Variability in speech perception scores among cochle-ar implant listeners may largely reflect the... more Variability in speech perception scores among cochle-ar implant listeners may largely reflect the variable efficacy of implant electrodes to convey stimulus information to the auditory nerve. In the present study, three metrics were applied to assess the quality of the electrode-neuron interface of individual co-chlear implant channels: the electrically evoked compound action potential (ECAP), the estimation of electrode position using computerized tomography (CT), and behavioral thresholds using focused stimulation. The primary motivation of this approach is to evaluate the ECAP as a site-specific measure of the electrode-neuron interface in the context of two peripheral factors that likely contribute to degraded perception: large electrode-to-modiolus distance and reduced neural density. Ten unilaterally implanted adults with Advanced Bionics HiRes90k devices participated. ECAPs were elicited with monopolar stimulation within a forward-masking paradigm to construct channel interaction functions (CIF), behavioral thresholds were obtained with quadrupolar (sQP) stimulation, and data from imaging provided estimates of electrode-to-modiolus distance and scalar location (scala tympani (ST), intermediate, or scala vestibuli (SV)) for each electrode. The width of the ECAP CIF was positively correlated with electrode-to-modiolus distance; both of these measures were also influenced by scalar position. The ECAP peak amplitude was negatively correlated with behavioral thresholds. Moreover, subjects with low behavioral thresholds and large ECAP amplitudes, averaged across electrodes, tended to have higher speech perception scores. These results suggest a potential clinical role for the ECAP in the objective assessment of individual cochlear implant channels, with the potential to improve speech perception outcomes.
Cochlear implant (CI) users usually exhibit marked across-electrode differences in detection thre... more Cochlear implant (CI) users usually exhibit marked across-electrode differences in detection thresholds with Bfocused^modes of stimulation, such as partialtripolar (pTP) mode. This may reflect differences either in local neural survival or in the distance of the electrodes from the modiolus. To shed light on these two explanations, we compared stimulus-detection thresholds and gap-detection thresholds (GDTs) at comfortably loud levels for at least four electrodes in each of ten Advanced Bionics CI users, using 1031-pps pulse trains. The electrodes selected for each user had a wide range of stimulus-detection thresholds in pTP mode. We also measured across-electrode variations in both stimulus-detection and gap-detection tasks in monopolar (MP) mode. Both stimulus-detection and gap-detection thresholds correlated across modes. However, there was no significant correlation between stimulus-detection and gap-detection thresholds in either mode. Hence, gap-detection thresholds likely tap a source of across-electrode variation additional to, or different from, that revealed by stimulusdetection thresholds. Stimulus-detection thresholds were significantly lower for apical than for basal electrodes in both modes; this was only true for gap detection in pTP mode. Finally, although the acrosselectrode standard deviation in stimulus-detection thresholds was greater in pTP than in MP mode, the reliability of these differences-assessed by dividing the across-electrode standard deviation by the standard deviation across adaptive runs for each electrode-was similar for the two modes; this metric was also similar across modes for gap detection. Hence across-electrode differences can be revealed using clinically available MP stimulation, with a reliability comparable to that observed with focused stimulation.
The 10 consonant-nucleus-consonant (CNC) word lists are considered the gold standard in the testi... more The 10 consonant-nucleus-consonant (CNC) word lists are considered the gold standard in the testing of cochlear implant (CI) users. However, variance in scores across lists could degrade the sensitivity and reliability of them to identify deficits in speech perception. This study examined the relationship between variability in performance among lists and the lexical characteristics of the words. Data are from 28 adult CI users. Each subject was tested on all 10 CNC word lists. Data were analyzed in terms of lexical characteristics, lexical frequency, neighborhood density, bi-, and tri-phonemic probabilities. To determine whether individual performance variability across lists can be reduced, the standard set of 10 phonetically balanced 50-word lists was redistributed into a new set of lists using two sampling strategies: (a) balancing with respect to word lexical frequency or (b) selecting words with equal probability. The mean performance on the CNC lists varied from 53.1% to 62.4...
The design of contemporary multichannel cochlear implants is predicated on the presumption that t... more The design of contemporary multichannel cochlear implants is predicated on the presumption that they activate multiple independent sectors of the auditory nerve array. The independence of these channels, however, is limited by the spread of activation from each intracochlear electrode across the auditory nerve array. In this study, we evaluated factors that influence intracochlear spread of activation using two types of intracochlear electrodes: (1) a clinical-type device consisting of a linear series of ring contacts positioned along a silicon elastomer carrier, and (2) a pair of visually placed (VP) ball electrodes that could be positioned independently relative to particular intracochlear structures, e.g., the spiral ganglion. Activation spread was estimated by recording multineuronal evoked activity along the cochleotopic axis of the central nucleus of the inferior colliculus (ICC). This activity was recorded using silicon-based singleshank, 16-site recording probes, which were fixed within the ICC at a depth defined by responses to acoustic tones. After deafening, electric stimuli consisting of single biphasic electric pulses were presented with each electrode type in various stimulation configurations (monopolar, bipolar, tripolar) and/or various electrode orientations (radial, off-radial, longitudinal). The results indicate that monopolar (MP) stimulation with either electrode type produced widepread excitation across the ICC. Bipolar (BP) stimulation with banded pairs of electrodes oriented longitudinally produced activation that was somewhat less broad than MP stimulation, and tripolar (TP) stimulation produced activation that was more restricted than MP or BP stimulation. Bipolar stimulation with radially oriented pairs of VP ball electrodes produced the most restricted activation. The activity patterns evoked by radial VP balls were comparable to those produced by pure tones in normal-hearing animals. Variations in distance between radially oriented VP balls had little effect on activation spread, although increases in interelectrode spacing tended to reduce thresholds. Bipolar stimulation with longitudinally oriented VP electrodes produced broad activation that tended to broaden as the separation between electrodes increased.
The Journal of the Acoustical Society of America, Mar 1, 2007
The efficacy of cochlear implants is limited by spatial and temporal interactions among channels.... more The efficacy of cochlear implants is limited by spatial and temporal interactions among channels. This study explores the spatially restricted tripolar electrode configuration and compares it to bipolar and monopolar stimulation. Measures of threshold and channel interaction were obtained from nine subjects implanted with the Clarion HiFocus-I electrode array. Stimuli were biphasic pulses delivered at 1020 pulses/s. Threshold increased from monopolar to bipolar to tripolar stimulation and was most variable across channels with the tripolar configuration. Channel interaction, quantified by the shift in threshold between single- and two-channel stimulation, occurred for all three configurations but was largest for the monopolar and simultaneous conditions. The threshold shifts with simultaneous tripolar stimulation were slightly smaller than with bipolar and were not as strongly affected by the timing of the two channel stimulation as was monopolar. The subjects' performances on clinical speech tests were correlated with channel-to-channel variability in tripolar threshold, such that greater variability was related to poorer performance. The data suggest that tripolar channels with high thresholds may reveal cochlear regions of low neuron survival or poor electrode placement.
7 other HighWire hosted articles, the first 5 are: This article has been cited by [PDF] [Full Tex... more 7 other HighWire hosted articles, the first 5 are: This article has been cited by [PDF] [Full Text] [Abstract] , June, 1 2003; 89 (6): 3190-3204. J Neurophysiol [PDF] [Full Text] [Abstract] , April, 27 2005; 25 (17): 4207-4216. J. Neurosci. Nandakumar S. Narayanan, Eyal Y. Kimchi and Mark Laubach Redundancy and Synergy of Neuronal Ensembles in Motor Cortex [PDF] [Full Text] [Abstract] , August, 1 2006; 96 (2): 512-521. J Neurophysiol Direct and Indirect Activation of Cortical Neurons by Electrical Microstimulation [PDF] [Full Text] [Abstract] , July, 18 2007; 27 (29): 7838-7846. [PDF] [Full Text] [Abstract] , July, 1 2008; 100 (1): 92-107. J Neurophysiol images of cochlear-implant stimuli: coding of stimulus channel and current level. J Neurophysiol 87: 493-507, 2002; 10.1152/jn.00211.2001.
7 other HighWire hosted articles, the first 5 are: This article has been cited by [PDF] [Full Tex... more 7 other HighWire hosted articles, the first 5 are: This article has been cited by [PDF] [Full Text] [Abstract] , June, 1 2003; 89 (6): 3190-3204. J Neurophysiol [PDF] [Full Text] [Abstract] , April, 27 2005; 25 (17): 4207-4216. J. Neurosci. Nandakumar S. Narayanan, Eyal Y. Kimchi and Mark Laubach Redundancy and Synergy of Neuronal Ensembles in Motor Cortex [PDF] [Full Text] [Abstract] , August, 1 2006; 96 (2): 512-521. J Neurophysiol Direct and Indirect Activation of Cortical Neurons by Electrical Microstimulation [PDF] [Full Text] [Abstract] , July, 18 2007; 27 (29): 7838-7846. [PDF] [Full Text] [Abstract] , July, 1 2008; 100 (1): 92-107. J Neurophysiol images of cochlear-implant stimuli: coding of stimulus channel and current level. J Neurophysiol 87: 493-507, 2002; 10.1152/jn.00211.2001.
Thesis (Ph. D.)--University of Michigan, 2001. Includes bibliographical references (leaves 157-16... more Thesis (Ph. D.)--University of Michigan, 2001. Includes bibliographical references (leaves 157-165). Microfilm.
The partial tripolar electrode configuration is a relatively novel stimulation strategy that can ... more The partial tripolar electrode configuration is a relatively novel stimulation strategy that can generate more spatially focused electric fields than the commonly used monopolar configuration. Focused stimulation strategies should improve spectral resolution in cochlear implant users, but may also be more sensitive to local irregularities in the electrodeeneuron interface. In this study, we develop a practical computer model of cochlear implant stimulation that can simulate neural activation in a simplified cochlear geometry and we relate the resulting patterns of neural activity to basic psychophysical measures. We examine how two types of local irregularities in the electrodeeneuron interface, variations in spiral ganglion nerve density and electrode position within the scala tympani, affect the simulated neural activation patterns and how these patterns change with electrode configuration. The model shows that higher partial tripolar fractions activate more spatially restricted populations of neurons at all current levels and require higher current levels to excite a given number of neurons. We find that threshold levels are more sensitive at high partial tripolar fractions to both types of irregularities, but these effects are not independent. In particular, at close electrodeeneuron distances, activation is typically more spatially localized which leads to a greater influence of neural dead regions.
Journal of the Association for Research in Otolaryngology : JARO, 2015
Cochlear implant (CI) users usually exhibit marked across-electrode differences in detection thre... more Cochlear implant (CI) users usually exhibit marked across-electrode differences in detection thresholds with "focused" modes of stimulation, such as partial-tripolar (pTP) mode. This may reflect differences either in local neural survival or in the distance of the electrodes from the modiolus. To shed light on these two explanations, we compared stimulus-detection thresholds and gap-detection thresholds (GDTs) at comfortably loud levels for at least four electrodes in each of ten Advanced Bionics CI users, using 1031-pps pulse trains. The electrodes selected for each user had a wide range of stimulus-detection thresholds in pTP mode. We also measured across-electrode variations in both stimulus-detection and gap-detection tasks in monopolar (MP) mode. Both stimulus-detection and gap-detection thresholds correlated across modes. However, there was no significant correlation between stimulus-detection and gap-detection thresholds in either mode. Hence, gap-detection threshol...
This study examines patterns of auditory cortical activity elicited by single-pulse cochlear impl... more 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 electrode...
Variability in speech perception scores among cochle-ar implant listeners may largely reflect the... more Variability in speech perception scores among cochle-ar implant listeners may largely reflect the variable efficacy of implant electrodes to convey stimulus information to the auditory nerve. In the present study, three metrics were applied to assess the quality of the electrode-neuron interface of individual co-chlear implant channels: the electrically evoked compound action potential (ECAP), the estimation of electrode position using computerized tomography (CT), and behavioral thresholds using focused stimulation. The primary motivation of this approach is to evaluate the ECAP as a site-specific measure of the electrode-neuron interface in the context of two peripheral factors that likely contribute to degraded perception: large electrode-to-modiolus distance and reduced neural density. Ten unilaterally implanted adults with Advanced Bionics HiRes90k devices participated. ECAPs were elicited with monopolar stimulation within a forward-masking paradigm to construct channel interaction functions (CIF), behavioral thresholds were obtained with quadrupolar (sQP) stimulation, and data from imaging provided estimates of electrode-to-modiolus distance and scalar location (scala tympani (ST), intermediate, or scala vestibuli (SV)) for each electrode. The width of the ECAP CIF was positively correlated with electrode-to-modiolus distance; both of these measures were also influenced by scalar position. The ECAP peak amplitude was negatively correlated with behavioral thresholds. Moreover, subjects with low behavioral thresholds and large ECAP amplitudes, averaged across electrodes, tended to have higher speech perception scores. These results suggest a potential clinical role for the ECAP in the objective assessment of individual cochlear implant channels, with the potential to improve speech perception outcomes.
Cochlear implant (CI) users usually exhibit marked across-electrode differences in detection thre... more Cochlear implant (CI) users usually exhibit marked across-electrode differences in detection thresholds with Bfocused^modes of stimulation, such as partialtripolar (pTP) mode. This may reflect differences either in local neural survival or in the distance of the electrodes from the modiolus. To shed light on these two explanations, we compared stimulus-detection thresholds and gap-detection thresholds (GDTs) at comfortably loud levels for at least four electrodes in each of ten Advanced Bionics CI users, using 1031-pps pulse trains. The electrodes selected for each user had a wide range of stimulus-detection thresholds in pTP mode. We also measured across-electrode variations in both stimulus-detection and gap-detection tasks in monopolar (MP) mode. Both stimulus-detection and gap-detection thresholds correlated across modes. However, there was no significant correlation between stimulus-detection and gap-detection thresholds in either mode. Hence, gap-detection thresholds likely tap a source of across-electrode variation additional to, or different from, that revealed by stimulusdetection thresholds. Stimulus-detection thresholds were significantly lower for apical than for basal electrodes in both modes; this was only true for gap detection in pTP mode. Finally, although the acrosselectrode standard deviation in stimulus-detection thresholds was greater in pTP than in MP mode, the reliability of these differences-assessed by dividing the across-electrode standard deviation by the standard deviation across adaptive runs for each electrode-was similar for the two modes; this metric was also similar across modes for gap detection. Hence across-electrode differences can be revealed using clinically available MP stimulation, with a reliability comparable to that observed with focused stimulation.
The 10 consonant-nucleus-consonant (CNC) word lists are considered the gold standard in the testi... more The 10 consonant-nucleus-consonant (CNC) word lists are considered the gold standard in the testing of cochlear implant (CI) users. However, variance in scores across lists could degrade the sensitivity and reliability of them to identify deficits in speech perception. This study examined the relationship between variability in performance among lists and the lexical characteristics of the words. Data are from 28 adult CI users. Each subject was tested on all 10 CNC word lists. Data were analyzed in terms of lexical characteristics, lexical frequency, neighborhood density, bi-, and tri-phonemic probabilities. To determine whether individual performance variability across lists can be reduced, the standard set of 10 phonetically balanced 50-word lists was redistributed into a new set of lists using two sampling strategies: (a) balancing with respect to word lexical frequency or (b) selecting words with equal probability. The mean performance on the CNC lists varied from 53.1% to 62.4...
The design of contemporary multichannel cochlear implants is predicated on the presumption that t... more The design of contemporary multichannel cochlear implants is predicated on the presumption that they activate multiple independent sectors of the auditory nerve array. The independence of these channels, however, is limited by the spread of activation from each intracochlear electrode across the auditory nerve array. In this study, we evaluated factors that influence intracochlear spread of activation using two types of intracochlear electrodes: (1) a clinical-type device consisting of a linear series of ring contacts positioned along a silicon elastomer carrier, and (2) a pair of visually placed (VP) ball electrodes that could be positioned independently relative to particular intracochlear structures, e.g., the spiral ganglion. Activation spread was estimated by recording multineuronal evoked activity along the cochleotopic axis of the central nucleus of the inferior colliculus (ICC). This activity was recorded using silicon-based singleshank, 16-site recording probes, which were fixed within the ICC at a depth defined by responses to acoustic tones. After deafening, electric stimuli consisting of single biphasic electric pulses were presented with each electrode type in various stimulation configurations (monopolar, bipolar, tripolar) and/or various electrode orientations (radial, off-radial, longitudinal). The results indicate that monopolar (MP) stimulation with either electrode type produced widepread excitation across the ICC. Bipolar (BP) stimulation with banded pairs of electrodes oriented longitudinally produced activation that was somewhat less broad than MP stimulation, and tripolar (TP) stimulation produced activation that was more restricted than MP or BP stimulation. Bipolar stimulation with radially oriented pairs of VP ball electrodes produced the most restricted activation. The activity patterns evoked by radial VP balls were comparable to those produced by pure tones in normal-hearing animals. Variations in distance between radially oriented VP balls had little effect on activation spread, although increases in interelectrode spacing tended to reduce thresholds. Bipolar stimulation with longitudinally oriented VP electrodes produced broad activation that tended to broaden as the separation between electrodes increased.
The Journal of the Acoustical Society of America, Mar 1, 2007
The efficacy of cochlear implants is limited by spatial and temporal interactions among channels.... more The efficacy of cochlear implants is limited by spatial and temporal interactions among channels. This study explores the spatially restricted tripolar electrode configuration and compares it to bipolar and monopolar stimulation. Measures of threshold and channel interaction were obtained from nine subjects implanted with the Clarion HiFocus-I electrode array. Stimuli were biphasic pulses delivered at 1020 pulses/s. Threshold increased from monopolar to bipolar to tripolar stimulation and was most variable across channels with the tripolar configuration. Channel interaction, quantified by the shift in threshold between single- and two-channel stimulation, occurred for all three configurations but was largest for the monopolar and simultaneous conditions. The threshold shifts with simultaneous tripolar stimulation were slightly smaller than with bipolar and were not as strongly affected by the timing of the two channel stimulation as was monopolar. The subjects&#39; performances on clinical speech tests were correlated with channel-to-channel variability in tripolar threshold, such that greater variability was related to poorer performance. The data suggest that tripolar channels with high thresholds may reveal cochlear regions of low neuron survival or poor electrode placement.
7 other HighWire hosted articles, the first 5 are: This article has been cited by [PDF] [Full Tex... more 7 other HighWire hosted articles, the first 5 are: This article has been cited by [PDF] [Full Text] [Abstract] , June, 1 2003; 89 (6): 3190-3204. J Neurophysiol [PDF] [Full Text] [Abstract] , April, 27 2005; 25 (17): 4207-4216. J. Neurosci. Nandakumar S. Narayanan, Eyal Y. Kimchi and Mark Laubach Redundancy and Synergy of Neuronal Ensembles in Motor Cortex [PDF] [Full Text] [Abstract] , August, 1 2006; 96 (2): 512-521. J Neurophysiol Direct and Indirect Activation of Cortical Neurons by Electrical Microstimulation [PDF] [Full Text] [Abstract] , July, 18 2007; 27 (29): 7838-7846. [PDF] [Full Text] [Abstract] , July, 1 2008; 100 (1): 92-107. J Neurophysiol images of cochlear-implant stimuli: coding of stimulus channel and current level. J Neurophysiol 87: 493-507, 2002; 10.1152/jn.00211.2001.
7 other HighWire hosted articles, the first 5 are: This article has been cited by [PDF] [Full Tex... more 7 other HighWire hosted articles, the first 5 are: This article has been cited by [PDF] [Full Text] [Abstract] , June, 1 2003; 89 (6): 3190-3204. J Neurophysiol [PDF] [Full Text] [Abstract] , April, 27 2005; 25 (17): 4207-4216. J. Neurosci. Nandakumar S. Narayanan, Eyal Y. Kimchi and Mark Laubach Redundancy and Synergy of Neuronal Ensembles in Motor Cortex [PDF] [Full Text] [Abstract] , August, 1 2006; 96 (2): 512-521. J Neurophysiol Direct and Indirect Activation of Cortical Neurons by Electrical Microstimulation [PDF] [Full Text] [Abstract] , July, 18 2007; 27 (29): 7838-7846. [PDF] [Full Text] [Abstract] , July, 1 2008; 100 (1): 92-107. J Neurophysiol images of cochlear-implant stimuli: coding of stimulus channel and current level. J Neurophysiol 87: 493-507, 2002; 10.1152/jn.00211.2001.
Thesis (Ph. D.)--University of Michigan, 2001. Includes bibliographical references (leaves 157-16... more Thesis (Ph. D.)--University of Michigan, 2001. Includes bibliographical references (leaves 157-165). Microfilm.
The partial tripolar electrode configuration is a relatively novel stimulation strategy that can ... more The partial tripolar electrode configuration is a relatively novel stimulation strategy that can generate more spatially focused electric fields than the commonly used monopolar configuration. Focused stimulation strategies should improve spectral resolution in cochlear implant users, but may also be more sensitive to local irregularities in the electrodeeneuron interface. In this study, we develop a practical computer model of cochlear implant stimulation that can simulate neural activation in a simplified cochlear geometry and we relate the resulting patterns of neural activity to basic psychophysical measures. We examine how two types of local irregularities in the electrodeeneuron interface, variations in spiral ganglion nerve density and electrode position within the scala tympani, affect the simulated neural activation patterns and how these patterns change with electrode configuration. The model shows that higher partial tripolar fractions activate more spatially restricted populations of neurons at all current levels and require higher current levels to excite a given number of neurons. We find that threshold levels are more sensitive at high partial tripolar fractions to both types of irregularities, but these effects are not independent. In particular, at close electrodeeneuron distances, activation is typically more spatially localized which leads to a greater influence of neural dead regions.
Journal of the Association for Research in Otolaryngology : JARO, 2015
Cochlear implant (CI) users usually exhibit marked across-electrode differences in detection thre... more Cochlear implant (CI) users usually exhibit marked across-electrode differences in detection thresholds with "focused" modes of stimulation, such as partial-tripolar (pTP) mode. This may reflect differences either in local neural survival or in the distance of the electrodes from the modiolus. To shed light on these two explanations, we compared stimulus-detection thresholds and gap-detection thresholds (GDTs) at comfortably loud levels for at least four electrodes in each of ten Advanced Bionics CI users, using 1031-pps pulse trains. The electrodes selected for each user had a wide range of stimulus-detection thresholds in pTP mode. We also measured across-electrode variations in both stimulus-detection and gap-detection tasks in monopolar (MP) mode. Both stimulus-detection and gap-detection thresholds correlated across modes. However, there was no significant correlation between stimulus-detection and gap-detection thresholds in either mode. Hence, gap-detection threshol...
This study examines patterns of auditory cortical activity elicited by single-pulse cochlear impl... more 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 electrode...
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