophysiologic, and psychologic variables from 48 postlingually deafened adults was tested in a prospective randomized clinical trial. Multivariate analyses were used to select and combine these preoperative variables in a predictive index that was significantly related to audiological outcome at 9 months. The preoperative variables included in the predictive index were duration of profound deafness, speech reading ability, residual hearing, cognitive ability, measures of compliance and engagement with treatment, and use of nonverbal communication strategies. The preoperative predictive index had correlations of .81 with the Iowa Sentences Test, and .78 with the NU6 word understanding scores, both obtained in a sound-only test Probability and percentile curves generated from these data offer considerable optimism in forecasting the range of likely audiological outcomes that would be realized by postlingually deafened adult candidates for multichannel cochlear implants.

Comparison of EAP thresholds with MAP levels in the nucleus 24 cochlear implant: data from children.

Objective: The purpose of this study is to examine the relationship between the electrically evoked compound action potential (EAP) thresholds and the MAP thresholds (T‐levels) and maximum comfort levels (C‐levels) in children implanted with the Nucleus 24 device. Design: EAP thresholds were measured using the Neural Response Telemetry system of the Nucleus 24 device. Twenty children implanted with the Nucleus 24 cochlear implant participated in this study. EAP thresholds were compared with the behavioral measures of T‐ and C‐level used to construct the MAP these children used on a daily basis. For these subjects, both EAP and MAP T‐ and C‐levels were obtained the same visit, which occurred at 3 to 5 mo postconnection. Results: EAP thresholds were shown to fall between MAP T‐ and C‐level for 18 of 20 subjects tested; however, considerable variability across subjects was noted. On average, EAP thresholds fell at 53% of the MAP dynamic range. Correlations between EAP threshold and MAP T‐ and C‐level improved substantially when combined with behavioral measures obtained from one electrode in the array. Conclusions: Moderate correlations were found between EAP thresholds and MAP T‐ and C‐levels for the children participating in this study. However, a technique is described for improving the accuracy of predictions of MAP T‐ and C‐levels based on EAP data combined with a small amount of behavioral information.

A longitudinal study of electrode impedance, the electrically evoked compound action potential, and behavioral measures in nucleus 24 cochlear implant users.

Objective The primary goal of this study was to examine changes that may occur in electrode impedance, electrically evoked compound action potential (EAP) threshold and slope of the EAP growth function, and behavioral measures of threshold (MAP T-level) and maximum comfort (C-level) over time in both adult and child cochlear implant users. Secondary goals were to determine whether changes in these measures are consistent between children and adults, and to determine whether behavioral measures (MAP T- and C-levels) and electrophysiologic measures (EAP thresholds) exhibit the same trends over time. Design Thirty-five children and 33 adults implanted with the Nucleus CI24M between November 1996 and August 1999 participated in this study. Subjects were included in this study if 1) they had used their implant for at least 1 yr after device connection, and 2) they had participated in the necessary data collection at a minimum number of the time intervals assessed in this study. EAP threshold, slope of the EAP growth function, and common ground electrode impedance measures were collected intraoperatively, at initial stimulation, and at several subsequent visits up to 2 yr post initial stimulation. MAP T- and C-levels were measured at initial stimulation and at the same time intervals as described above. Results Changes in electrode impedance, EAP thresholds, and slope of the EAP growth function from measures made intraoperatively, at initial stimulation, and at 1 to 2 mo post initial stimulation were similar in both children and adults. Beyond the 1- to 2-mo visit, children exhibited significant increases in electrode impedance, EAP thresholds, slope, and MAP T-levels, whereas these same measures in adults remained relatively stable. EAP thresholds in children stabilized by the 3- to 8-mo visit, and electrode impedance stabilized by the 6- to 8-mo visit, while slope of the EAP growth function, MAP T-levels, and MAP C-levels were stable by 1 yr post initial stimulation. C-levels in adults increased up to 1 yr post initial stimulation; however, the amount of increase was much smaller than that seen in children. In both children and adults, longitudinal trends in EAP thresholds mirrored T-level more closely than C-level. Conclusions The results of this study suggest that peripheral changes occur in many children that do not generally occur in adults within the first year of cochlear implant use. One implication of these results is that if EAP thresholds are to be used to assist in programming the speech processor for children, it is best to make those measures at the same time interval as device programming rather than using measures made intraoperatively or at the initial programming session to set MAP levels at later visits.

Binaural cochlear implants placed during the same operation

Objective To evaluate the binaural listening advantages for speech in quiet and in noise and to localize sound when independently programmed binaural cochlear implants are used, and to determine whether ears with different hearing ability and duration of profound deafness perform differently with cochlear implants as well as to what extent preimplant psychophysical and physiologic assessment could be predictive of performance. Study Design Prospective study in which patients were prospectively selected to undergo bilateral implantation during a single surgical procedure at a tertiary referral center. All testing was performed with patients using their right, left, or both cochlear implants. Preimplant and intraoperative measures used electrical stimulation at the round window and stimulation through the cochlear implant. Results Bilateral implantation during the same operation did not cause any postoperative problems such as severe vertigo or ataxia. At 1 year, results of speech testing in quiet demonstrated a binaural advantage for 2 of 10 subjects. Speech-in-noise testing demonstrated that two implants were beneficial for two individuals. All subjects benefited from a head shadow effect when an ear with a better signal-to-noise ratio was available. The ability to localize sound was improved with binaural implants in all subjects. Preimplant psychophysical or physiologic measures were not predictive of eventual speech perception performance. Conclusion Binaural cochlear implants can assist in the localization of sounds and have the potential in some individuals to improve speech understanding in quiet and in noise.

Two‐tone suppression in auditory‐nerve fibers: Extension of a stimulus‐response relationship

Average discharge rate of single auditory‐nerve fibers in cats was measured in response to one‐ and two‐tone stimuli. One component (the ’’suppressor tone’’) of each two‐tone stimulus was at a frequency (f2) which produced two‐tone suppression at some stimulus levels. The other component (excitor tone) produced an increase in rate above the spontaneous rate when presented alone. Fractional response was defined as the driven rate to the two‐tone stimulus divided by the driven rate to the excitor alone. Fractional response is thus a quantitative measure of the amount of suppression produced by a suppressor tone. A number of qualitative differences were found in the dependence of fractional response for f2≳CF and f2<CF. For suppressor tone frequencies greater than CF, fractional response depends only on the ratio of suppressor to excitor levels (P2/P1) for a range of excitor levels (P1). For P1 large enough to drive a unit into saturation, fractional response increases with P1. For f2<CF, however, fractional...

Response Properties of the Refractory Auditory Nerve Fiber

The refractory characteristics of auditory nerve fibers limit their ability to accurately encode temporal information. Therefore, they are relevant to the design of cochlear prostheses. It is also possible that the refractory property could be exploited by prosthetic devices to improve information transfer, as refractoriness may enhance the nerves stochastic properties. Furthermore, refractory data are needed for the development of accurate computational models of auditory nerve fibers. We applied a two-pulse forward-masking paradigm to a feline model of the human auditory nerve to assess refractory properties of single fibers. Each fiber was driven to refractoriness by a single (masker) current pulse delivered intracochlearly. Properties of firing efficiency, latency, jitter, spike amplitude, and relative spread (a measure of dynamic range and stochasticity) were examined by exciting fibers with a second (probe) pulse and systematically varying the masker-probe interval (MPI). Responses to monophasic cathodic current pulses were analyzed. We estimated the mean absolute refractory period to be about 330 micros and the mean recovery time constant to be about 410 micros. A significant proportion of fibers (13 of 34) responded to the probe pulse with MPIs as short as 500 micros. Spike amplitude decreased with decreasing MPI, a finding relevant to the development of computational nerve-fiber models, interpretation of gross evoked potentials, and models of more central neural processing. A small mean decrement in spike jitter was noted at small MPI values. Some trends (such as spike latency-vs-MPI) varied across fibers, suggesting that sites of excitation varied across fibers. Relative spread was found to increase with decreasing MPI values, providing direct evidence that stochastic properties of fibers are altered under conditions of refractoriness.