Search PubMed⌕ Search

Biomedical subjects

Robert V Shannon

Publications and source records attributed to Robert V Shannon.

17 recordsLinked to original sources
PubMed2006

Frequency transposition around dead regions simulated with a noiseband vocoder.

In sensorineural hearing loss, damage to inner hair cells or the auditory nerve may result in dead regions in the cochlea, where the information transmission is disrupted. In cochlear implants, similar dead regions might appear if the spiral ganglia do not function. Shannon et al. [J. Assoc. Res. Otolaryngol. 3, 185-199 (2002)] simulated dead regions of varying size and location using a noiseband vocoder. Phoneme recognition by normal-hearing subjects was measured under two frequency-place mapping conditions: the frequency range corresponding to the dead region was (1) removed or (2) reassigned to bands adjacent to the dead region to simulate the off-frequency stimulation of neurons at the edge of a dead region. The present study extends the results of Shannon et al. by including a frequency transposition mapping condition, where the overall acoustic input frequency range was distributed over the entire remaining nondead region. The frequency transposed map provided more acoustic information when compared to the map with the frequency range corresponding to the dead region removed. However, speech perception did not improve for many simulated dead region conditions, possibly due to the spectral distortions in the frequency-place mapping.

Acoustic Stimulation↗
PubMed2005

Effects of electrode design and configuration on channel interactions.

A potential shortcoming of existing multichannel cochlear implants is electrical-field summation during simultaneous electrode stimulation. Electrical-field interactions can disrupt the stimulus waveform prior to neural activation. To test whether speech intelligibility can be degraded by electrical-field interaction, speech recognition performance and interaction were examined for three Clarion electrode arrays: the pre-curved, enhanced bipolar electrode array, the enhanced bipolar electrode with an electrode positioner, and the Hi-Focus electrode with a positioner. Channel interaction was measured by comparing stimulus detection thresholds for a probe signal in the presence of a sub-threshold perturbation signal as a function of the separation between the two simultaneously stimulated electrodes. Correct identification of vowels, consonants, and words in sentences was measured with two speech strategies: one which used simultaneous stimulation and another which used sequential stimulation. Speech recognition scores were correlated with measured electrical-field interaction for the strategy which used simultaneous stimulation but not the strategy which used sequential stimulation. Higher speech recognition scores with the simultaneous strategy were generally associated with lower levels of electrical-field interaction. Electrical-field interaction accounted for as much as 70% of the variance in speech recognition scores, suggesting that electrical-field interaction is a significant contributor to the variability found across patients who use simultaneous strategies.

Adult↗
PubMed2005

Effects of stimulation mode, level and location on forward-masked excitation patterns in cochlear implant patients.

In multi-channel cochlear implants, electrical current is delivered to appropriate electrodes in the cochlea to approximate the spatial representation of speech. Theoretically, electrode configurations that restrict the current spread within the cochlea (e.g., bi- or tri-polar stimulation) may provide better spatial selectivity, and in turn, better speech recognition than configurations that produce a broader current spread (e.g., monopolar stimulation). However, the effects of electrode configuration on supra-threshold excitation patterns have not been systematically studied in cochlear implant patients. In the present study, forward-masked excitation patterns were measured in cochlear implant patients as functions of stimulation mode, level and location within the cochlea. All stimuli were 500 pulses-per-second biphasic pulse trains (200 micros/phase, 20 micros inter-phase gap). Masker stimuli were 200 ms in duration; the bi-polar configuration was varied from narrow (BP+1) to wide (BP+17), depending on the test condition. Probe stimuli were 20 ms in duration and the masker-probe delay was 5 ms; the probe configuration was fixed at BP+1. The results indicated that as the distance between the active and return electrodes in a bi-polar pair was increased, the excitation pattern broadened within the cochlea. When the distance between active and return electrodes was sufficiently wide, two peaks were often observed in the excitation pattern, comparable to non-overlapping electric fields produced by widely separated dipoles. Analyses of the normalized data showed little effect of stimulation level on the shape of the excitation pattern.

Adult↗
PubMed2005

Effects of stimulation rate on speech recognition with cochlear implants.

Phoneme and speech recognition were measured as a function of stimulation pulse rate in 12 listeners with three types of cochlear implants. Identification of consonants and vowels and recognition of words and sentences were measured in 5 Clarion C1 subjects fit with continuous interleaved sampling (CIS) processors having 4 or 8 electrodes, 4 Nucleus 24 subjects fit with CIS processors having 4, 8, 12 or 16 electrodes and 3 Clarion C2 subjects fit with CIS processors with 4, 8, 12 and 16 electrodes. Stimulation rates ranged from 200 to more than 5000 Hz per electrode, depending on the device, number of electrodes used and stimulation strategy. Listeners were also tested on the same materials with their original processor prior to receiving the experimental processors. All testing was done in quiet listening conditions with essentially no practice with the experimental processor prior to data collection. Listeners scored the highest with their original processor. Little difference in speech understanding was observed for listener scores with processors using different stimulation rates. Speech recognition was significantly poorer only at the lowest stimulation rate and at high rates that used noninterleaved pulses. Speech recognition was similar for processors using 8, 12 or 16 electrodes. Only 4-electrode processors produced a significantly poorer performance. These results suggest that patients with present commercial implants are not able to make full use of the number of channels of spectral information delivered by the present speech processors. In addition, the results show no significant change in performance as a function of stimulation rate, suggesting that high stimulation rates do not result in improved access to temporal cues in speech, at least under quiet listening conditions.

Adult↗
PubMed2005

Open set speech perception with auditory brainstem implant?

OBJECTIVE: Only a small percentage of auditory brainstem implant (ABI) recipients treated for neurofibromatosis type 2 (NF2) have proved capable of identifying words using only the sound from the ABI. Recently, the ABI was applied to a series of patients with no cochlear nerve or with cochlear disorders that could not benefit from a cochlear implant (i.e., cochlear nerve aplasia or posttraumatic avulsion) or whose benefit was or would be severely compromised. A significant number of these patients have proven capable of understanding speech, including effortless telephone use. In the present study, a series of psychophysical tests were administered to determine the cause of the difference in performance between tumor (T) and nontumor (NT) ABI patients. STUDY DESIGN: Retrospective case review. SETTING: Tertiary referral center. PATIENTS: Twenty patients with ABIs participated in the investigation. Ten were NF2 patients and 10 NT subjects. Patient ages ranged from 24 to 61 years. Eleven were males and nine females. INTERVENTION: Auditory rehabilitation in auditory disconnection caused by cochlea or cochlear nerve disorders. RESULTS: There was a significant correlation between modulation detection thresholds and speech understanding and a significant difference in modulation detection between T and NT patients. CONCLUSIONS: The difference in modulation detection between the two groups suggests a difference in the survival of specific cells in the cochlear nucleus that support modulation. The pattern of results indicates a separate pathway of auditory processing that is specialized for modulated sounds, and that pathway is critical for speech understanding. In NF2 patients, the tumor and surgery may selectively damage this pathway, resulting in poor speech recognition with prosthetic stimulation.

Adolescent↗
PubMed2005

Interactions between cochlear implant electrode insertion depth and frequency-place mapping.

While new electrode designs allow deeper insertion and wider coverage in the cochlea, there is still considerable variation in the insertion depth of the electrode array among cochlear implant users. The present study measures speech recognition as a function of insertion depth, varying from a deep insertion of 10 electrodes at 28.8 mm to a shallow insertion of a single electrode at 7.2 mm, in four Med-El Combi 40+ users. Short insertion depths were simulated by inactivating apical electrodes. Speech recognition increased with deeper insertion, reaching an asymptotic level at 21.6 or 26.4 mm depending on the frequency-place map used. Başkent and Shannon [J. Acoust. Soc. Am. 116, 3130-3140 (2004)] showed that speech recognition by implant users was best when the acoustic input frequency was matched onto the cochlear location that normally processes that frequency range, minimizing the spectral distortions in the map. However, if an electrode array is not fully inserted into the cochlea, a matched map will result in the loss of considerable low-frequency information. The results show a strong interaction between the optimal frequency-place mapping and electrode insertion depth. Consistent with previous studies, frequency-place matching produced better speech recognition than compressing the full speech range onto the electrode array for full insertion ranges (20 to 25 mm from the round window). For shallower insertions (16.8 and 19.2 mm) a mild amount of frequency-place compression was better than truncating the frequency range to match the basal cochlear location. These results show that patients with shallow electrode insertions might benefit from a map that assigns a narrower frequency range than patients with full insertions.

Adult↗
PubMed2004

The number of spectral channels required for speech recognition depends on the difficulty of the listening situation.

Cochlear implants provide a limited number of electrodes, each of which represents a channel of spectral information. Studies have shown that implant recipients are not receiving all of the information from the channels presented to their implant. The present paper provides a quantitative framework for evaluating how many spectral channels of information are necessary for speech recognition. Speech and melody recognition data from previous studies with cochlear implant simulations are compared as a function of the number of spectral channels of information. A quantitative model is applied to the results. Speech recognition performance increases as the number of spectral channels increases. A sigmoid function best describes this increase when plotted as a function of the log number of channels. As speech materials become more difficult, the function shifts to the right, indicating that more spectral channels of information are required. A model proposed by Plomp provides a single index to relate the difficulty of the task to the number of spectral channels needed for moderate recognition performance. In conclusion, simple sentence recognition in quiet can be achieved with only 3-4 channels of spectral information, while more complex materials can require 30 or more channels for an equivalent level of performance. The proposed model provides a single index that not only quantifies the number of functional channels in a cochlear implant, but also predicts the level of performance for different listening tasks.

Audiometry, Speech↗
PubMed2004

Frequency-place compression and expansion in cochlear implant listeners.

In multichannel cochlear implants, low frequency information is delivered to apical cochlear locations while high frequency information is delivered to more basal locations, mimicking the normal acoustic tonotopic organization of the auditory nerves. In clinical practice, little attention has been paid to the distribution of acoustic input across the electrodes of an individual patient that might vary in terms of spacing and absolute tonotopic location. In normal-hearing listeners, Başkent and Shannon (J. Acoust. Soc. Am. 113, 2003) simulated implant signal processing conditions in which the frequency range assigned to the array was systematically made wider or narrower than the simulated stimulation range in the cochlea, resulting in frequency-place compression or expansion, respectively. In general, the best speech recognition was obtained when the input acoustic information was delivered to the matching tonotopic place in the cochlea with least frequency-place distortion. The present study measured phoneme and sentence recognition scores with similar frequency-place manipulations in six Med-El Combi 40+ implant subjects. Stimulation locations were estimated using the Greenwood mapping function based on the estimated electrode insertion depth. Results from frequency-place compression and expansion with implants were similar to simulation results, especially for postlingually deafened subjects, despite the uncertainty in the actual stimulation sites of the auditory nerves. The present study shows that frequency-place mapping is an important factor in implant performance and an individual implant patient's map could be optimized with functional tests using frequency-place manipulations.

Acoustic Stimulation↗
PubMed2004

The multichannel auditory brainstem implant: how many electrodes make sense?

OBJECT: Development of multichannel auditory brainstem implant (ABI) systems has been based in part on the assumption that audiological outcome can be optimized by increasing the number of available electrodes. In this paper the authors critically analyze this assumption on the basis of a retrospective clinical study performed using the Nucleus 22 ABI surface electrode array. METHODS: The perceptual performances of 61 patients with neurofibromatosis Type 2 were tested approximately 6 weeks after an eight-electrode ABI had been implanted. Of eight implanted electrodes 5.57 +/- 2.57 (mean +/- standard deviation [SD] provided auditory sensations when stimulated. Electrodes were deactivated when stimulation resulted in significant nonauditory side effects or no auditory sensation at all, and also when they failed to provide distinctive pitch sensations. The mean (+/- SD) scores for patients with ABIs were the following: sound-only consonant recognition, 20.4 +/- 14.3 (range 0-65%); vowel recognition, 28.8 +/- 18% (range 0-67%); Monosyllable Trochee Spondee (MTS) word recognition 41.1 +/- 25.3% (range 0-100%); and sentence recognition, 5.3 +/- 11.4% (range 0-64%). Performance in patients in whom between one and three electrodes provided auditory sensation was significantly poorer than that in patients with between four and eight functional electrodes in the vowel, MTS word, and City University of New York (CUNY) sentence recognition tests. The correlation between performance and electrode number did not reach the 0.05 level of significance with respect to the sound effect, consonant, and MTS stress-pattern recognition tests, probably because a satisfactory performance in these tests can be obtained only with temporal cues, that is, without any information about the frequency of the sounds. In the MTS word and the CUNY sentence recognition tests, performance was optimal in the patients with eight functional electrodes. Although all top performers had more than three functional auditory electrodes, no further improvement (asymptotic performance) was seen in those with five or more active electrodes in the consonant, vowel, and sound effect recognition tests. CONCLUSIONS: A minimum of three spectral channels, programmed in the appropriate individual tonotopic order seem to be required for satisfactory speech recognition in most patients with ABI. Due to the limited access to the tonotopic frequency gradient of the cochlear nucleus with surface stimulation, patients with ABI do not receive a wide range of spectral cues (frequency information) with multielectrode (> 5) surface arrays.

Acoustic Stimulation↗
PubMed2003

Speech recognition under conditions of frequency-place compression and expansion.

In normal acoustic hearing the mapping of acoustic frequency information onto the appropriate cochlear place is a natural biological function, but in cochlear implants it is controlled by the speech processor. The cochlear tonotopic range of the implant is determined by the length and insertion depth of the electrode array. Conventional cochlear implant electrode arrays are designed for an insertion of 25 mm inside the round window and the active electrodes occupy 16 mm, which would place the electrodes in a cochlear region corresponding to an acoustic frequency range of 500-6000 Hz. However, some implant speech processors map an acoustic frequency range from 150 to 10000 Hz onto these electrodes. While this mapping preserves the entire range of acoustic frequency information, it also results in a compression of the tonotopic pattern of speech information delivered to the brain. The present study measured the effects of such a compression of frequency-to-place mapping on speech recognition using acoustic simulations. Also measured were the effects of an expansion of the frequency-to-place mapping, which produces an expanded representation of speech in the cochlea. Such an expanded representation might improve speech recognition by improving the relative spatial (tonotopic) resolution, like an "acoustic fovea." Phoneme and sentence recognition was measured as a function of linear (in terms of cochlear distance) frequency-place compression and expansion. These conditions were presented to normal-hearing listeners using a noise-band vocoder, simulating cochlear implant electrodes with different insertion depths and different number of electrode channels. The cochlear tonotopic range was held constant by employing the same noise carrier bands for each condition, while the analysis frequency range was either compressed or expanded relative to the carrier frequency range. For each condition, the result was compared to that of the perfect frequency-place match, where the carrier and the analysis bands were perfectly matched. Speech recognition in the matched conditions was generally better than any condition of frequency-place expansion and compression, even when the matched condition eliminated a considerable amount of acoustic information. This result suggests that speech recognition, at least without training, is dependent on the mapping of acoustic frequency information onto the appropriate cochlear place.

Adult↗
PubMed2003

Use of a multichannel auditory brainstem implant for neurofibromatosis type 2.

Neurofibromatosis type 2 (NF 2) typically results in deafness due to disruption of the cochlear nerves, making peripheral devices such as cochlear implants ineffective. Auditory brainstem implants (ABIs), for direct electrical stimulation of the cochlear nucleus, have been used to provide auditory stimulation in this group of patients. Currently, 141 patients have been implanted in our institution, most recently using an advanced multichannel device. We report results of a recent series of 86 patients who received ABIs. Of this group, 60 had successful implantation, recovered from surgery, responded successfully to stimulation and underwent a full course of device programming and audiologic testing. This group had significant improvement in scores on several audiologic tests compared to baseline. When used to augment lip reading, improvement was also seen. The degree of improvement varied considerably among patients. ABI is a useful device for deaf patients with NF 2. As measured by audiologic testing, many patients receive substantial benefit with regard to sound and speech comprehension.

Brain Stem↗
PubMed2002

Holes in hearing.

Previous experiments have demonstrated that the correct tonotopic representation of spectral information is important for speech recognition. However, in prosthetic devices, such as hearing aids and cochlear implants, there may be a frequency/place mismatch due in part to the signal processing of the device and in part to the pathology that caused the hearing loss. Local regions of damaged neurons may create a "hole" in the tonotopic representation of spectral information, further distorting the frequency-to-place mapping. The present experiment was performed to quantitatively assess the impact of spectral holes on speech recognition. Speech was processed by a 20-band processor: SPEAK for cochlear implant (CI) listeners, and a 20-band noise processor for normal-hearing (NH) listeners. Holes in the tonotopic representation (from 1.5 to 6 mm in extent) were created by eliminating electrodes or noise carrier bands in the basal, middle, or apical regions of the cochlea. Vowel, consonant, and sentence recognition were measured as a function of the location and size of the hole. In addition, the spectral information that would normally be represented in the hole region was either: (1) dropped, (2) assigned to the apical side of the hole, (3) assigned to the basal side of the hole, or (4) split evenly to both sides of the hole. In general, speech features that are highly dependent on spectral cues (consonant place, vowel identity) were more affected by the presence of tonotopic holes than temporal features (consonant voicing and manner). Holes in the apical region were more damaging than holes in the basal or middle regions. A similar pattern of performance was observed for NH and CI listeners, suggesting that the loss of spectral information was the primary cause of the effects. The Speech Intelligibility Index was able to account for both NH and CI listeners' results. No significant differences were observed among the four conditions that redistributed the spectral information around the hole, suggesting that rerouting spectral information around a hole was no better than simply dropping it.

Adult↗
PubMed2002

The relative importance of amplitude, temporal, and spectral cues for cochlear implant processor design.

Speech understanding with cochlear implants has improved steadily over the last 25 years, and the success of implants has provided a powerful tool for understanding speech recognition in general. Comparing speech recognition in normal-hearing listeners and in cochlear-implant listeners has revealed many important lessons about the types of information necessary for good speech recognition--and some of the lessons are surprising. This paper presents a summary of speech perception research over the last 25 years with cochlear-implant and normal-hearing listeners. As long as the speech is audible, even the relatively severe amplitude distortion has only a mild effect on intelligibility. Temporal cues appear to be useful for speech intelligibility only up to about 20 Hz. Whereas temporal information above 20 Hz may contribute to improved quality, it contributes little to speech understanding. In contrast, the quantity and quality of spectral information appear to be critical for speech understanding. Only four spectral "channels" of information can produce good speech understanding, but more channels are required for difficult listening situations. Speech understanding is sensitive to the placement of spectral information along the cochlea. In prosthetic devices, in which the spectral information can be delivered to any cochlear location, it is critical to present spectral information to the normal acoustic tonotopic location for that information. If there is a shift or distortion of 2 to 3 mm between frequency and cochlear place, speech recognition is decreased dramatically.

Cochlear Implants↗
PubMed2002

Frequency mapping in cochlear implants.

OBJECTIVE: To understand the short-term ("acute") effects of parametric variations to the frequency-to-electrode mapping on phoneme identification by Nucleus-22 cochlear implant listeners. METHODS: Phoneme recognition was measured in five Nucleus-22 cochlear implant listeners using custom four-channel continuous interleaved sampler (CIS) processors. For the four-channel processors, speech signals were band-pass filtered into four broad frequency bands. The temporal envelope in each band was extracted by half-wave rectification and low-pass filtering at 160 Hz. The extracted envelope was then transformed to electric currents by a power function with an exponent of 0.2. The resulting electric currents were delivered to four electrode pairs (18,22), (13,17), (8,12), (3,7). The effect of frequency-to-electrode mapping was investigated by systematically varying the parameters of band-pass filters while fixing the electrode locations. Experiment 1 measured phoneme recognition as a function of the slope of band-pass filters. The slope of band-pass filters varied from 48 dB/octave to 6 dB/octave; the corner frequencies of band-pass filters were not varied. Experiment 2 measured phoneme recognition as a function of the distribution of band-pass filters across a fixed overall frequency range. The frequency divisions of a fixed overall frequency range were systematically varied from a logarithmic to a linear distribution. Experiment 3 measured phoneme recognition as a function of the bandwidth of the band-pass filters. The bandwidth of each filter varied from 0.2 to 2 octaves; the center frequencies for each band were not varied. No practice or feedback was provided for subjects in all experiments. RESULTS: The slope of the band-pass filters had little effect on both vowel and consonant recognition. A slight performance drop was observed for only the shallowest slope condition (6 dB/octave). In contrast, the distribution of the band-pass filters had a strong effect on vowel recognition but a weak effect on consonant recognition. Best performance was achieved when a logarithmic or near-logarithmic frequency distribution was used to divide the overall frequency range. The bandwidth of the band-pass filters had a moderate effect on both vowel and consonant recognition. Vowel scores dropped significantly when the bandwidth of filters was too broad, whereas consonant scores dropped significantly when a narrower bandwidth was used. CONCLUSION: Under "acute" testing conditions, phoneme recognition with a four-channel CIS strategy seems to be only mildly affected by the slope of the band-pass filters, but can be significantly affected by the distribution of filters as well as the bandwidth of the filters. Optimal or near-optimal performance can be achieved with a logarithmic frequency distribution. Vowels are more susceptible to broad bandwidths, whereas consonants are more susceptible to narrow bandwidths.

Adult↗
PubMed2002

Perceptual learning following changes in the frequency-to-electrode assignment with the Nucleus-22 cochlear implant.

The goal of the present study was to investigate the time course of adaptation by experienced cochlear implant users to a shifted frequency-to-electrode assignment in their speech processors. Speech recognition performance of three Nucleus-22 cochlear implant users was measured over a 3-month period, during which the implant listeners continuously wore "experimental" speech processors that were purposely shifted by 2-4 mm in terms of the frequency-to-electrode assignment relative to their normal processor. Baseline speech performance was measured with each subject's clinically assigned speech processor just prior to implementation of the experimental processor. Baseline speech performance was measured again after the 3-month test period, immediately following the reinstallation of the clinically assigned processor settings. Speech performance with the experimental processor was measured four times during the first week, and weekly thereafter over the 3-month period. Results showed that the experimental processor produced significantly lower performance on all measures of speech recognition immediately following implementation. Over the 3-month test period, consonant and HINT sentence recognition with the experimental processors gradually approached a performance level comparable to but still significantly below the baseline and postexperiment measures made with the clinically assigned processor. However, vowel and TIMIT sentence recognition with the experimental processors remained far below the level of the baseline measures even at the end of the 3-month experimental period. There was no significant change in performance with the clinically assigned processor before or after fitting with the experimental processor. The results suggest that a long-time exposure to a new pattern of stimulation may not be able to compensate for the deficit in performance caused by a 2-4-mm shift in the tonotopic location of stimulation, at least within a 3-month period.

Adult↗
PubMed2002

Multichannel auditory brainstem implant: update on performance in 61 patients.

OBJECT: Neurofibromatosis Type 2 (NF2) has typically resulted in deafness after surgical removal of bilateral vestibular schwannomas (VSs). Cochlear implants are generally ineffective for this kind of deafness because of the loss of continuity in the auditory nerve after tumor removal. The first auditory brainstem implant (ABI) in such a patient was performed in 1979 at the House Ear Institute, and this individual continues to benefit from electrical stimulation of the cochlear nucleus complex. In 1992, an advanced multichannel ABI was developed and a series of patients with NF2 received this implant to study the safety and efficacy of the device. METHODS: At the time of first- or second-side VS removal, patients received an eight-electrode array applied to the surface of the cochlear nucleus within the confines of the lateral recess of the fourth ventricle. The device was activated approximately 6 weeks after implantation. and patients were tested every 3 months for the 1st year after the initial stimulation, and annually thereafter. The protocol included a comprehensive battery of psychophysical and speech perception tests. CONCLUSIONS: The multichannel ABI proved to be effective and safe in providing useful auditory sensations in most patients with NF2. The ABI improved patients' ability to communicate compared with the lipreading-only condition, it allowed the detection and recognition of many environmental sounds, and in some cases it provided significant ability to understand speech by using just the sound from the ABI (with no lipreading cues). Its performance in most patients has continued to improve for up to 8 years after implantation.

Adolescent↗