Biomedical subjects

H J McDermott

Publications and source records attributed to H J McDermott.

At least 19 recordsLinked to original sources

PubMed2001

Loudness summation for pulsatile electrical stimulation of the cochlea: effects of rate, electrode separation, level, and mode of stimulation.

The aim of these two experiments was to gain systematic data on the amount of loudness summation measured for dual-electrode stimuli with varying temporal and spatial separation of current pulses. Loudness summation is important in the implementation of speech processing strategies for implantees. However, the loudness mapping functions used in current speech processors utilize psychophysical data (thresholds and comfortable loudness levels) derived using single-electrode stimuli, and do not take into account the temporal and spatial patterns of the speech processor output. In the first experiment, the current reduction required to equalize the loudness of a dual-electrode stimulus to that of its component (and equally loud) single-electrode stimuli was measured for three electrode separations (0.75, 2.25, and 7.5 mm), three repetition rates (250, 500, and 1000 Hz), and two loudness levels (comfortably loud, and mid-dynamic range). It was found that electrode separation had little effect on loudness summation, except for interactions with level and rate effects at the smallest separation. More current adjustment (in dB) was required for higher rates and lower levels of stimulation. The second experiment investigated the effects of mode (monopolar versus bipolar) and pulse duration on loudness summation. More current adjustment was required in bipolar mode than in monopolar mode at the lower level only. The main effects in both experiments, and their interactions, are consistent with a loudness model in which the neural excitation density is first obtained by temporal integration of excitation at each cochlear place, then converted to specific loudness via a nonlinear relationship, and finally integrated over cochlear place to obtain the loudness. The two important features which affect the loudness relationships in dual-electrode stimulation in this model are the shape of the excitation density function and the amount by which the neural spike probability per pulse is reduced in areas of overlapping excitation due to refractory effects.

Adult↗

PubMed2001

Preliminary results with the AVR ImpaCt frequency-transposing hearing aid.

Currently, the only behind-the-ear hearing aid that provides a frequency transposition function is the ImpaCt DSR675, recently introduced by AVR Communications Ltd. of Israel. In tests with three hearing-impaired adults, the performance of the ImpaCt aid(s) was compared with that of each subject's own (nontransposing) hearing aids. Recognition of monosyllabic words and medial consonants did not differ significantly between the two types of aids. This suggests that the transposition function of the ImpaCt was not effective at providing these subjects with increased high-frequency speech information, at least for the programmable parameters applied in the experiments. However, the subjects' understanding of sentences in a competing noise was significantly poorer with the ImpaCt than with the subjects' own aids. In that test, the ImpaCt aids were programmed to attenuate parts of the noise. The decreased sentence recognition may have resulted from this program, which effectively reduced the bandwidth of the ImpaCt aids.

Acoustic Stimulation↗

PubMed2000

Injury-induced reorganization in adult auditory cortex and its perceptual consequences.

Restricted cochlear lesions in adult animals result in a reorganization of auditory cortex such that the cortical region deprived of its normal input by the lesion is occupied by expanded representations of adjacent cochlear loci, and thus of the frequencies represented at those loci. Analogous injury-induced reorganization is seen in somatosensory, visual and motor cortices of adult animals after restricted peripheral lesions. The occurrence of such reorganization in a wide range of species (including simian primates), and across different sensory systems and forms of peripheral lesion, suggests that it would also occur in humans with similar lesions. Direct evidence in support of this suggestion is provided by a small body of functional imaging evidence in the somatosensory and auditory systems. Although such reorganization does not seem to have a compensatory function, such a profound change in the pattern of cortical activation produced by stimuli exciting peri-lesion parts of the receptor epithelium would be expected to have perceptual consequences. However, there is only limited psychophysical evidence for perceptual effects that might be attributable to injury-induced cortical reorganization, and very little direct evidence for the correlation between the perceptual phenomena and the occurrence of reorganization.

Animals↗

PubMed2000

Preferred listening levels: the effect of background noise for moderate-to-profoundly hearing impaired aid users.

The effects of background noise on the choice of preferred listening level for moderately-to-profoundly hearing impaired adults were investigated. During three listening conditions: 'quiet' (isolated monosyllables), 'suppress' (speech with noise attenuated in the gaps between words), and 'steady' (speech with continuous noise), subjects chose a level that maximized intelligibility without being uncomfortable while listening through a hearing aid. The group mean preferred level for 'suppress' was significantly higher than for 'steady', indicating that the level of noise between words influenced these subjects' choice of listening level. Average word recognition scores were higher in 'suppress' than in 'steady', but the difference was not statistically significant. Subjective loudness and quality ratings indicated no reduction in comfort or quality at the higher level for 'suppress'. The results suggest that a hearing aid that attenuated the level of background noise between words, or temporarily increased the gain during each word, would enable some listeners to tolerate higher levels of speech, thereby improving audibility and possibly speech perception while maintaining listening comfort.

Adult↗

PubMed2000

The relationship between speech perception and electrode discrimination in cochlear implantees.

Speech Intelligibility Index (SII) procedures were used to measure the amount of speech information perceived in five frequency bands (170-570, 570-1170, 1170-1768, 1768-2680, and 2680-5744 Hz) by 15 users of the Cochlear Ltd. CI-22M implant and Spectra-22/SPEAK processor. The speech information perceived was compared to that perceived by normal-hearing listeners. The ability of these subjects to discriminate between stimulation on adjacent electrodes corresponding to each frequency band was also investigated, using a 4IFC procedure with random current level variations of between 0% and 60% of the dynamic range. Relative to normal-hearing listeners, speech information was, on average, significantly more reduced in the four frequency regions between 170 and 2680 Hz than in the region 2680-5744 Hz. There was a significant correlation between electrode discrimination ability (when the random level variation encompassed 20% or more of the dynamic range) and the amount of speech information perceived in the four frequency regions between 170 and 2680 Hz. There was no such correlation in the region 2680-5744 Hz, regardless of the extent of random level variation. These results indicate that speech information in the low to medium frequencies is more difficult for implantees to perceive, that this difficulty is correlated with the difficulty in discriminating electrode place in the presence of random loudness variations, and that fine spectral discrimination may be relatively more important in the vowel-formant regions than in higher frequency regions.

Adolescent↗

PubMed2000

Speech perception with steeply sloping hearing loss: effects of frequency transposition.

Six adults with a very steeply sloping high-frequency hearing loss listened to monosyllabic words in several conditions. In the first condition, their ability to identify phonemes with a signal-to-noise ratio of 6 dB was measured. Results were similar to those of normally hearing subjects listening to the same material through low-pass filters having comparable cut-off frequencies. In the remaining two conditions, four of the hearing-impaired subjects, and a control group of five normally hearing subjects, listened to speech in quiet with and without frequency transposition. The transposition lowered all speech frequencies by a factor of 0.6. Specific auditory training with transposed speech materials different from the materials used in the tests of speech perception was provided in 10 sessions, each of one hour's duration, which were scheduled at weekly intervals. Despite this training, no significant differences were found between the two conditions in these subjects' recognition of words. It is concluded that such a frequency-transposition scheme, if implemented in a wearable hearing aid, would be unlikely to benefit people with a sloping hearing impairment of this type.

Adult↗

PubMed1999

Improvements in speech perception with use of the AVR TranSonic frequency-transposing hearing aid.

Five adults with sensorineural hearing impairment participated in a trial comparing the performance of the AVR TranSonic frequency-transposing hearing aid with that of their own conventional aids. They used the TranSonic for approximately 12 weeks, during which time systematic changes were made to the transposition parameters. Speech perception was assessed with each setting of those parameters and with the participants' own hearing aids. Four participants obtained significantly higher scores with the TranSonic than with their own aids on at least one of the tests. However, analysis of the consonant confusions suggested that the improvement resulted mostly from the TranSonic's low-frequency electro-acoustic characteristics. There was only limited evidence for 2 of the participants that the frequency-lowering function was effective at improving speech perception.

Adult↗

PubMed1999

Control of hearing-aid saturated sound pressure level by frequency-shaped output compression limiting.

To fit a hearing aid successfully, it is important to set the Saturated Sound Pressure Level (SSPL) or Maximum Power Output (MPO) appropriately. The SSPL should be low enough to prevent sounds from being amplified to uncomfortable loudness, and yet high enough to maximize speech intelligibility and signal quality. To help attain an optimum SSPL setting, a novel output compression limiting scheme, with shapable MPO (ShaMPO), has been devised. In ShaMPO, the SSPL is shaped across frequencies in accordance with the individual user's loudness discomfort levels (LDLs). The contributions of different frequency regions to loudness are controlled by summing the amplified signal power relative to the LDLs across frequencies, and using this signal to control a wideband compressor. This scheme and a conventional output compression limiting (AGCo) scheme have been implemented in a digital hearing aid. Ten subjects, with moderately-severe to profound sensorineural hearing losses, participated in a study comparing speech intelligibility and listening comfort for the two schemes. Results showed that there were no significant differences in the speech perception scores between AGCo and ShaMPO, even when the speech was presented at 80 dBA, at which level both schemes were in compression much of the time. However, an examination of how subjects selected the SSPL for the two schemes revealed that, in many instances, AGCo would permit some sounds with compact spectra to be amplified above LDL, whereas ShaMPO would not. Thus the ShaMPO scheme can improve listening comfort for some intense sounds without a loss of speech intelligibility. In contrast, half the subjects found speech at 80 dBA to be uncomfortably loud when listening through their own aids.

Aged↗

PubMed1999

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

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

Adult↗

PubMed1998

Loudness perception with pulsatile electrical stimulation: the effect of interpulse intervals.

The effect of interpulse intervals on the perception of loudness of biphasic current pulse trains was investigated in eight adult cochlear implantees at three different stimulus levels encompassing the psychophysical dynamic range. Equal-loudness contours and thresholds were obtained for stimuli in which two biphasic pulses were presented in a fixed repetition period (4 and 20 ms), and also for single-pulse/period stimuli with rates varying between 20 and 750 Hz. All stimuli were of 500-ms duration, and the phase durations of each pulse were 100 microseconds or less. The results of these experiments were consistent with predictions of a three-stage model of loudness perception, consisting of a peripheral refractory effect function, a sliding central integration time window, and a central equal-loudness decision device. Application of the model to the data allowed the estimation of neural refractory characteristics of the subjects' remaining peripheral neural population. The average neural spike probability for a 50-Hz stimulus was predicted to be about 0.77, with an associated neural refractory time of 7.3 ms. These predictions did not vary systematically with level, implying that the effect of increasing current level on loudness results more from recruitment of neurons than from any increase in average spike probability.

Adult↗

PubMed1998

Loudness perception and frequency discrimination in subjects with steeply sloping hearing loss: possible correlates of neural plasticity.

Loudness functions and frequency difference limens (DLFs) were measured in five subjects with steeply sloping high-frequency sensorineural hearing loss. The stimuli were pulsed pure tones encompassing a range of frequencies. Loudness data were obtained using a 2AFC matching procedure with a 500-Hz reference presented at a number of levels. DLFs were measured using a 3AFC procedure with intensities randomized within 6 dB around an equal-loudness level. Results showed significantly shallower loudness functions near the cutoff frequency of the loss than at a lower frequency, where hearing thresholds were near normal. DLFs were elevated, on average, relative to DLFs measured using the same procedure in five normally hearing subjects, but showed a local reduction near the cutoff frequency in most subjects with high-frequency loss. The loudness data are generally consistent with recent models that describe loudness perception in terms of peripheral excitation patterns that are presumably restricted by a steeply sloping hearing loss. However, the DLF data are interpreted with reference to animal experiments that have shown reorganization in the auditory cortex following the introduction of restricted cochlear lesions. Such reorganization results in an increase in the spatial representation of lesion-edge frequencies, and is comparable with the functional reorganization observed in animals following frequency-discrimination training. It is suggested that similar effects may occur in humans with steeply sloping high-frequency hearing loss, and therefore, the local reduction in DLFs in our data may reflect neural plasticity.

Acoustic Stimulation↗

PubMed1997

Musical pitch perception with electrical stimulation of the cochlea.

Studies were undertaken to investigate the ability of a user of the Nucleus multi-electrode cochlear implant to judge pitch in the context of musical intervals. The subject had qualified as a musical instrument tuner before he received his implant, and was able to judge the intervals between electrical sensations with neither training nor the guidance of familiar melodies. The procedures used were interval estimation, and interval production by the method of adjustment. The pitch of the electrical stimulation was controlled by varying the pulse repetition rate, the active electrode position, or two combinations of these parameters. Further studies employed sinusoidally amplitude modulated pulse trains with varying modulation frequency. The results showed that rate or modulation frequency could convey musical pitch information over a limited range (approximately two octaves). The data were directly comparable with the relationship between musical intervals and frequency for normal hearing. The pitch related to electrode place varied in accordance with the tonotopic organization of the cochlea, and also appeared to be able to support musical intervals. When both place and rate varied together, the place-related pitch was generally dominant. In all cases, the judgement of intervals tended to diverge from their acoustic counterparts as the intervals became larger.

Acoustics↗

PubMed1996

The perceptual dimensions of single-electrode and nonsimultaneous dual-electrode stimuli in cochlear implantees.

The perceptual dimensions evoked by dual-electrode stimulation were investigated in four cochlear implantees. The dual-electrode stimulation consisted of biphasic current pulse trains, such that two intracochlear electrodes each received one pulse in a 250-Hz cycle. The experiments tested the hypothesis that perceptual qualities would be altered when there was an increased likelihood of interactions occurring between the two electrode places. The parameters of the stimulation which were manipulated to test this hypothesis were the time delay between the pulses on the two electrodes (2 ms and a value between 0.62 and 0.92 ms), the distance between the component electrode rings of each bipolar pair (3.75 mm and either 2.25 or 1.5 mm), and the distance between the two bipolar pairs in the dual-electrode stimuli (from 0.75 to 12.0 mm). Five set of 15 loudness balanced stimuli were created, each set having different stimulation parameters. These stimuli consisted of five single-electrode stimuli (a 250-Hz pulse train on each of five electrodes) and the ten dual-electrode stimuli formed by the combinations of those five electrodes. Two perceptual dissimilarity matrices were obtained for each subject and each set, and were analyzed using repeated nonmetric multidimensional scaling techniques. The resultant "stimulus spaces" were then examined to see how many perceptual dimensions the stimuli were best described by, and to what perceptual or stimulus qualities the dimensions might correspond. The results showed that the percept evoked by dual-electrode stimulation contained two main dimensions. Increasing the width of the current path to create substantially overlapping stimulation areas, or altering the temporal delay between the two electrodes, had very little effect on this percept. The position of the stimuli in the two-dimensional space was related to the distances of the two component electrodes along the electrode array. These results are relevant to speech processing strategies and electrode design for cochlear implants, as they imply that the creation of discrete, nonoverlapping areas of stimulation may not cause the percepts to be more distinct, and therefore may not necessarily lead to better speech perception.

Aged↗

PubMed1996

The perception of temporal patterns for electrical stimulation presented at one or two intracochlear sites.

The question of how well the temporal structure in pulsatile electrical stimulation is perceived, and the nature of the information that may be conveyed by this temporal structure, is of importance to the further development of speech processing strategies for cochlear implants. The two experiments described here investigated the perception of temporal fine structure in amplitude modulated 1-kHz pulse trains, both when a single electrode position was used, and when the pulses alternated between two electrode positions. Five subjects with the Mini System 22 implant took part in these experiments. The amplitude modulations were constructed so that all dual-electrode stimuli had the same temporal pattern on each individual electrode but differed in the aggregate temporal pattern A hypothesis was investigated that subjects perceive the aggregate temporal pattern rather than the pattern at each individual electrode place, only when the electrodes are less than a critical distance apart. The first of these two experiments used a four-interval forced-choice task to measure the ability of subjects to detect changes in the aggregate temporal pattern. At electrode distances greater than 3 to 4 mm, subjects could no longer perceive the aggregate pattern, confirming the hypothesis. The second experiment used a single-interval pitch estimation task to test the hypothesis that the perceptual differences in temporal patterns measured in the previous experiment were classified similarly to rate pitch differences by the subjects. The results confirmed this hypothesis, and showed that the pitch of the modulated stimuli could be predicted by the expected inter-pulse intervals in the excited neural population.

Adult↗

PubMed1996

Loudness ratio production by cochlear implantees using the spectral maxima sound processor.

The loudness growth characteristics of five users of the Mini System 22 implant and the Spectral Maxima Sound Processor were measured and compared with those of five normally hearing subjects. The main objective was to evaluate the suitability of the function employed in the sound processor that converts acoustic input levels of electrical stimulation. The method of loudness ratio production was used. The average result of the normally hearing subjects for halving and doubling loudness was 10.8 dB. The results of three of the implantees were comparable with those of the normally hearing subjects (average 8.83 dB), while those of the remaining subjects were quantitatively and qualitatively different. Investigation showed that altering the amplitude conversion function in the sound processor would not make the results of these two implantees more like those of the other subjects. However, it is possible that their responses were affected by the limited input dynamic range of the sound processor.

Adult↗

PubMed1995

Pitch matching of amplitude-modulated current pulse trains by cochlear implantees: the effect of modulation depth.

An experiment was conducted with four cochlear implantees, which investigated the pitch evoked by amplitude-modulating current pulse trains delivered to a single cochlear location. These stimuli produce a pitch percept which may be similar to that of acoustic stimuli such as modulated noise for modulation frequencies in the range 80-300 Hz, approximately. The experiments investigated the effect of modulation depth on the way pitch was matched to that of unmodulated pulse trains. The method of constant stimuli was used, in which the variable parameter was the rate of the unmodulated stimulus. The modulated stimuli comprised pulses having one of two possible current values, with the higher value occurring once in every modulation period. The results showed that the matched rate fell exponentially from a value close to the carrier rate towards a value equal to the modulation frequency as the modulation depth increased. The results were compared to the predictions of a simple model in which the matched rate corresponded to a weighted average of carrier and modulation frequencies, with the weightings proportional to the number of neurons firing at each of these frequencies. The results agreed with the predictions of the model reasonably well, except in cases where the carrier rate was 700 Hz or higher, and for one subject at the highest intensity level.

Adult↗

PubMed1995

Evaluation of the Nucleus Spectra 22 processor and new speech processing strategy (SPEAK) in postlinguistically deafened adults.

A new speech processing strategy (SPEAK) has been compared with the previous Multipeak (MPEAK) strategy in a study with 24 postlinguistically deafened adults. The results show that performance with the SPEAK coding strategy was significantly better for 58.31% of subjects on closed-set consonant identification, for 33.3% of subjects on closed-set vowel identification and open-set monosyllabic word recognition, and for 81.8% of subjects on open-set sentence recognition in quiet and in competing noise (+ 10 dB signal-to-noise ratio). By far the largest improvement observed was for sentence recognition in noise, with the mean score across subjects for the SPEAK strategy twice that obtained with MPEAK.

Adult↗

PubMed1995

Loudness summation for two channels of stimulation in cochlear implants: effects of spatial and temporal separation.

An experiment with four implantees with the Mini System 22 device was undertaken to measure the loudness summation across two channels of stimulation, with stimuli in which the current pulses were delivered alternately to each channel. The effects of varying spatial separation, temporal separation, and extent of stimulation were investigated. It was found that the absolute amount of summation varied among subjects, and was in general independent of electrode separation, except for a reduction at zero separation. Widening of the spatial extent of the stimulation did not have a consistent effect. There was a reduction in summation for all subjects at zero electrode separation when the time between the two pulses was increased from less than 1 millisecond to 2 milliseconds. In conclusion, loudness summation did not appear to be highly dependent on parameters that affect the spatial current spread in the cochlea. Further study of the effect of temporal parameters on loudness may help to quantify interaction between stimulation channels.

Adult↗