PubMed Health⌕ Search

Biomedical subjects

Paul J Abbas

Publications and source records attributed to Paul J Abbas.

17 recordsLinked to original sources
PubMed2006

Electrical excitation of the acoustically sensitive auditory nerve: single-fiber responses to electric pulse trains.

Nearly all studies on auditory-nerve responses to electric stimuli have been conducted using chemically deafened animals so as to more realistically model the implanted human ear that has typically been profoundly deaf. However, clinical criteria for implantation have recently been relaxed. Ears with "residual" acoustic sensitivity are now being implanted, calling for the systematic evaluation of auditory-nerve responses to electric stimuli as well as combined electric and acoustic stimuli in acoustically sensitive ears. This article presents a systematic investigation of single-fiber responses to electric stimuli in acoustically sensitive ears. Responses to 250 pulse/s electric pulse trains were collected from 18 cats. Properties such as threshold, dynamic range, and jitter were found to differ from those of deaf ears. Other types of fiber activity observed in acoustically sensitive ears (i.e., spontaneous activity and electrophonic responses) were found to alter the temporal coding of electric stimuli. The electrophonic response, which was shown to greatly change the information encoded by spike intervals, also exhibited fast adaptation relative to that observed in the "direct" response to electric stimuli. More complex responses, such as "buildup" (increased responsiveness to successive pulses) and "bursting" (alternating periods of responsiveness and unresponsiveness) were observed. Our findings suggest that bursting is a response unique to sustained electric stimulation in ears with functional hair cells.

Animals↗
PubMed2006

Improved noise reduction in single fiber auditory neural responses using template subtraction.

When recording single-unit responses from neural systems, a common problem is the accurate detection of spikes (action potentials) in the presence of competing unwanted (noise) signals. While some sources of noise can be readily dealt with through filtering or established "template subtraction" techniques, other sources present a more difficult problem. In particular, noise components introduced by power supplies, which contain harmonics of the power-line frequency, can be particularly troublesome in that they can mimic the shape of the desired spikes. The aforementioned standard techniques typically fail to effectively deal with such "noise". In this study, we propose the use of a novel template-subtraction scheme that involves estimating the power-line noise waveform and using cross-correlation techniques to subtract it from the recordings. This technique requires two key steps: (1) cross-correlation analysis of each recorded waveform to extract a robust representation of the power-line noise waveform and (2) a second level of cross-correlation to successfully subtract that representation from each recorded waveform. This paper describes this algorithm and provides examples of its implementation using actual recorded waveforms that were contaminated with these power-line noise signals. An improvement (reduction) in the noise level is reported, as are suggestions for future implementation of this strategy.

Action Potentials↗
PubMed2006

The relation between electrophysiologic channel interaction and electrode pitch ranking in cochlear implant recipients.

The primary goal of this study was to examine the relation between electrophysiologic measures of channel interaction and the ability to discriminate pitch between electrodes in a psychophysical pitch-ranking task. It was hypothesized that cochlear implant recipients should perform better on an electrode pitch-ranking task when using electrodes with less channel interaction as measured with the electrically evoked compound action potential (ECAP). The width of the ECAP channel interaction function was compared with the slope of the pitch-ranking function for 10 adult Nucleus 24 recipients. Results showed no significant correlation between electrode pitch-ranking ability and width of the ECAP channel interaction function for individual subjects or for group data. Additionally, there was no significant correlation between speech perception performance and either pitch-ranking ability or width of the ECAP channel interaction function. These results suggest that the width of the ECAP interaction function may not be an accurate predictor of the ability to discriminate between electrodes on the basis of pitch.

Action Potentials↗
PubMed2006

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.

Action Potentials↗
PubMed2005

Design, analysis and simulation for development of the first clinical micro-CT scanner.

In this article, we propose to develop the first clinical micro-CT (CMCT) system for human temporal bone imaging in vivo. This CMCT system consists of medical CT and micro-CT scanners either as separate components or in a combination, a cross-modality registration mechanism such as a facial surface scanner, and associated software. This system integrates the strengths of state-of-the-art medical CT and micro-CT techniques to achieve a spatial resolution that is much higher than currently available for inner ear imaging at acceptable dose levels. Our design, analysis, and simulation results demonstrate that the CMCT system is feasible for inner ear imaging and other clinical applications. For example, the CMCT system has the potential to improve the safety of guiding cochlear implant electrodes within the inner ear and assist the placement of inner ear microcatheters for delivery of gene modification therapy or administration of neurotrophic factors. Imaging of microarchitectures of the cancellous bone would be also an important application.

Animals↗
PubMed2005

Effects of acoustic noise on the auditory nerve compound action potentials evoked by electric pulse trains.

This study investigated the effects of acoustic noise on the auditory nerve compound action potentials in response to electric pulse trains. Subjects were adult guinea pigs, implanted with a minimally invasive electrode to preserve acoustic sensitivity. Electrically evoked compound action potentials (ECAP) were recorded from the auditory nerve trunk in response to electric pulse trains both during and after the presentation of acoustic white noise. Simultaneously presented acoustic noise produced a decrease in ECAP amplitude. The effect of the acoustic masker on the electric probe was greatest at the onset of the acoustic stimulus and it was followed by a partial recovery of the ECAP amplitude. Following cessation of the acoustic noise, ECAP amplitude recovered over a period of approximately 100-200 ms. The effects of the acoustic noise were more prominent at lower electric pulse rates (interpulse intervals of 3 ms and higher). At higher pulse rates, the ECAP adaptation to the electric pulse train alone was larger and the acoustic noise, when presented, produced little additional effect. The observed effects of noise on ECAP were the greatest at high electric stimulus levels and, for a particular electric stimulus level, at high acoustic noise levels.

Action Potentials↗
PubMed2005

An analysis of the impact of auditory-nerve adaptation on behavioral measures of temporal integration in cochlear implant recipients.

The objective of this study was to determine the impact that auditory-nerve adaptation has on behavioral measures of temporal integration in Nucleus 24 cochlear implant recipients. It was expected that, because the auditory nerve serves as the input to central temporal integrator, a large degree of auditory-nerve adaptation would reduce the amount of temporal integration. Neural adaptation was measured by tracking amplitude changes of the electrically evoked compound action potential (ECAP) in response to 1000-pps biphasic pulse trains of varying durations. Temporal integration was measured at both suprathreshold and threshold levels by an adaptive procedure. Although varying degrees of neural adaptation and temporal integration were observed across individuals, results of this investigation revealed no correlation between the degree of neural adaptation and psychophysical measures of temporal integration.

Acoustic Stimulation↗
PubMed2004

Response of the auditory nerve to sinusoidal electrical stimulation: effects of high-rate pulse trains.

Electrical stimulation of the auditory nerve produces highly synchronized responses. As a consequence, electrical stimulation may result in a narrow dynamic range of hearing and poor temporal representation of an input signal. The electrically evoked compound action potential (ECAP) is an electrophysiologic response used for neural assessment in individuals with auditory prostheses. Because the ECAP arises from the activity of a population of auditory nerve fibers, within- and across-fiber synchrony should be evident in the responses. Due to its clinical relevance and reflection of neural response properties, the ECAP is used in the present study to examine changes in neural synchrony. Empirical and modeled single-fiber data indicate that stimulation with electrical pulses of a sufficiently high rate may induce stochastic neural response behaviors. This study investigated the effects of adding high-rate conditioning pulses (5000 pps) on the ECAP in response to 100 Hz electrical sinusoids. The results showed that high-rate conditioning pulses increased response amplitudes at low sinusoidal levels and decreased the amplitudes at high sinusoidal levels, indicating a decrease in the slope of the ECAP growth functions to sinusoidal stimuli. The results are consistent with a hypothesis that high-rate conditioning pulses increase single-fiber relative spread (RS) in response to sinusoidal stimuli, and the effect is highly dependent on the level of the high-rate conditioning pulses.

Action Potentials↗
PubMed2004

Intracochlear and extracochlear ECAPs suggest antidromic action potentials.

With experimental animals, the electrically evoked compound action potential (ECAP) can be recorded from multiple sites (e.g., round window, intracranial and intracochlear sites). However, human ECAPs are typically recorded from intracochlear electrodes of the implanted array. To bridge this difference, we obtained ECAPs from cats using both intracochlear and nerve-trunk recording sites. We also sought to determine how recording the site influences the acquired evoked potential and how those differences may provide insight into basic excitation properties. In the main experiment, ECAPs were recorded from four acutely deafened cats after implanting a Nucleus-style banded electrode array. Potentials were recorded from an electrode positioned on the nerve trunk and an intracochlear electrode. We manipulated stimulus level, electrode configuration (monopolar vs bipolar) and stimulus polarity, variables that influence the site of excitation. Intracochlear ECAPs were found to be an order of magnitude greater than those obtained with the nerve-trunk electrode. Also, compared with the nerve-trunk potentials, the intracochlear ECAPs more closely resembled those obtained from humans in that latencies were shorter and the waveform morphology was typically biphasic (a negative peak followed by a positive peak). With anodic monophasic stimuli, the ECAP had a unique positive-to-negative morphology which we attributed to antidromic action potentials resulting from a relatively central site of excitation. We also collected intracochlear ECAPs from twenty Nucleus 24 implant users. Compared with the feline ECAPs, the human potentials had smaller amplitudes and longer latencies. It is not clear what underlies these differences, although several factors are considered.

Action Potentials↗
PubMed2004

Feasibility of using silicon-substrate recording electrodes within the auditory nerve.

The use of penetrating, silicon-substrate (i.e., "thin-film") probes within a cross-section of a sensory nerve offers the possibility of assessing the pattern and extent of fiber excitation within the nerve. We used acute cat preparations to assess the feasibility of this technique for recordings within the auditory nerve trunk. Four probe configurations fabricated by the University of Michigan Center for Neural Communication Technology were evaluated using acoustic and electric stimuli. Our main concerns were the nature of the recorded potentials and the degree of spatial selectivity provided by these probes. We also made some basic assessments of electrode-tissue compatibility. The recorded potentials were characterized as field potentials with varying degrees of spatial selectivity. In some cases, responses to pure tones demonstrated good spatial selectivity, with unique responses recorded by different electrode sites. When electrode sites were positioned at different longitudinal positions along the nerve trunk, responses with latencies characteristic of each site were recorded. These results indicate that thin-film electrodes are capable of providing spatially specific response information from sensory nerves. However, in the case of feline auditory nerves, place-specific responses were inconsistently observed, making it difficult to use this technique to obtain detailed cochleotopic maps of neural excitation. More productive results may be possible from other peripheral nerves with less complex spatial arrangements of fibers.

Acoustic Stimulation↗
PubMed2004

Sensitivity and specificity of averaged electrode voltage measures in cochlear implant recipients.

OBJECTIVE: The primary purpose of this study was to evaluate the diagnostic accuracy of averaged electrode voltages (AEVs) for identifying malfunctioning electrodes in cochlear implant users. DESIGN: AEVs were measured using common ground (N = 33 ears) and monopolar stimulation modes (N = 40 ears) in adult subjects implanted with the Nucleus 24M or Nucleus 24R(CS) cochlear implant. AEV measures were then compared with common ground electrode impedance measures as well as to subjective judgments of electrode sound quality to determine the sensitivity and specificity of AEVs. RESULTS: Results showed excellent specificity for both modes of AEV testing: 97.9% for common ground and 99.8% for monopolar. Sensitivity for common ground AEV testing was 91.7%; however the sensitivity of monopolar AEVs was very poor, at 7.7%. Monopolar AEVs accurately identified open circuits but missed all short circuits. These results are consistent with the sensitivity of impedance measures made with each of the two stimulation modes. CONCLUSIONS: Common ground AEVs are more reliable than monopolar AEVs for identifying malfunctioning electrodes.

Adolescent↗
PubMed2004

Effects of electrode-to-fiber distance on temporal neural response with electrical stimulation.

This paper presents an analysis of the effects of the electrode-to-fiber distance on the temporal response properties of an auditory nerve fiber stimulated by electric current pulses. This analysis was based upon results from a computational model of a mammalian auditory nerve fiber axon having 50 nodes of Ranvier, each consisting of 130 stochastic sodium channels and 50 stochastic potassium channels, making it possible to represent the temporal fluctuations of action potential initiation and conduction. A monopolar stimulus electrode was located above a central (26th) node at electrode-to-fiber distances of 1, 4, and 7 mm, while the recording electrode was located at the 36th node. Action potentials (spikes) were generated by the biophysical model using the Crank-Nicholson method to solve a diffusive partial differential equation. By observing the occurrence times of spikes in response to 2000 cathodic monophasic stimulus pulses, temporal jitter (i.e., the standard deviation of spike times) was calculated and the poststimulus time (PST) histogram was generated as well. Furthermore, by computing the PST histogram for each initiation node as functions of space (node number) and time (PST), it was shown that spike initiation was distributed not only spatially but also temporally for stimulus levels producing firing efficiencies (FEs) near 0.5. However, at levels producing FEs near 0.99, while temporal variations approached zero, the spatial distribution of initiating nodes was comparable to that observed for the FE near 0.5. As temporal fluctuations are important for speech coding in cochlear implants, we conclude that spatial characteristics of the electrode-auditory nerve fiber interface may play a significant role in influencing these stochastic temporal processes.

Action Potentials↗
PubMed2003

Electrode configuration influences action potential initiation site and ensemble stochastic response properties.

The configuration of intracochlear electrodes used to electrically stimulate the auditory nerve influences the ensemble fiber response. For example, monopolar stimulation produces lower thresholds and greater spread of excitation than does bipolar stimulation. We used two approaches to investigate how the ensemble of auditory-nerve fibers responds to stimulation delivered by different electrode configurations. As the electrically evoked compound action potential (ECAP) reflects the ensemble response of the nerve, we used its morphology and changes with stimulus level to assess issues related to site-of-excitation and fiber recruitment. In our first approach, feline ECAPs were obtained using a nucleus-style banded electrode array. ECAP latency functions indicated that bipolar stimulation can initiate action potentials at more peripheral sites than does monopolar stimulation. We observed double-peaked ECAPs with bipolar and tripolar stimulation, suggesting excitation of both peripheral and central neural processes. Finally, we observed in some cases a tendency for monopolar stimulation to produce wider ECAP potentials, consistent with the notion that monopolar stimulation excites a broader spatial extent of the fiber population. In our second approach, we applied a simple model to published surveys of single-fiber responses to provide insight into the stochastic properties of the ensemble response. Our results suggest that broader recruitment of fiber activity produced by monopolar stimulation results in a population response with more probabilistic response characteristics and ensemble spike jitter. These observations and our ECAP results are consistent with reports of perceptual advantages attributed to monopolar or other less-focused modes of stimulation.

Animals↗
PubMed2003

Auditory response to intracochlear electric stimuli following furosemide treatment.

The influence of functional hair cells on electrical stimulation of the auditory nerve is an important issue as individuals with significant residual hearing are now cochlear implant candidates. Previous work has shown that chemical deafening during the course of acute experiments changes the auditory nerve's responses to electrical stimulation [Third Quarterly Progress Report, NIH contract N01-DC-9-2106 (2000), Final Report, NIH Contract N01-DC-9-2106 (2002)]. This study extended that work by investigating the changes and subsequent recovery following furosemide injections which reversibly impair hair-cell function [Hear. Res. (1980) 79-89; Hear. Res. 14 (1984) 305-314, J. Physiol. 347 (1984) 685-696; Hear. Res. 71 (1993) 202-207]. Acoustic sensitivity of guinea pig subjects was repeatedly monitored with the click-evoked compound action potential. Responses to single biphasic electric pulses and biphasic electric pulse trains delivered by a monopolar intracochlear electrode were also repeatedly assessed using the electrically evoked compound action potential (ECAP). Our measures demonstrated a clear relationship between the state of hair-cell function and ECAP responses, as changes in the latter coincided with the loss or recovery of acoustic sensitivity. ECAP growth functions demonstrated increased slope and increased maximum (saturation) amplitude. Both trends were reversible and followed approximately the time course of post-furosemide hearing recovery. Additional changes were observed using electric pulse-train stimulation: (1) the magnitude of ECAP amplitude alternation (observed in response to successive stimulus pulses) increased, (2) the degree of ECAP adaptation (measured 80-100 ms after pulse-train onset) increased, and (3) the degree of refractoriness (measured by the ratio of ECAP amplitudes to the second and first pulses) tended to increase. All these trends are consistent with the hypothesis that functional hair cells desynchronize the population of auditory nerve fibers, thereby changing the electrically evoked responses. Viable hair cells may therefore provide positive effects on auditory response to electric stimuli delivered to implant patients with residual hearing, as they may enhance the random activity of the stimulated nerve.

Animals↗
PubMed2002

ABR thresholds to tonebursts gated with Blackman and linear windows in adults with high-frequency sensorineural hearing loss.

OBJECTIVE: The goal of this study was to determine whether tonebursts gated on and off using a nonlinear, exact-Blackman-gating function would be a more frequency-specific stimulus for auditory brain stem response audiometry than the more traditional 2-1-2 cycle linearly gated toneburst. DESIGN: Toneburst ABRs were recorded in 10 adults with normal hearing and in 18 adults with sloping high-frequency sensorineural hearing loss. It was hypothesized that any advantage of the Blackman stimuli for frequency-specific threshold assessment should be evident in hearing-impaired subjects with hearing loss confined to the 2000 to 4000 Hz frequency region since spectral splatter in the toneburst stimuli could lead to an underestimation of hearing loss based on the ABR thresholds. ABR stimuli consisted of 2000- and 4000-Hz 2-1-2 (rise-plateau-fall) cycle linearly gated tonebursts and 1-0-1 msec exact-Blackman-gated tonebursts. An additional 0.5-0-0.5 msec 4000-Hz Blackman-gated toneburst was used to investigate whether the difference in rise/fall characteristics of the linearly and Blackman-gated tonebursts could account for any differences in ABR results at 4000 Hz. The ABR toneburst stimuli were calibrated behaviorally in 15 adults with normal hearing. RESULTS: In the normal-hearing listeners toneburst-ABR thresholds generally exceeded behavioral thresholds by 10 to 13 dB for all stimuli. Correlations of 0.85 to 0.96 were obtained between 2000 and 4000 Hz toneburst ABR thresholds and pure-tone audiometric thresholds in the hearing-impaired listeners. Results were similar for Blackman- and linearly gated stimuli. CONCLUSIONS: There were no clear differences between Blackman- and linearly gated tonebursts in terms of how well ABR thresholds predicted pure-tone thresholds at 2000 and 4000 Hz. In general audiometric thresholds were predicted with good accuracy (+/-15 dB) by the toneburst ABR thresholds. The 4000-Hz audiometric threshold was underestimated in one subject with a very steeply sloping hearing loss by both Blackman- and linearly gated toneburst ABR thresholds, indicating that ipsilateral masking such as notched noise would be needed to ensure frequency specificity in this and similar cases.

Acoustic Stimulation↗
PubMedDate not supplied

Channel interaction in cochlear implant users evaluated using the electrically evoked compound action potential.

One likely determinant of performance with a cochlear implant is the degree of interaction that occurs when overlapping subsets of nerve fibers are stimulated by various electrodes of a multielectrode array. The electrically evoked compound action potential (ECAP) can be used to assess physiological channel interaction. This paper describes results from two different methods of analysis of ECAP channel interaction measures made by the Nucleus neural response telemetry system. Using a forward-masking stimulus paradigm, masker and probe pulses are delivered through different electrodes. The response to the probe is then dependent on the extent of overlap in the stimulated neural populations. The amplitude of response to the probe as a function of masker electrode position then reflects the degree of overlap between the population of neurons responding to the masker and those stimulated by the probe. Results demonstrate large variations across individual implant users as well as across electrodes within an individual. In general, the degree of interaction is shown to be dependent on stimulus level.

Action Potentials↗