Effects of deafness and cochlear implant use on temporal response characteristics in cat primary auditory cortex

Effects of chronic implantation and long-term stimulation of a cochlear implant in the partial hearing cat model

The expansion of criteria for cochlear implantation has resulted in increasing numbers of cochlear implant subjects having some level of residual hearing. The present study examined the effects of implantation surgery and long-term electrical stimulation on residual hearing in a partially deafened cat model. Eighteen animals were partially deafened, implanted and chronically stimulated. Implantation resulted in a pronounced loss evident 2-weeks post implantation of up to 30-40 dB at 4 & 8 kHz which was statistically significant (2-way RM ANOVA (Time, Frequency): p(Time) = 0.001; p(Frequency) < 0.001; p(Time x Frequency) < 0.001)). Chronic stimulation resulted in a significant (RM ANOVA: p(Time) = 0.030) ongoing hearing loss, with 5 animals (∼30%) exhibiting an increase in threshold of 20 dB or more. Different loss profiles were evident with peripheral and central hearing assessments suggests that changes in 'central gain' may be occurring. Despite significant loss of hair cells and spiral ganglion neurons and distinct fibrous tissue growth in the scala tympani following implantation and long-term electrical stimulation, there were no significant correlations with any histological measures and ongoing hearing loss. The partially deafened, chronically stimulated cat model provides a clinically relevant model in which to further investigate the cause of the delayed hearing loss following cochlear implant surgery and use.

Cochlear Implant Research and Development in the Twenty-first Century: A Critical Update

Cochlear implants (CIs) are the world's most successful sensory prosthesis and have been the subject of intense research and development in recent decades. We critically review the progress in CI research, and its success in improving patient outcomes, from the turn of the century to the present day. The review focuses on the processing, stimulation, and audiological methods that have been used to try to improve speech perception by human CI listeners, and on fundamental new insights in the response of the auditory system to electrical stimulation. The introduction of directional microphones and of new noise reduction and pre-processing algorithms has produced robust and sometimes substantial improvements. Novel speech-processing algorithms, the use of current-focusing methods, and individualised (patient-by-patient) deactivation of subsets of electrodes have produced more modest improvements. We argue that incremental advances have and will continue to be made, that collectively these may substantially improve patient outcomes, but that the modest size of each individual advance will require greater attention to experimental design and power. We also briefly discuss the potential and limitations of promising technologies that are currently being developed in animal models, and suggest strategies for researchers to collectively maximise the potential of CIs to improve hearing in a wide range of listening situations.

Perceptual learning of pitch provided by cochlear implant stimulation rate

Cochlear implant users hear pitch evoked by stimulation rate, but discrimination diminishes for rates above 300 Hz. This upper limit on rate pitch is surprising given the remarkable and specialized ability of the auditory nerve to respond synchronously to stimulation rates at least as high as 3 kHz and arguably as high as 10 kHz. Sensitivity to stimulation rate as a pitch cue varies widely across cochlear implant users and can be improved with training. The present study examines individual differences and perceptual learning of stimulation rate as a cue for pitch ranking. Adult cochlear implant users participated in electrode psychophysics that involved testing once per week for three weeks. Stimulation pulse rate discrimination was measured in bipolar and monopolar configurations for apical and basal electrodes. Base stimulation rates between 100 and 800 Hz were examined. Individual differences were quantified using psychophysically derived metrics of spatial tuning and temporal integration. This study examined distribution of measures across subjects, predictive power of psychophysically derived metrics of spatial tuning and temporal integration, and the effect of training on rate discrimination thresholds. Psychophysical metrics of spatial tuning and temporal integration were not predictive of stimulation rate discrimination, but discrimination thresholds improved at lower frequencies with training. Since most clinical devices do not use variable stimulation rates, it is unknown to what extent recipients may learn to use stimulation rate cues if provided in a clear and consistent manner.

Age-Related Changes in Temporal Resolution Revisited: Electrophysiological and Behavioral Findings From Cochlear Implant Users

OBJECTIVES

The mechanisms underlying age-related changes in speech perception are still unclear, most likely multifactorial and often can be difficult to parse out from the effects of hearing loss. Age-related changes in temporal resolution (i.e., the ability to track rapid changes in sounds) have long been associated with speech perception declines exhibited by many older individuals. The goals of this study were as follows: (1) to assess age-related changes in temporal resolution in cochlear implant (CI) users, and (2) to examine the impact of changes in temporal resolution and cognition on the perception of speech in noise. In this population, it is possible to bypass the cochlea and stimulate the auditory nerve directly in a noninvasive way. Additionally, CI technology allows for manipulation of the temporal properties of a signal without changing its spectrum.

DESIGN

Twenty postlingually deafened Nucleus CI users took part in this study. They were divided into groups of younger (18 to 40 years) and older (68 to 82 years) participants. A cross-sectional study design was used. The speech processor was bypassed and a mid-array electrode was used for stimulation. We compared peripheral and central physiologic measures of temporal resolution with perceptual measures obtained using similar stimuli. Peripherally, temporal resolution was assessed with measures of the rate of recovery of the electrically evoked compound action potential (ECAP), evoked using a single pulse and a pulse train as maskers. The acoustic change complex (ACC) to gaps in pulse trains was used to assess temporal resolution more centrally. Psychophysical gap detection thresholds were also obtained. Cognitive assessment included two tests of processing speed (Symbol Search and Coding) and one test of working memory (Digit Span Test). Speech perception was tested in the presence of background noise (QuickSIN test). A correlational design was used to explore the relationship between temporal resolution, cognition, and speech perception.

RESULTS

The only metric that showed significant age effects in temporal processing was the ECAP recovery function recorded using pulse train maskers. Younger participants were found to have faster rates of neural recovery following presentation of pulse trains than older participants. Age was not found to have a significant effect on speech perception. When results from both groups were combined, digit span was the only measure significantly correlated with speech perception performance.

CONCLUSIONS

In this sample of CI users, few effects of advancing age on temporal resolution were evident. While this finding would be consistent with a general lack of aging effects on temporal resolution, it is also possible that aging effects are influenced by processing peripheral to the auditory nerve, which is bypassed by the CI. However, it is known that cross-fiber neural synchrony is improved with electrical (as opposed to acoustic) stimulation. This change in neural synchrony may, in turn, make temporal cues more robust/perceptible to all CI users. Future studies involving larger sample sizes should be conducted to confirm these findings. Results of this study also add to the growing body of literature that suggests that working memory is important for the perception of degraded speech.

Influence of storage on the quality of conventional CT and µCT-imaging for the middle and inner cat ear

The aim of this study was to analyze whether different fixation methods such as freezing or formaldehyde storage for different periods of time have an influence on the recognition of anatomical relevant structures in the middle and inner ear of the cat with conventional computed tomography (cCT) and micro-computed tomography (µCT). Besides, effects of freeze-thaw cycles on determined structures of the ear were investigated by means of histological slices. Three veterinarians with different radiologic expertise evaluated the scans of 30 dissected cat ears anonymously and scored predefined structures in a five-point scale with reference to visually sharp reproducibility and perfect image quality. The total scores of the different fixation groups as well as the ears within a group were compared with each other. Furthermore, an intra-reader examination including an evaluation of the identifiability of specified structures was performed for both imaging methods. cCT as well as µCT-scans have a very low variation coefficient of 1.6% and 2.3%, respectively. The results for the alterations between the different fixation methods show that the changes for cCT-scans are negligible, as the percentage alteration compared to fresh samples ranges in a very small interval with values from 1.0% better to 1.2% worse. µCT-scans are more influenced by the fixation method with a range from 1.3% better to 6.9% worse values. The scans mostly deteriorated after two freeze-thaw cycles (1.8% worse) and after storing the samples for 1 (2.4% worse), respectively, and 3 weeks in formaldehyde (6.9% worse).

Restoration of spatial hearing in adult cochlear implant users with single-sided deafness

In recent years, cochlear implants (CIs) have been provided in growing numbers to people with not only bilateral deafness but also to people with unilateral hearing loss, at times in order to alleviate tinnitus. This study presents audiological data from 15 adult participants (ages 48 ± 12 years) with single sided deafness. Results are presented from 9/15 adults, who received a CI (SSD-CI) in the deaf ear and were tested in Acoustic or Acoustic + CI hearing modes, and 6/15 adults who are planning to receive a CI, and were tested in the unilateral condition only. Testing included (1) audiometric measures of threshold, (2) speech understanding for CNC words and AzBIO sentences, (3) tinnitus handicap inventory, (4) sound localization with stationary sound sources, and (5) perceived auditory motion. Results showed that when listening to sentences in quiet, performance was excellent in the Acoustic and Acoustic + CI conditions. In noise, performance was similar between Acoustic and Acoustic + CI conditions in 4/6 participants tested, and slightly worse in the Acoustic + CI in 2/6 participants. In some cases, the CI provided reduced tinnitus handicap scores. When testing sound localization ability, the Acoustic + CI condition resulted in improved sound localization RMS error of 29.2° (SD: ±6.7°) compared to 56.6° (SD: ±16.5°) in the Acoustic-only condition. Preliminary results suggest that the perception of motion direction, whereby subjects are required to process and compare directional cues across multiple locations, is impaired when compared with that of normal hearing subjects.

Voice Discrimination by Adults with Cochlear Implants: the Benefits of Early Implantation for Vocal-Tract Length Perception

Cochlear implant (CI) users find it extremely difficult to discriminate between talkers, which may partially explain why they struggle to understand speech in a multi-talker environment. Recent studies, based on findings with postlingually deafened CI users, suggest that these difficulties may stem from their limited use of vocal-tract length (VTL) cues due to the degraded spectral resolution transmitted by the CI device. The aim of the present study was to assess the ability of adult CI users who had no prior acoustic experience, i.e., prelingually deafened adults, to discriminate between resynthesized "talkers" based on either fundamental frequency (F0) cues, VTL cues, or both. Performance was compared to individuals with normal hearing (NH), listening either to degraded stimuli, using a noise-excited channel vocoder, or non-degraded stimuli. Results show that (a) age of implantation was associated with VTL but not F0 cues in discriminating between talkers, with improved discrimination for those subjects who were implanted at earlier age; (b) there was a positive relationship for the CI users between VTL discrimination and speech recognition score in quiet and in noise, but not with frequency discrimination or cognitive abilities; (c) early-implanted CI users showed similar voice discrimination ability as the NH adults who listened to vocoded stimuli. These data support the notion that voice discrimination is limited by the speech processing of the CI device. However, they also suggest that early implantation may facilitate sensory-driven tonotopicity and/or improve higher-order auditory functions, enabling better perception of VTL spectral cues for voice discrimination.

Induced cortical responses require developmental sensory experience

Sensory areas of the cerebral cortex integrate the sensory inputs with the ongoing activity. We studied how complete absence of auditory experience affects this process in a higher mammal model of complete sensory deprivation, the congenitally deaf cat. Cortical responses were elicited by intracochlear electric stimulation using cochlear implants in adult hearing controls and deaf cats. Additionally, in hearing controls, acoustic stimuli were used to assess the effect of stimulus mode (electric versus acoustic) on the cortical responses. We evaluated time-frequency representations of local field potential recorded simultaneously in the primary auditory cortex and a higher-order area, the posterior auditory field, known to be differentially involved in cross-modal (visual) reorganization in deaf cats. The results showed the appearance of evoked (phase-locked) responses at early latencies (<100 ms post-stimulus) and more abundant induced (non-phase-locked) responses at later latencies (>150 ms post-stimulus). In deaf cats, substantially reduced induced responses were observed in overall power as well as duration in both investigated fields. Additionally, a reduction of ongoing alpha band activity was found in the posterior auditory field (but not in primary auditory cortex) of deaf cats. The present study demonstrates that induced activity requires developmental experience and suggests that higher-order areas involved in the cross-modal reorganization show more auditory deficits than primary areas.

Congenital deafness affects deep layers in primary and secondary auditory cortex

Congenital deafness leads to functional deficits in the auditory cortex for which early cochlear implantation can effectively compensate. Most of these deficits have been demonstrated functionally. Furthermore, the majority of previous studies on deafness have involved the primary auditory cortex; knowledge of higher-order areas is limited to effects of cross-modal reorganization. In this study, we compared the cortical cytoarchitecture of four cortical areas in adult hearing and congenitally deaf cats (CDCs): the primary auditory field A1, two secondary auditory fields, namely the dorsal zone and second auditory field (A2); and a reference visual association field (area 7) in the same section stained either using Nissl or SMI-32 antibodies. The general cytoarchitectonic pattern and the area-specific characteristics in the auditory cortex remained unchanged in animals with congenital deafness. Whereas area 7 did not differ between the groups investigated, all auditory fields were slightly thinner in CDCs, this being caused by reduced thickness of layers IV-VI. The study documents that, while the cytoarchitectonic patterns are in general independent of sensory experience, reduced layer thickness is observed in both primary and higher-order auditory fields in layer IV and infragranular layers. The study demonstrates differences in effects of congenital deafness between supragranular and other cortical layers, but similar dystrophic effects in all investigated auditory fields.

Synaptic distribution and plasticity in primary auditory cortex (A1) exhibits laminar and cell-specific changes in the deaf

The processing sequence through primary auditory cortex (A1) is impaired by deafness as evidenced by reduced neuronal activation in A1 of cochlear-implanted deaf cats. Such a loss of neuronal excitation should be manifest as changes in excitatory synaptic number and/or size, for which the post-synaptic correlate is the dendritic spine. Therefore, the present study sought evidence for this functional disruption using Golgi-Cox/light microscopic techniques that examined spine-bearing neurons and their dendritic spine features across all laminae in A1 of early-deaf (ototoxic lesion <1 month; raised into adulthood >16 months) and hearing cats. Surprisingly, in the early-deaf significant increases in spine density and size were observed in the supragranular layers, while significant reductions in spine density were observed for spiny non-pyramidal, but not pyramidal, neurons in the granular layer. No changes in dendritic spine density consistent with loss of excitatory inputs were seen for infragranular neurons. These results indicate that long-term early-deafness induces plastic changes in the excitatory circuitry of A1 that are laminar and cell-specific. An additional finding was that, unlike the expected abundance of stellate neurons that characterize the granular layer of other primary sensory cortices, pyramidal neurons predominate within layer 4 of A1. Collectively, these observations are important for understanding how neuronal connectional configurations contribute to region-specific processing capabilities in normal brains as well as those with altered sensory experiences.

Effect of current focusing on the sensitivity of inferior colliculus neurons to amplitude-modulated stimulation

In multichannel cochlear implants (CIs), current is delivered to specific electrodes along the cochlea in the form of amplitude-modulated pulse trains, to convey temporal and spectral cues. Our previous studies have shown that focused multipolar (FMP) and tripolar (TP) stimulation produce more restricted neural activation and reduced channel interactions in the inferior colliculus (IC) compared with traditional monopolar (MP) stimulation, suggesting that focusing of stimulation could produce better transmission of spectral information. The present study explored the capability of IC neurons to detect modulated CI stimulation with FMP and TP stimulation compared with MP stimulation. The study examined multiunit responses of IC neurons in acutely deafened guinea pigs by systematically varying the stimulation configuration, modulation depth, and stimulation level. Stimuli were sinusoidal amplitude-modulated pulse trains (carrier rate of 120 pulses/s). Modulation sensitivity was quantified by measuring modulation detection thresholds (MDTs), defined as the lowest modulation depth required to differentiate the response of a modulated stimulus from an unmodulated one. Whereas MP stimulation showed significantly lower MDTs than FMP and TP stimulation (P values <0.05) at stimulation ≤2 dB above threshold, all stimulation configurations were found to have similar modulation sensitivities at 4 dB above threshold. There was no difference found in modulation sensitivity between FMP and TP stimulation. The present study demonstrates that current focusing techniques such as FMP and TP can adequately convey amplitude modulation and are comparable to MP stimulation, especially at higher stimulation levels, although there may be some trade-off between spectral and temporal fidelity with current focusing stimulation.