Chronic intracochlear electrical stimulation in the neonatally deafened cat. II. Temporal properties of neurons in the inferior colliculus

Investigation of inner ear drug delivery with a cochlear catheter in piglets as a representative model for human cochlear pharmacokinetics

Hearing impairment is the most common sensory disorder in humans, and yet hardly any medications are licensed for the treatment of inner ear pathologies. Intricate pharmacokinetic examinations to better understand drug distribution within this complex organ could facilitate the development of novel therapeutics. For such translational research projects, animal models are indispensable, but differences in inner ear dimensions and other anatomical features complicate the transfer of experimental results to the clinic. The gap between rodents and humans may be bridged using larger animal models such as non-human primates. However, their use is challenging and impeded by administrative, regulatory, and financial hurdles. Other large animal models with more human-like inner ear dimensions are scarce. In this study, we analyzed the inner ears of piglets as a potential representative model for the human inner ear and established a surgical approach for intracochlear drug application and subsequent apical sampling. Further, controlled intracochlear delivery of fluorescein isothiocyanate-dextran (FITC-d) was carried out after the insertion of a novel, clinically applicable CE-marked cochlear catheter through the round window membrane. Two, six, and 24 hours after a single injection with this device, the intracochlear FITC-d distribution was determined in sequential perilymph samples. The fluorometrically assessed concentrations two hours after injection were compared to the FITC-d content in control groups, which either had been injected with a simple needle puncture through the round window membrane or the cochlear catheter in combination with a stapes vent hole. Our findings demonstrate not only significantly increased apical FITC-d concentrations when using the cochlear catheter but also higher total concentrations in all perilymph samples. Additionally, the concentration decreased after six and 24 hours and showed a more homogenous distribution compared to shorter observation times.

Temporal Pitch Sensitivity in an Animal Model: Psychophysics and Scalp Recordings : Temporal Pitch Sensitivity in Cat

Cochlear implant (CI) users show limited sensitivity to the temporal pitch conveyed by electric stimulation, contributing to impaired perception of music and of speech in noise. Neurophysiological studies in cats suggest that this limitation is due, in part, to poor transmission of the temporal fine structure (TFS) by the brainstem pathways that are activated by electrical cochlear stimulation. It remains unknown, however, how that neural limit might influence perception in the same animal model. For that reason, we developed non-invasive psychophysical and electrophysiological measures of temporal (i.e., non-spectral) pitch processing in the cat. Normal-hearing (NH) cats were presented with acoustic pulse trains consisting of band-limited harmonic complexes that simulated CI stimulation of the basal cochlea while removing cochlear place-of-excitation cues. In the psychophysical procedure, trained cats detected changes from a base pulse rate to a higher pulse rate. In the scalp-recording procedure, the cortical-evoked acoustic change complex (ACC) and brainstem-generated frequency following response (FFR) were recorded simultaneously in sedated cats for pulse trains that alternated between the base and higher rates. The range of perceptual sensitivity to temporal pitch broadly resembled that of humans but was shifted to somewhat higher rates. The ACC largely paralleled these perceptual patterns, validating its use as an objective measure of temporal pitch sensitivity. The phase-locked FFR, in contrast, showed strong brainstem encoding for all tested pulse rates. These measures demonstrate the cat's perceptual sensitivity to pitch in the absence of cochlear-place cues and may be valuable for evaluating neural mechanisms of temporal pitch perception in the feline animal model of stimulation by a CI or novel auditory prostheses.

Threshold sound conditioning in the treatment of sensorineural hearing loss

OBJECTIVES/HYPOTHESIS

Sensorineural hearing loss is one of the most common human disorders, with increasing incidence in elderly patients, severely restricting normal activities, and lowering quality of life. The introduction of sound conditioning has the potential to activate auditory pathway plasticity and improve basal frequency hearing. Our objective was to evaluate the safety and efficacy of threshold sound conditioning (TSC). The null hypothesis in this study was that TSC does not have a significant effect on auditory threshold amelioration.

METHODS

Pure tone audiometry (PTA) was performed and hearing thresholds were measured once at baseline, and a second time following TSC intervention. Data were analyzed using an intention-to treat design.

RESULTS

The TSC group (78%) significantly differed from the control group (44%) on auditory threshold amelioration; P = .008091 in DV1, P = .000546 in DV2 by Scheffe's post hoc test. Female subjects (77%) showed a significant difference in DV1 from male subjects (47%); P = .025468 in DV1 by Scheffe's post hoc test. Older subjects (75%) showed no significant difference from younger subjects (53%); P = .139149 in DV1, P = .082920 in DV2 by Scheffe's post hoc test.

CONCLUSIONS

We observed a significant improvement in a narrow band frequency threshold in this randomized controlled prospective clinical study in a broad range of subjects. These data have important clinical implications since there is no current long-term therapy for this widespread and growing disability. Additional physiologic, mechanistic, and molecular studies are necessary to fully elucidate the pathophysiology and mechanism of action of TSC.

LEVEL OF EVIDENCE

1a.

Chronic Deafness Degrades Temporal Acuity in the Electrically Stimulated Auditory Pathway

Electrical stimulation of the auditory nerve with a penetrating intraneural (IN) electrode in acutely deafened cats produces much more restricted spread of excitation than is obtained in that preparation with a conventional cochlear implant (CI) as reported by Middlebrooks and Snyder (J Assoc Res Otolaryngol 8:258–279, 2007). That suggests that a future auditory prosthesis employing IN stimulation might offer human patients greater frequency selectivity than is available with a present-day CI. Nevertheless, it is a concern that the electrical field produced by an IN electrode might be too restricted to produce adequate stimulation of the partially depopulated auditory nerve of a deaf patient. We evaluated this by testing responses to IN and CI stimulation in adult-deafened cats. Activation of the auditory pathway was monitored by recording from the central nucleus of the inferior colliculus (ICC). Cats deaf for 153–277 days exhibited a ~ 30 % loss of auditory nerve fibers compared to cats deaf for < 18 h. Contrary to our concern, measures of thresholds and dynamic ranges showed no significant deafness-related impairment of excitation by IN or CN stimulation. Surprisingly, however, temporal acuity decreased dramatically in these adult-deafened cats, as demonstrated by a marked decrease in the maximum rate of electrical cochlear stimulation to which ICC neurons synchronized to IN or CI stimulation. For instance, half of ICC neurons synchronized to IN stimulation up to 203 pulses per second (pps) in acute deafness, whereas that number dropped to 79 pps for chronic deafness. Such a loss of temporal acuity might contribute to the poor sensitivity to temporal fine structure that has been reported in human CI users. Seemingly, the degraded temporal acuity that we observed in cats was even worse than the fine-structure sensitivity of human CI users, suggesting that most patients experience some improvement of temporal acuity resulting from restoration of patterned auditory nerve stimulation by a CI.

Passive stimulation and behavioral training differentially transform temporal processing in the inferior colliculus and primary auditory cortex

In profoundly deaf cats, behavioral training with intracochlear electric stimulation (ICES) can improve temporal processing in the primary auditory cortex (AI). To investigate whether similar effects are manifest in the auditory midbrain, ICES was initiated in neonatally deafened cats either during development after short durations of deafness (8 wk of age) or in adulthood after long durations of deafness (≥3.5 yr). All of these animals received behaviorally meaningless, "passive" ICES. Some animals also received behavioral training with ICES. Two long-deaf cats received no ICES prior to acute electrophysiological recording. After several months of passive ICES and behavioral training, animals were anesthetized, and neuronal responses to pulse trains of increasing rates were recorded in the central (ICC) and external (ICX) nuclei of the inferior colliculus. Neuronal temporal response patterns (repetition rate coding, minimum latencies, response precision) were compared with results from recordings made in the AI of the same animals (Beitel RE, Vollmer M, Raggio MW, Schreiner CE. J Neurophysiol 106: 944-959, 2011; Vollmer M, Beitel RE. J Neurophysiol 106: 2423-2436, 2011). Passive ICES in long-deaf cats remediated severely degraded temporal processing in the ICC and had no effects in the ICX. In contrast to observations in the AI, behaviorally relevant ICES had no effects on temporal processing in the ICC or ICX, with the single exception of shorter latencies in the ICC in short-deaf cats. The results suggest that independent of deafness duration passive stimulation and behavioral training differentially transform temporal processing in auditory midbrain and cortex, and primary auditory cortex emerges as a pivotal site for behaviorally driven neuronal temporal plasticity in the deaf cat.

NEW & NOTEWORTHY

Behaviorally relevant vs. passive electric stimulation of the auditory nerve differentially affects neuronal temporal processing in the central nucleus of the inferior colliculus (ICC) and the primary auditory cortex (AI) in profoundly short-deaf and long-deaf cats. Temporal plasticity in the ICC depends on a critical amount of electric stimulation, independent of its behavioral relevance. In contrast, the AI emerges as a pivotal site for behaviorally driven neuronal temporal plasticity in the deaf auditory system.

Temporal properties of inferior colliculus neurons to photonic stimulation in the cochlea

Infrared neural stimulation (INS) may be beneficial in auditory prostheses because of its spatially selective activation of spiral ganglion neurons. However, the response properties of single auditory neurons to INS and the possible contributions of its optoacoustic effects are yet to be examined. In this study, the temporal properties of auditory neurons in the central nucleus of the inferior colliculus (ICC) of guinea pigs in response to INS were characterized. Spatial selectivity of INS was observed along the tonotopically organized ICC. Trains of laser pulses and trains of acoustic clicks were used to evoke single unit responses in ICC of normal hearing animals. In response to INS, ICC neurons showed lower limiting rates, longer latencies, and lower firing efficiencies. In deaf animals, ICC neurons could still be stimulated by INS while unresponsive to acoustic stimulation. The site and spatial selectivity of INS both likely shaped the temporal properties of ICC neurons.

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

We have previously shown that neonatal deafness of 7-13 months duration leads to loss of cochleotopy in the primary auditory cortex (AI) that can be reversed by cochlear implant use. Here we describe the effects of a similar duration of deafness and cochlear implant use on temporal processing. Specifically, we compared the temporal resolution of neurons in AI of young adult normal-hearing cats that were acutely deafened and implanted immediately prior to recording with that in three groups of neonatally deafened cats. One group of neonatally deafened cats received no chronic stimulation. The other two groups received up to 8 months of either low- or high-rate (50 or 500 pulses per second per electrode, respectively) stimulation from a clinical cochlear implant, initiated at 10 weeks of age. Deafness of 7-13 months duration had no effect on the duration of post-onset response suppression, latency, latency jitter, or the stimulus repetition rate at which units responded maximally (best repetition rate), but resulted in a statistically significant reduction in the ability of units to respond to every stimulus in a train (maximum following rate). None of the temporal response characteristics of the low-rate group differed from those in acutely deafened controls. In contrast, high-rate stimulation had diverse effects: it resulted in decreased suppression duration, longer latency and greater jitter relative to all other groups, and an increase in best repetition rate and cut-off rate relative to acutely deafened controls. The minimal effects of moderate-duration deafness on temporal processing in the present study are in contrast to its previously-reported pronounced effects on cochleotopy. Much longer periods of deafness have been reported to result in significant changes in temporal processing, in accord with the fact that duration of deafness is a major factor influencing outcome in human cochlear implantees.

Coding of electric pulse trains presented through cochlear implants in the auditory midbrain of awake rabbit: comparison with anesthetized preparations

Cochlear implant (CI) listeners show limits at high frequencies in tasks involving temporal processing such as rate pitch and interaural time difference discrimination. Similar limits have been observed in neural responses to electric stimulation in animals with CI; however, the upper limit of temporal coding of electric pulse train stimuli in the inferior colliculus (IC) of anesthetized animals is lower than the perceptual limit. We hypothesize that the upper limit of temporal neural coding has been underestimated in previous studies due to the confound of anesthesia. To test this hypothesis, we developed a chronic, awake rabbit preparation for single-unit studies of IC neurons with electric stimulation through CI. Stimuli were periodic trains of biphasic pulses with rates varying from 20 to 1280 pulses per second. We found that IC neurons in awake rabbits showed higher spontaneous activity and greater sustained responses, both excitatory and suppressive, at high pulse rates. Maximum pulse rates that elicited synchronized responses were approximately two times higher in awake rabbits than in earlier studies with anesthetized animals. Here, we demonstrate directly that anesthesia is a major factor underlying these differences by monitoring the responses of single units in one rabbit before and after injection of an ultra-short-acting barbiturate. In general, the physiological rate limits of IC neurons in the awake rabbit are more consistent with the psychophysical limits in human CI subjects compared with limits from anesthetized animals.

Unilateral hearing during development: hemispheric specificity in plastic reorganizations

The present study investigates the hemispheric contributions of neuronal reorganization following early single-sided hearing (unilateral deafness). The experiments were performed on ten cats from our colony of deaf white cats. Two were identified in early hearing screening as unilaterally congenitally deaf. The remaining eight were bilaterally congenitally deaf, unilaterally implanted at different ages with a cochlear implant. Implanted animals were chronically stimulated using a single-channel portable signal processor for two to five months. Microelectrode recordings were performed at the primary auditory cortex under stimulation at the hearing and deaf ear with bilateral cochlear implants. Local field potentials (LFPs) were compared at the cortex ipsilateral and contralateral to the hearing ear. The focus of the study was on the morphology and the onset latency of the LFPs. With respect to morphology of LFPs, pronounced hemisphere-specific effects were observed. Morphology of amplitude-normalized LFPs for stimulation of the deaf and the hearing ear was similar for responses recorded at the same hemisphere. However, when comparisons were performed between the hemispheres, the morphology was more dissimilar even though the same ear was stimulated. This demonstrates hemispheric specificity of some cortical adaptations irrespective of the ear stimulated. The results suggest a specific adaptation process at the hemisphere ipsilateral to the hearing ear, involving specific (down-regulated inhibitory) mechanisms not found in the contralateral hemisphere. Finally, onset latencies revealed that the sensitive period for the cortex ipsilateral to the hearing ear is shorter than that for the contralateral cortex. Unilateral hearing experience leads to a functionally-asymmetric brain with different neuronal reorganizations and different sensitive periods involved.

Functional and structural changes throughout the auditory system following congenital and early-onset deafness: implications for hearing restoration

The absence of auditory input, particularly during development, causes widespread changes in the structure and function of the auditory system, extending from peripheral structures into auditory cortex. In humans, the consequences of these changes are far-reaching and often include detriments to language acquisition, and associated psychosocial issues. Much of what is currently known about the nature of deafness-related changes to auditory structures comes from studies of congenitally deaf or early-deafened animal models. Fortunately, the mammalian auditory system shows a high degree of preservation among species, allowing for generalization from these models to the human auditory system. This review begins with a comparison of common methods used to obtain deaf animal models, highlighting the specific advantages and anatomical consequences of each. Some consideration is also given to the effectiveness of methods used to measure hearing loss during and following deafening procedures. The structural and functional consequences of congenital and early-onset deafness have been examined across a variety of mammals. This review attempts to summarize these changes, which often involve alteration of hair cells and supporting cells in the cochleae, and anatomical and physiological changes that extend through subcortical structures and into cortex. The nature of these changes is discussed, and the impacts to neural processing are addressed. Finally, long-term changes in cortical structures are discussed, with a focus on the presence or absence of cross-modal plasticity. In addition to being of interest to our understanding of multisensory processing, these changes also have important implications for the use of assistive devices such as cochlear implants.

Single-sided deafness leads to unilateral aural preference within an early sensitive period

Unilateral deafness has a high incidence in children. In addition to children who are born without hearing in one ear, children with bilateral deafness are frequently equipped only with one cochlear implant, leaving the other ear deaf. The present study investigates the effects of such single-sided deafness during development in the congenitally deaf cat. The investigated animals were either born with unilateral deafness or received a cochlear implant in one ear and were subjected to chronic monaural stimulation. In chronically stimulated animals, implantation ages were at the following three critical developmental points: 'early' during the peak of functional cortical synaptogenesis in deaf animals; 'intermediate' at the age when synaptic activity in the deaf cats dropped to the level of hearing control cats and finally, 'late' at the age when the evoked synaptic activity fell below the level of hearing control cats. After periods of unilateral hearing, local field potentials were recorded from the cortical surface using a microelectrode at ∼100 recording positions. Stimulation was with cochlear implants at both ears. The measures evaluated were dependent only on the symmetry of aural input: paired differences of onset latencies and paired relations of peak amplitudes of local field potentials. A massive reorganization of aural preference in favour of the hearing ear was found in these measures if the onset of unilateral hearing was early (before or around the peak of functional synaptogenesis). The effect was reduced if onset of unilateral hearing was in the intermediate period, and it disappeared if the onset was late. In early onset of unilateral deafness, the used ear became functionally dominant with respect to local field potential onset latency and amplitude. This explains the inferior outcome of implantations at the second-implanted ear compared with first-implanted ear in children. However, despite a central disadvantage for the deaf ear, it still remained capable of activating the auditory cortex. Appropriate training may thus help to improve the performance at the second-implanted ear. In conclusion, periods of monaural stimulation should be kept as short as possible, and training focused on the deaf ear should be introduced after delayed second implantation in children.

Development of brainstem-evoked responses in congenital auditory deprivation

To compare the development of the auditory system in hearing and completely acoustically deprived animals, naive congenitally deaf white cats (CDCs) and hearing controls (HCs) were investigated at different developmental stages from birth till adulthood. The CDCs had no hearing experience before the acute experiment. In both groups of animals, responses to cochlear implant stimulation were acutely assessed. Electrically evoked auditory brainstem responses (E-ABRs) were recorded with monopolar stimulation at different current levels. CDCs demonstrated extensive development of E-ABRs, from first signs of responses at postnatal (p.n.) day 3 through appearance of all waves of brainstem response at day 8 p.n. to mature responses around day 90 p.n.. Wave I of E-ABRs could not be distinguished from the artifact in majority of CDCs, whereas in HCs, it was clearly separated from the stimulus artifact. Waves II, III, and IV demonstrated higher thresholds in CDCs, whereas this difference was not found for wave V. Amplitudes of wave III were significantly higher in HCs, whereas wave V amplitudes were significantly higher in CDCs. No differences in latencies were observed between the animal groups. These data demonstrate significant postnatal subcortical development in absence of hearing, and also divergent effects of deafness on early waves II–IV and wave V of the E-ABR.

Developmental neuroplasticity after cochlear implantation

Cortical development is dependent on stimulus-driven learning. The absence of sensory input from birth, as occurs in congenital deafness, affects normal growth and connectivity needed to form a functional sensory system, resulting in deficits in oral language learning. Cochlear implants bypass cochlear damage by directly stimulating the auditory nerve and brain, making it possible to avoid many of the deleterious effects of sensory deprivation. Congenitally deaf animals and children who receive implants provide a platform to examine the characteristics of cortical plasticity in the auditory system. In this review, we discuss the existence of time limits for, and mechanistic constraints on, sensitive periods for cochlear implantation and describe the effects of multimodal and cognitive reorganization that result from long-term auditory deprivation.

Inner ear protection and regeneration: a 'historical' perspective

PURPOSE OF REVIEW

In evaluating strategies to preserve or regenerate the cochlea, understanding the process of labyrinthine injury on a cellular and molecular level is crucial. Examination of inner ear injury reveals mechanism-specific types of damage, often at specific areas within the cochlea. Site-specific interventions can then be considered.

RECENT FINDINGS

The review will briefly summarize the historical perspective of advancements in hearing science through 2006. Areas of research covered include hair cell protection, hair cell regeneration, spiral ganglion cell regeneration, and stria vascularis metabolic regulation.

SUMMARY

The review will briefly summarize the early development of a few such site-specific interventions for inner ear functional rehabilitation, for work done prior to 2006. The outstanding reviews of cutting edge research from this year's and last year's Hearing Science section of Current Opinion in Otolaryngology - Head and Neck Surgery can then be understood and appreciated in a more informed manner.

Unimodal and cross-modal plasticity in the ‘deaf’ auditory cortex

Congenital auditory deprivation leads to deficits in the auditory cortex. The present review focuses on central aspects of auditory deprivation: development, plasticity, corticocortical interactions, and cross-modal reorganization. We compile imaging data from human subjects, electroencephalographic data from cochlear implanted children, and animal research on congenital deafness. Behavioral, electroencephalographic, and imaging data in humans correspond well to data behavioral and neurophysiological data obtained from congenitally deaf cats. The available data indicate that auditory deprivation leads to 'decoupling' of the primary auditory cortex from cognitive modulation of higher-order auditory areas. Higher-order auditory areas undergo a strong cross-modal reorganization and take-over new functions. Due to these and other deficits of intrinsic microcircuitry, the cortical column can not integrate bottom-up and top-down influences in deaf auditory cortex. In the ultimate consequence perceptual learning is compromised, resulting in sensitive periods.

Combined electro-acoustic stimulation: a beneficial union?

BACKGROUND

The most pressing problem facing cochlear implant research is no longer making artificial hearing a reality. Instead, it is to develop devices that can more clearly reflect the capabilities of the human auditory system. Current cochlear implants rarely provide adequate pitch perception. As hearing loss commonly affects higher, more than lower frequencies, a possible solution is to preserve acoustic hearing at low frequencies by inserting a short electrode array and thus deliver combined electro-acoustic stimulation (EAS).

OBJECTIVE OF REVIEW

To determine whether individuals with severe-to-profound high-frequency hearing loss have realised this predicted benefit of combined EAS, over conventional cochlear implants, with respect to pitch.

TYPE OF REVIEW

A systematic review of publications pertaining to the benefits of combined EAS over conventional cochlear implantation, with specific reference to pitch perception.

SEARCH STRATEGY

A systematic literature search was conducted across multiple databases and supplemented by searching the reference lists of relevant trials and identified reviews.

RESULTS

The included studies suggest an overall benefit of combined EAS, over conventional cochlear implants, with respect to pitch. In addition, (i) 13% sustained a total loss of low-frequency hearing post-implantation of the short electrode array and, (ii) 24% had >20 dB hearing loss across all frequencies and/or total hearing loss.

CONCLUSIONS

For patients with severe-to-profound high-frequency hearing loss combined EAS appears to offer a significant, everyday, long-term benefit. However, further clinical trials with larger numbers of candidates are necessary to confirm this finding. The risks involved cannot be ignored, but there is potential for a variety of strategies to improve the safety margin.

Spatial selectivity to intracochlear electrical stimulation in the inferior colliculus is degraded after long-term deafness in cats

In an animal model of electrical hearing in prelingually deaf adults, this study examined the effects of deafness duration on response thresholds and spatial selectivity (i.e., cochleotopic organization, spatial tuning and dynamic range) in the central auditory system to intracochlear electrical stimulation. Electrically evoked auditory brain stem response (EABR) thresholds and neural response thresholds in the external (ICX) and central (ICC) nuclei of the inferior colliculus were estimated in cats after varying durations of neonatally induced deafness: in animals deafened <1.5 yr (short-deafened unstimulated, SDU cats) with a mean spiral ganglion cell (SGC) density of approximately 45% of normal and in animals deafened >2.5 yr (long-deafened, LD cats) with severe cochlear pathology (mean SGC density <7% of normal). LD animals were subdivided into unstimulated cats and those that received chronic intracochlear electrical stimulation via a feline cochlear implant. Acutely deafened, implanted adult cats served as controls. Independent of their stimulation history, LD animals had significantly higher EABR and ICC thresholds than SDU and control animals. Moreover, the spread of electrical excitation was significantly broader and the dynamic range significantly reduced in LD animals. Despite the prolonged durations of deafness the fundamental cochleotopic organization was maintained in both the ICX and the ICC of LD animals. There was no difference between SDU and control cats in any of the response properties tested. These findings suggest that long-term auditory deprivation results in a significant and possibly irreversible degradation of response thresholds and spatial selectivity to intracochlear electrical stimulation in the auditory midbrain.

Cochlear implant electrode configuration effects on activation threshold and tonotopic selectivity

The multichannel design of contemporary cochlear implants (CIs) is predicated on the assumption that each channel activates a relatively restricted and independent sector of the deaf auditory nerve array, just as a sound within a restricted frequency band activates a restricted region of the normal cochlea The independence of CI channels, however, is limited; and the factors that determine their independence, the relative overlap of the activity patterns that they evoke, are poorly understood. In this study, we evaluate the spread of activity evoked by cochlear implant channels by monitoring activity at 16 sites along the tonotopic axis of the guinea pig inferior colliculus (IC). "Spatial tuning curves" (STCs) measured in this way serve as an estimate of activation spread within the cochlea and the ascending auditory pathways. We contrast natural stimulation using acoustic tones with two kinds of electrical stimulation either (1) a loose fitting banded array consisting of a cylindrical silicone elastomer carrier with a linear series of ring contacts; or (2) a space-filling array consisting of a tapered silicone elastomer carrier that is designed to fit snugly into the guinea pig scala tympani with a linear series of ball contacts positioned along it Spatial tuning curves evoked by individual acoustic tones, and by activation of each contact of each array as a monopole, bipole or tripole were recorded. Several channel configurations and a wide range of electrode separations were tested for each array, and their thresholds and selectivity were estimated. The results indicate that the tapered space-filling arrays evoked more restricted activity patterns at lower thresholds than did the banded arrays. Monopolar stimulation (one intracochlear contact activated with an extracochlear return) using either array evoked broad activation patterns that involved the entire recording array at current levels <6dBSL, but at relatively low thresholds. Bi- and tri-polar configurations of both array types evoked more restricted activity patterns, but their thresholds were higher than those of monopolar configurations. Bipolar and tripolar configurations with closely spaced contacts evoked activity patterns that were comparable to those evoked by pure tones. As the spacing of bipolar electrodes was increased (separations >1mm), the activity patterns became broader and evoked patterns with two distinct threshold minima, one associated with each contact.

Binaural interactions of electrically and acoustically evoked responses recorded from the inferior colliculus of guinea pigs

Binaural interactions within the inferior colliculus (IC) elicited by electric and acoustic stimuli were investigated in this study. Using a guinea pig model, binaural acoustic stimuli were presented with different time delays, as were combinations of binaural electric and acoustic stimuli. Averaged evoked potentials were measured using electrodes inserted into the central nucleus of the IC to obtain the binaural interaction component (BIC), computed by subtracting the sum of the two monaural responses from the binaural response. The BICs to acoustic-acoustic stimulation and electric-acoustic stimulation were found to be similar. The BIC amplitude increased with stimulus intensity, but the shapes of the delay functions were similar across the levels tested. The gross-potential data are thus consistent with the thesis that the central auditory system processes binaural electric and acoustic stimuli in a similar manner. These results suggest that the binaural auditory system can process combinations of electric and acoustic stimulation presented across ears and that evoked gross potentials may be used to measure such interaction.

Evidence of a tonotopic organization of the auditory cortex in cochlear implant users

Deprivation from normal sensory input has been shown to alter tonotopic organization of the human auditory cortex. In this context, cochlear implant subjects provide an interesting model in that profound deafness is made partially reversible by the cochlear implant. In restoring afferent activity, cochlear implantation may also reverse some of the central changes related to deafness. The purpose of the present study was to address whether the auditory cortex of cochlear implant subjects is tonotopically organized. The subjects were thirteen adults with at least 3 months of cochlear implant experience. Auditory event-related potentials were recorded in response to electrical stimulation delivered at different intracochlear electrodes. Topographic analysis of the auditory N1 component (approximately 85 ms latency) showed that the locations on the scalp and the relative amplitudes of the positive/negative extrema differ according to the stimulated electrode, suggesting that distinct sets of neural sources are activated. Dipole modeling confirmed electrode-dependent orientations of these sources in temporal areas, which can be explained by nearby, but distinct sites of activation in the auditory cortex. Although the cortical organization in cochlear implant users is similar to the tonotopy found in normal-hearing subjects, some differences exist. Nevertheless, a correlation was found between the N1 peak amplitude indexing cortical tonotopy and the values given by the subjects for a pitch scaling task. Hence, the pattern of N1 variation likely reflects how frequencies are coded in the brain.

Sensitivity to interaural time differences in the inferior colliculus with bilateral cochlear implants

Bilateral cochlear implantation attempts to increase performance over a monaural prosthesis by harnessing the binaural processing of the auditory system. Although many bilaterally implanted human subjects discriminate interaural time differences (ITDs), a major cue for sound localization and signal detection in noise, their performance is typically poorer than that of normal-hearing listeners. We developed an animal model of bilateral cochlear implantation to study neural ITD sensitivity for trains of electric current pulses delivered via bilaterally implanted intracochlear electrodes. We found that a majority of single units in the inferior colliculus of acutely deafened, anesthetized cats are sensitive to ITD and that electric ITD tuning is as sharp as found for acoustic stimulation with broadband noise in normal-hearing animals. However, the sharpness and shape of ITD tuning often depended strongly on stimulus intensity; some neurons had dynamic ranges of ITD sensitivity as low as 1 dB. We also found that neural ITD sensitivity was best at pulse rates below 100 Hz and decreased with increasing pulse rate. This rate limitation parallels behavioral ITD discrimination in bilaterally implanted individuals. The sharp neural ITD sensitivity found with electric stimulation at the appropriate intensity is encouraging for the prospect of restoring the functional benefits of binaural hearing in bilaterally implanted human subjects and suggests that neural plasticity resulting from previous deafness and deprivation of binaural experience may play a role in the poor ITD discrimination with current bilateral implants.

Auditory prosthesis with a penetrating nerve array

Contemporary auditory prostheses ("cochlear implants") employ arrays of stimulating electrodes implanted in the scala tympani of the cochlea. Such arrays have been implanted in some 100,000 profoundly or severely deaf people worldwide and arguably are the most successful of present-day neural prostheses. Nevertheless, most implant users show poor understanding of speech in noisy backgrounds, poor pitch recognition, and poor spatial hearing, even when using bilateral implants. Many of these limitations can be attributed to the remote location of stimulating electrodes relative to excitable cochlear neural elements. That is, a scala tympani electrode array lies within a bony compartment filled with electrically conductive fluid. Moreover, scala tympani arrays typically do not extend to the apical turn of the cochlea in which low frequencies are represented. In the present study, we have tested in an animal model an alternative to the conventional cochlear implant: a multielectrode array implanted directly into the auditory nerve. We monitored the specificity of stimulation of the auditory pathway by recording extracellular unit activity at 32 sites along the tonotopic axis of the inferior colliculus. The results demonstrate the activation of specific auditory nerve populations throughout essentially the entire frequency range that is represented by characteristic frequencies in the inferior colliculus. Compared to conventional scala tympani stimulation, thresholds for neural excitation are as much as 50-fold lower and interference between electrodes stimulated simultaneously is markedly reduced. The results suggest that if an intraneural stimulating array were incorporated into an auditory prosthesis system for humans, it could offer substantial improvement in hearing replacement compared to contemporary cochlear implants.

Design and fabrication of multichannel cochlear implants for animal research

The effectiveness of multichannel cochlear implants depends on the activation of perceptually distinct regions of the auditory nerve. Increased information transfer is possible as the number of channels and dynamic range are increased and electrical and neural interaction among channels is reduced. Human and animal studies have demonstrated that specific design features of the intracochlear electrode directly affect these performance factors. These features include the geometry, size, and orientation of the stimulating sites, proximity of the device to spiral ganglion neurons, shape and position of the insulating carrier, and the stimulation mode (monopolar, bipolar, etc.). Animal studies to directly measure the effects of changes in electrode design are currently constrained by the lack of available electrodes that model contemporary clinical devices. This report presents methods to design and fabricate species-specific customizable electrode arrays. We have successfully implanted these arrays in guinea pigs and cats for periods of up to 14 months and have conducted acute electrophysiological experiments in these animals. Modifications enabling long-term intracochlear drug infusion are also described. Studies using these scale model arrays will improve our understanding of how these devices function in human subjects and how we can best optimize future cochlear implants.

Neurotrophic effects of GM1 ganglioside and electrical stimulation on cochlear spiral ganglion neurons in cats deafened as neonates

Previous studies have shown that electrical stimulation of the cochlea by a cochlear implant promotes increased survival of spiral ganglion (SG) neurons in animals deafened early in life (Leake et al. [1999] J Comp Neurol 412:543-562). However, electrical stimulation only partially prevents SG degeneration after deafening and other neurotrophic agents that may be used along with an implant are of great interest. GM1 ganglioside is a glycosphingolipid that has been reported to be beneficial in treating stroke, spinal cord injuries, and Alzheimer's disease. GM1 activates trkB signaling and potentiates neurotrophins, and exogenous administration of GM1 has been shown to reduce SG degeneration after hearing loss. In the present study, animals were deafened as neonates and received daily injections of GM1, beginning either at birth or after animals were deafened and continuing until the time of cochlear implantation. GM1-treated and deafened control groups were examined at 7-8 weeks of age; additional GM1 and no-GM1 deafened control groups received a cochlear implant at 7-8 weeks of age and at least 6 months of unilateral electrical stimulation. Electrical stimulation elicited a significant trophic effect in both the GM1 group and the no-GM1 group as compared to the contralateral, nonstimulated ears. The results also demonstrated a modest initial improvement in SG density with GM1 treatment, which was maintained by and additive with the trophic effect of subsequent electrical stimulation. However, in the deafened ears contralateral to the implant SG soma size was severely reduced several months after withdrawal of GM1 in the absence of electrical activation.

Cochlear implants: cortical plasticity in congenital deprivation

Congenital auditory deprivation (deafness) leads to a dysfunctional intrinsic cortical microcircuitry. This chapter reviews these deficits with a particular emphasis on layer-specific activity within the primary auditory cortex. Evidence for a delay in activation of supragranular layers and reduction in activity in infragranular layers is discussed. Such deficits indicate the incompetence of the primary auditory cortex to not only properly process thalamic input and generate output within the infragranular layers, but also incorporate top-down modulations from higher order auditory cortex into the processing within primary auditory cortex. Such deficits are the consequence of a misguided postnatal development. Maturation of primary auditory cortex in deaf animals shows evidence of a developmental delay and further alterations in gross synaptic currents, spread of activation, and morphology of local field potentials recorded at the cortical surface. Additionally, degenerative changes can be observed. When hearing is initiated early in life (e.g., by chronic cochlear-implant stimulation), many of these deficits are counterbalanced. However, plasticity of the auditory cortex decreases with increasing age, so that a sensitive period for plastic adaptation can be demonstrated within the second to sixth months of life in the deaf cat. Potential molecular mechanisms of the existence of sensitive period are discussed. Data from animal research may be compared to electroencephalographic data obtained from cochlear-implanted congenitally deaf children. After cochlear implantation in humans, three phases of plastic adaptation can be observed: a fast one, taking place within the first few weeks after implantation, showing no sensitive period; a slower one, taking place within the first months after implantation (a sensitive period up to 4 years of age); and possibly a third, and the longest one, related to increasing activation of higher order cortical areas.

Cochlear implants: the view from the brain

The cochlear implant arguably is the most successful neural prosthesis. Studies of the responses of the central auditory system to prosthetic electrical stimulation of the cochlea are revealing the success with which electrical stimulation of a deaf ear can mimic acoustic stimulation of a normal-hearing ear. Understanding of the physiology of central auditory structures can lead to improved restoration of hearing with cochlear implants. In turn, the cochlear implant can be exploited as an experimental tool for examining central hearing mechanisms isolated from the effects of cochlear mechanics and transduction.

Degradation of temporal resolution in the auditory midbrain after prolonged deafness is reversed by electrical stimulation of the cochlea

In an animal model of prelingual deafness, we examined the anatomical and physiological effects of prolonged deafness and chronic electrical stimulation on temporal resolution in the adult central auditory system. Maximum following frequencies (Fmax) and first spike latencies of single neurons responding to electrical pulse trains were evaluated in the inferior colliculus of two groups of neonatally deafened cats after prolonged periods of deafness (>2.5 yr): the first group was implanted with an intracochlear electrode and studied acutely (long-deafened unstimulated, LDU); the second group (LDS) received a chronic implant and several weeks of electrical stimulation (pulse rates > or =300 pps). Acutely deafened and implanted adult cats served as controls. Spiral ganglion cell density in all long-deafened animals was markedly reduced (mean <5.8% of normal). Both long-term deafness and chronic electrical stimulation altered temporal resolution of neurons in the central nucleus (ICC) but not in the external nucleus. Specifically, LDU animals exhibited significantly poorer temporal resolution of ICC neurons (lower Fmax, longer response latencies) as compared with control animals. In contrast, chronic stimulation in LDS animals led to a significant increase in temporal resolution. Changes in temporal resolution after long-term deafness and chronic stimulation occurred broadly across the entire ICC and were not correlated with its tonotopic gradient. These results indicate that chronic electrical stimulation can reverse the degradation in temporal resolution in the auditory midbrain after long-term deafness and suggest the importance of factors other than peripheral pathology on plastic changes in the temporal processing capabilities of the central auditory system.

Topographic spread of inferior colliculus activation in response to acoustic and intracochlear electric stimulation

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 single-shank, 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.

Influence of auditory deprivation upon the tonopic organization in the inferior colliculus: a Fos immunocytochemical study in the rat

The frequency organization in the inferior colliculus of neonatally-deafened rats was investigated using electrical stimulation of the cochlea and immunoreactivity for Fos as a marker of neuronal activity. An electrode implanted either at the base or at the apex of the right cochlea delivered a unique 45-min stimulation at two different level intensities and at two time points, i.e. either at 4 weeks or at 4 months. In 4-week-old rats stimulated at 5x threshold, a site-for-site organization was observed since basal or apical stimulation induced a strong labelling in the ventro-medial or in the dorsolateral part of the left inferior colliculus, respectively. In 4-month-old rats, stimulation of the base induced an extremely weak Fos labelling without any specific location in the left inferior colliculus while stimulation of the apex induced a diffuse labelling with two discrete bands being distinguishable in the left inferior colliculus. In 4-week-old rats stimulated at 15x threshold, basal stimulation elicited a diffuse Fos-like immunoreactivity in the left inferior colliculus while apical stimulation yielded a response restricted to the dorsal part of the left inferior colliculus. In 4-month-old rats, no response was detected in the left inferior colliculus after stimulation of the basal part of the cochlea. Stimulation of the apex could still induce a labelling in the dorsolateral left inferior colliculus. Thus, the inferior colliculus exhibits an adult-like tonotopic organization early on independently of any acoustic stimulation. Prolonged absence of auditory input dramatically alters this organization in the inferior colliculus, especially for high frequencies. From a clinical standpoint, these results could argue for early implantation in deaf children.

Plastic changes in the central auditory system after hearing loss, restoration of function, and during learning

Traditionally the auditory system was considered a hard-wired sensory system; this view has been challenged in recent years in light of the plasticity of other sensory systems, particularly the visual and somatosensory systems. Practical experience in clinical audiology together with the use of prosthetic devices, such as cochlear implants, contributed significantly to the present view on the plasticity of the central auditory system, which was originally based on data obtained in animal experiments. The loss of auditory receptors, the hair cells, results in profound changes in the structure and function of the central auditory system, typically demonstrated by a reorganization of the projection maps in the auditory cortex. These plastic changes occur not only as a consequence of mechanical lesions of the cochlea or biochemical lesions of the hair cells by ototoxic drugs, but also as a consequence of the loss of hair cells in connection with aging or noise exposure. In light of the aging world population and the increasing amount of noise in the modern world, understanding the plasticity of the central auditory system has its practical consequences and urgency. In most of these situations, a common denominator of central plastic changes is a deterioration of inhibition in the subcortical auditory nuclei and the auditory cortex. In addition to the processes that are elicited by decreased or lost receptor function, the function of nerve cells in the adult central auditory system may dynamically change in the process of learning. A better understanding of the plastic changes in the central auditory system after sensory deafferentation, sensory stimulation, and learning may contribute significantly to improvement in the rehabilitation of damaged or lost auditory function and consequently to improved speech processing and production.

The effects of chronic intracochlear electrical stimulation on inferior colliculus spatial representation in adult deafened cats

Previous studies have shown that chronic electrical stimulation through a cochlear implant causes significant alterations in the central auditory system of neonatally deafened cats. The goal of this study was to investigate the effects of chronic stimulation in the mature auditory system. Normal hearing adult animals were deafened by ototoxic drugs and received daily electrical stimulation over periods of 4-6 months. In terminal physiology experiments, response thresholds to pulsatile and sinusoidal signals were recorded within the inferior colliculus (IC). Using previously established methods, spatial tuning curves (STCs; threshold vs. IC depth functions) were constructed, and their widths measured to infer spatial selectivity. The IC spatial representations were similar for pulsatile and sinusoidal stimulation when phase duration was taken into consideration. However, sinusoidal signals consistently elicited much lower thresholds than pulsatile signals, a difference not solely attributable to differences in charge/phase. The average STC width was significantly broader in the adult deafened/stimulated animals than in controls (adult deafened/unstimulated cats), suggesting that electrical stimulation can induce spatial expansion of the IC representation of the chronically stimulated cochlear sector. Further, results in these adult animals were not significantly different from results in neonatally deafened, early stimulated animals, suggesting that a similar degree of plasticity was induced within the auditory midbrains of mature animals.

Inner ear lesion alters acoustically induced c-Fos expression in the rat auditory rhomboencephalic brainstem

The pattern of c-Fos expression was mapped in the adult rat's brain following unilateral cochlear lesions. In normal and cochlear lesioned rats, c-Fos expression was induced with sound stimuli. Acoustic stimulation consisted of pulses of four tones. An additional control group consisted of non-stimulated rats. In the cochlear nuclei (CN), c-Fos activation was scarce in isolated rats and increased strongly following sound stimulation. Following unilateral cochlear lesion, acoustically driven expression was decreased in all CN in both the lesioned and the untreated sides. The ventromedial periolivary nucleus and the rostral periolivary nucleus showed c-Fos activation in isolated conditions and were strongly activated following sound stimulation. The rest of the superior olivary complex showed no c-Fos activation in isolated rats and a weak activation following sound stimulation. Following unilateral cochlear lesions, acoustically driven expression was decreased in some, but not all superior olivary nuclei in both the lesioned and the untreated sides. In the lateral lemniscus complex, c-Fos activation was scarce in isolated rats and increased strongly after stimulation. Following unilateral cochlear lesion, acoustically driven expression decreased bilaterally in all nuclei. We have found that unilateral inner ear lesions lead to bilateral impairment of the capability of acoustic pathway neurons, to being c-Fos-activated following sound stimulation.

Deafness-induced changes in the auditory pathway: implications for cochlear implants

A profound sensorineural hearing loss induces significant pathological and atrophic changes within the cochlea and central auditory pathway. We describe these deafness-induced morphological and functional changes following controlled lesions of the cochlea in experimental animals. Such changes are generally consistent with the limited number of reports describing deafness-induced changes observed in human material. The implications of these pathophysiological changes within the auditory pathway on cochlear implant function are discussed. Finally, the plastic response of the deafened auditory system to electrical stimulation of the auditory nerve is reviewed in light of the clinical implications for cochlear implant recipients.

Auditory detection and discrimination in deaf cats: psychophysical and neural thresholds for intracochlear electrical signals

More than 30,000 hearing-impaired human subjects have learned to use cochlear implants for speech perception and speech discrimination. To understand the basic mechanisms underlying the successful application of contemporary speech processing strategies, it is important to investigate how complex electrical stimuli delivered to the cochlea are processed and represented in the central auditory system. A deaf animal model has been developed that allows direct comparison of psychophysical thresholds with central auditory neuronal thresholds to temporally modulated intracochlear electrical signals in the same animals. Behavioral detection thresholds were estimated in neonatally deafened cats for unmodulated pulse trains (e.g., 30 pulses/s or pps) and sinusoidal amplitude-modulated (SAM) pulse trains (e.g., 300 pps, SAM at 30 Hz; 300/30 AM). Animals were trained subsequently in a discrimination task to respond to changes in the modulation frequency of successive SAM signals (e.g., 300/8 AM vs. 300/30 AM). During acute physiological experiments, neural thresholds to pulse trains were estimated in the inferior colliculus (IC) and the primary auditory cortex (A1) of the anesthetized animals. Psychophysical detection thresholds for unmodulated and SAM pulse trains were virtually identical. Single IC neuron thresholds for SAM pulse trains showed a small but significant increase in threshold (0.4 dB or 15.5 microA) when compared with thresholds for unmodulated pulse trains. The mean difference between psychophysical and minimum neural thresholds within animals was not significant (mean = 0.3 dB). Importantly, cats also successfully discriminated changes in the modulation frequencies of the SAM signals. Performance on the discrimination task was not affected by carrier rate (100, 300, 500, 1,000, or 1,500 pps). These findings indicate that 1) behavioral and neural response thresholds are based on detection of the peak pulse amplitudes of the modulated and unmodulated signals, and 2) discrimination of successive SAM pulse trains is based on temporal resolution of the envelope frequencies. Overall, our animal model provides a robust framework for future studies of behavioral discrimination and central neural temporal processing of electrical signals applied to the deaf cochlea by a cochlear implant.

Cochlear electrical stimulation: influence of age of implantation on Fos immunocytochemical reactions in inferior colliculi and dorsal cochlear nuclei of the rat

The influence of age at the time of implantation of a stimulating electrode unilaterally in the inner ear on central auditory pathways was investigated in rats deafened shortly after birth. Immunoreactivity for Fos served as a functional marker of neuronal activity. Electrodes were implanted in the left cochlea of rats aged 3 weeks or 4 months. Stimulation lasted 45 minutes, then rats were sacrificed and tissues processed for immunocytochemistry. The younger animals showed significantly more neurons with Fos immunoreactivity bilaterally in the dorsal cochlear nuclei (DCN) and inferior colliculi (IC) than the older rats or control animals with normal hearing receiving the same stimulation. Activity was more prominent in the left DCN and right IC. The results show that electrical stimulation of the inner ear is more effective in younger animals in eliciting gene expression associated with development of a functional network in the auditory pathways. This suggests that deaf children should be provided with cochlear implants as early as possible.

The effect of electrode configuration and duration of deafness on threshold and selectivity of responses to intracochlear electrical stimulation

This report examines the effects of intracochlear electrode configuration and mode of stimulation (bipolar or monopolar) on neural threshold and spatial selectivity in the inferior colliculus (IC) of the cat. Single and multiunit IC recordings were made in three groups of animals; acutely deafened adults (controls), neonatally deafened animals studied at 6 to 18 months of age and neonatally deafened cats studied at 2.5 to 6.5 years. Response thresholds were plotted versus IC depth to measure the spatial distribution of responses. The response selectivity for each stimulating configuration was defined as the width of the resulting spatial tuning curve (STC) measured at 6 dB above threshold. Spiral ganglion cell (SG) survival was examined histologically in all neonatally deafened animals and correlated with physiological results. Animals studied at less than 1.5 years had SG densities of 23.5%-64.4% of normal (mean=42.7%) while animals studied at greater than 2.5 years had densities of 5.1%-18.3% of normal (mean=9.9%). Electrophysiological results include the following. (1) Monopolar thresholds were 7-8 dB lower than bipolar thresholds in the same animals. (2) Varying the configuration of bipolar contacts (measured as radial, offset radial and longitudinal pairs) did not systematically affect IC threshold in either controls or short-term neonatally deafened animals. In contrast, the long-term neonatally deafened animals showed a difference in threshold with each configuration. (3) The spatial distributions (Q(6 dB)) of responses to bipolar stimulation were approximately 40% more restricted than those for monopolar stimulation. (4) The spatial selectivity of neonatally deafened animals studied at ages up to 1.5 years was equal to that of control animals with normal auditory experience. However, selectivity was degraded in the older animals. (5) Selectivity was decreased in some animals with the longitudinal bipolar configuration and multiple response peaks were seen in several cases using this stimulus configuration.

Plasticity of bat's central auditory system evoked by focal electric stimulation of auditory and/or somatosensory cortices

Recent findings indicate that the corticofugal system would play an important role in cortical plasticity as well as collicular plasticity. To understand the role of the corticofugal system in plasticity, therefore, we studied the amount and the time course of plasticity in the inferior colliculus (IC) and auditory cortex (AC) evoked by focal electrical stimulation of the AC and also the effect of electrical stimulation of the somatosensory cortex on the plasticity evoked by the stimulation of the AC. In adult big brown bats (Eptesicus fuscus), we made the following major findings. 1) Electric stimulation of the AC evokes best frequency (BF) shifts, i.e., shifts in frequency-response curves of collicular and cortical neurons. These BF shifts start to occur within 2 min, reach a maximum (or plateau) at 30 min, and then recover approximately 180 min after a 30-min-long stimulus session. When the stimulus session is lengthened from 30 to 90 min, the plateau lasts approximately 60 min, but BF shifts recover approximately 180 min after the session. 2) The electric stimulation of the somatosensory cortex delivered immediately after that of the AC, as in fear conditioning, evokes a dramatic lengthening of the recovery period of the cortical BF shifts but not that of the collicular BF shift. The electric stimulation of the somatosensory cortex delivered before that of the AC, as in backward conditioning, has no effect on the collicular and cortical BF shifts. 3) Electric stimulation of the AC evokes BF shifts not only in the ipsilateral IC and AC but also in the contralateral IC and AC. BF shifts are smaller in amount and shorter in recovery time for contralateral collicular and cortical neurons than for ipsilateral ones. Our findings support the hypothesis that the AC and the corticofugal system have an intrinsic mechanism for reorganization of the IC and AC, that the reorganization is highly specific to a value of an acoustic parameter (frequency), and that the reorganization is augmented by excitation of nonauditory sensory cortex that makes the acoustic stimulus behaviorally relevant to the animal through associative learning.

The influence of age at implantation on performance with a cochlear implant in children

OBJECTIVE

This study involved the assessment of speech recognition abilities as a function of age at implantation and length of cochlear implant use in children who received the Nucleus CI22M cochlear implant.

STUDY DESIGN

Two separate analyses were performed. The first analysis involved the assessment of speech recognition performance as a function of length of time with a cochlear implant in 48 patients evaluated at 7 years of age. The second analysis involved the assessment of speech recognition performance as a function of age at implantation in 53 patients evaluated 36 months after implantation. Patients were divided into four groups based on length of implant use or age at implantation, and the results were analyzed by a repeated-measures analysis of variance.

SETTING

This study was carried out at a tertiary academic medical center.

PATIENTS

Patients consisted of children implanted with a Nucleus Multi Channel cochlear implant programmed with the SPEAK encoding strategy. Their ages at the time of evaluation ranged from 5.5 to 7.8 years. Their ages at implantation ranged from 2.4 to 14.5 years.

INTERVENTIONS

All patients received a Nucleus Multi Channel cochlear implant programmed with the SPEAK encoding strategy. Word and sentence recognition tests were administered at various ages and at several postimplantation intervals.

MAIN OUTCOME MEASURES

Performance as a function of length of cochlear implant use and as a function of age at implantation.

RESULTS

Patients performed significantly better as length of cochlear implant use increased and age at implantation decreased. When patients were tested at a fixed postimplantation time interval (36 months), there was an overall trend for patients who received the implant at a younger age to perform better in spite of being younger at the time of evaluation. However, these effects were not statistically significant for all speech recognition tests that were administered.

CONCLUSIONS

These results confirm previous findings indicating continued improvement of speech recognition with time in implanted children. Furthermore, the results support the concept of the advantage of a younger age at implantation.

Effects of chronic stimulation on auditory nerve survival in ototoxically deafened animals

For almost 10 years, chronic stimulation has been known to affect spiral ganglion cell (SGC) survival in the deaf ear. However, the reported effects of chronic stimulation vary across preparations and studies. In this review, the effects of chronic stimulation on the deafened auditory periphery are examined, and variables that may impact on the efficacy of chronic stimulation are identified. The effects of deafening on the unstimulated peripheral and central auditory system are also described, as the deafened, unstimulated system is the canvas upon which stimulation-mediated effects are imposed. Discrepancies in the effects of chronic stimulation across studies may be attributable in large part to the combined effects of the deafening method and the post-deafening delay prior to chronic stimulation, which vary across studies. Emphasis is placed on the need to consider the natural progression of SGC loss following deafening in the absence of chronic stimulation, as the rate of SGC loss almost certainly affects both the efficacy of stimulation, and the impact of any delay between deafening and initiation of stimulation. The differences across preparations complicate direct comparison of protective efficacy of stimulation. At the same time, these differences can be used to our advantage, aiding characterization of the effects of different factors on the efficacy of chronic stimulation as a neuroprotective intervention.

Effects of deafferentation on the electrophysiology of ventral cochlear nucleus neurons

When cochlear pathology impairs the afferent innervation of the ventral cochlear nucleus (VCN), electrical responses of the auditory brainstem are altered and changes in cell and synaptic morphology are observed. However, the impact of deafferentation on the electrical properties of cells in the VCN is unknown. We examined the electrical properties of single neurons in the anterior and posterior VCN following bilateral cochlear removal in young rats. In control animals, two populations of cells were distinguished: those with a linear subthreshold current-voltage relationship and repetitive firing of action potentials with regular interspike intervals (type I), and those with rectifying subthreshold current-voltage relationships and phasic firing of 1-3 action potentials (type II). Measures of action potential shape further distinguished these two groups. Two weeks following cochlear removal, both electrical response patterns were still seen. Type I cells showed a higher input resistance. Deafferented single-spiking type II cells were slightly more depolarized, had smaller action potentials, smaller afterhyperpolarizations and shorter membrane time constants, whereas multiple-spiking type II cells were apparently unaffected. These changes in the electrical properties of VCN neurons following cochlear injury may adversely affect central processing of sounds presented acoustically or electrically by prostheses.

Plasticity in central representations in the inferior colliculus induced by chronic single- vs. two-channel electrical stimulation by a cochlear implant after neonatal deafness

The goal of this research is to examine the functional consequences of patterned electrical stimulation delivered by a cochlear implant in the deafened developing auditory system. In previous electrophysiological experiments conducted in the inferior colliculus (IC), we have demonstrated that the precise cochleotopic organization of the central nucleus (ICC) develops normally in neonatally deafened unstimulated cats and is unaltered despite the lack of normal auditory input during development. However, these studies also showed that chronic electrical stimulation delivered at a single intracochlear location by one bipolar channel of a cochlear implant induces significant expansion of the central representation of the stimulated cochlear sector and degrades the cochleotopic organization of the IC. This report presents additional data from a new experimental series of neonatally deafened cats that received chronic stimulation on two adjacent bipolar intracochlear channels of a cochlear implant. Results suggest that competing inputs elicited by electrical stimulation delivered by two adjacent channels can maintain the selective representations of each activated cochlear sector within the central auditory system and prevent the expansion seen after single-channel stimulation. Alternating stimulation of two channels and use of highly controlled electrical signals may be particularly effective in maintaining or even sharpening selectivity of central representations of stimulated cochlear sectors. In contrast, simultaneous stimulation using two channels of a model analog cochlear implant processor in one experimental animal failed to maintain channel selectivity and resulted in marked expansion and fusion of the central representations of the stimulated channels. This potentially important preliminary result suggests that under some conditions the central auditory system may be unable to discriminate simultaneous, overlapping inputs from adjacent cochlear implant channels as distinct.

Congenital auditory deprivation reduces synaptic activity within the auditory cortex in a layer-specific manner

The present study investigates the functional deficits of naive auditory cortices in adult congenitally deaf cats. For this purpose, their auditory system was stimulated electrically using cochlear implants. Synaptic currents in cortical layers were revealed using current source density analyses. They were compared with synaptic currents found in electrically stimulated hearing cats. The naive auditory cortex showed significant deficits in synaptic activity in infragranular cortical layers. Furthermore, there was also a deficit of synaptic activities at longer latencies (>30 ms). The 'cortical column' was not activated in the well-defined sequence found in normal hearing cats. These results demonstrate functional deficits as a consequence of congenital auditory deprivation. Similar deficits are likely in congenitally deaf children.

Modulation of responses and frequency tuning of thalamic and collicular neurons by cortical activation in mustached bats

In the Jamaican mustached bat, Pteronotus parnellii parnellii, one of the subdivisions of the primary auditory cortex is disproportionately large and over-represents sound at approximately 61 kHz. This area, called the Doppler-shifted constant frequency (DSCF) processing area, consists of neurons extremely sharply tuned to a sound at approximately 61 kHz. We found that a focal activation of the DSCF area evokes highly specific corticofugal modulation in the inferior colliculus and medial geniculate body. Namely a focal activation of cortical DSCF neurons tuned to, say, 61. 2 kHz with 0.2-ms-long, 100-nA electric pulses drastically increases the excitatory responses of thalamic and collicular neurons tuned to 61.2 kHz without shifting their best frequencies (BFs). However, it decreases the excitatory responses of subcortical neurons tuned to frequencies slightly higher or lower than 61.2 kHz and shifts their BFs away from 61.2 kHz. The BF shifts are symmetrical and centrifugal around 61.2 kHz. These corticofugal effects are larger on thalamic neurons than on collicular neurons. The cortical electrical stimulation sharpens the frequency-tuning curves of subcortical neurons. These corticofugal effects named "egocentric selection" last </=2.5 h after the cessation of a 7-min-long cortical electrical stimulation. In the mustached bat, corticofugal modulation serves to increase the contrast in neural representation of sound at approximately 61 kHz, which is an important component of an echo bearing velocity information. It is also most likely that the corticofugal system plays an important role in plasticity of the central auditory system. Another subdivision of the auditory cortex of the mustached bat is called the FM-FM area. This area consists of delay-tuned combination-sensitive neurons, called FM-FM neurons, and has the echo-delay axis for the systematic representation of target distances. A focal electric stimulation of the FM-FM area evokes changes in the responses of collicular and thalamic FM-FM neurons. These changes are basically the same as those described in the present paper. Therefore corticofugal modulation takes place for frequency domain analysis in exactly the same way as it does in time domain analysis.

Maturation of the mismatch negativity: effects of profound deafness and cochlear implant use

The use of cochlear implants to restore auditory sensation in deaf children is increasing, with a trend toward earlier implantation. However, little is known about how auditory deprivation and subsequent cochlear implant use affect the maturing human central auditory system. Our previous studies have demonstrated that the obligatory auditory evoked potentials (AEPs) of implanted children are very different from those of normal-hearing children. Unlike the obligatory potentials, which primarily reflect neural responses to stimulus onset, the mismatch negativity (MMN) provides a neurophysiological measure of auditory short-term memory and discrimination processes. The purpose of this investigation is to review our studies of the effects of auditory deprivation due to profound deafness and cochlear implant use on the maturation of the MMN in children, placed in the context of overall age-related changes in the AEPs. The development and application of a statistical technique to assess the MMN in individuals is also reviewed. Results show that although the morphology of the obligatory AEPs is substantially altered by the absence of a normal N(1) peak, the MMN is robustly present in a group of implanted children who have good spoken language perception through their device. Differences exist in the scalp distribution of the MMN between implanted and normal-hearing children. Specifically, the MMN appears to be more symmetrical in amplitude over both hemispheres, whereas it is initially much larger over the contralateral hemisphere in normal-hearing children. These findings suggest that, compared to N(1), the MMN is a better measure of basic auditory processes necessary for the development of spoken language perception skills in profoundly deaf children and adults who use a cochlear implant.

Reorganization of the frequency map of the auditory cortex evoked by cortical electrical stimulation in the big brown bat

In a search phase of echolocation, big brown bats, Eptesicus fuscus, emit biosonar pulses at a rate of 10/s and listen to echoes. When a short acoustic stimulus was repetitively delivered at this rate, the reorganization of the frequency map of the primary auditory cortex took place at and around the neurons tuned to the frequency of the acoustic stimulus. Such reorganization became larger when the acoustic stimulus was paired with electrical stimulation of the cortical neurons tuned to the frequency of the acoustic stimulus. This reorganization was mainly due to the decrease in the best frequencies of the neurons that had best frequencies slightly higher than those of the electrically stimulated cortical neurons or the frequency of the acoustic stimulus. Neurons with best frequencies slightly lower than those of the acoustically and/or electrically stimulated neurons slightly increased their best frequencies. These changes resulted in the over-representation of repetitively delivered acoustic stimulus. Because the over-representation resulted in under-representation of other frequencies, the changes increased the contrast of the neural representation of the acoustic stimulus. Best frequency shifts for over-representation were associated with sharpening of frequency-tuning curves of 25% of the neurons studied. Because of the increases in both the contrast of neural representation and the sharpness of tuning, the over-representation of the acoustic stimulus is accompanied with an improvement of analysis of the acoustic stimulus.

Electrical cochlear stimulation in the deaf cat: comparisons between psychophysical and central auditory neuronal thresholds

Cochlear prostheses for electrical stimulation of the auditory nerve ("electrical hearing") can provide auditory capacity for profoundly deaf adults and children, including in many cases a restored ability to perceive speech without visual cues. A fundamental challenge in auditory neuroscience is to understand the neural and perceptual mechanisms that make rehabilitation of hearing possible in these deaf humans. We have developed a feline behavioral model that allows us to study behavioral and physiological variables in the same deaf animals. Cats deafened by injection of ototoxic antibiotics were implanted with either a monopolar round window electrode or a multichannel scala tympani electrode array. To evaluate the effects of perceptually significant electrical stimulation of the auditory nerve on the central auditory system, an animal was trained to avoid a mild electrocutaneous shock when biphasic current pulses (0.2 ms/phase) were delivered to its implanted cochlea. Psychophysical detection thresholds and electrical auditory brain stem response (EABR) thresholds were estimated in each cat. At the conclusion of behavioral testing, acute physiological experiments were conducted, and threshold responses were recorded for single neurons and multineuronal clusters in the central nucleus of the inferior colliculus (ICC) and the primary auditory cortex (A1). Behavioral and neurophysiological thresholds were evaluated with reference to cochlear histopathology in the same deaf cats. The results of the present study include: 1) in the cats implanted with a scala tympani electrode array, the lowest ICC and A1 neural thresholds were virtually identical to the behavioral thresholds for intracochlear bipolar stimulation; 2) behavioral thresholds were lower than ICC and A1 neural thresholds in each of the cats implanted with a monopolar round window electrode; 3) EABR thresholds were higher than behavioral thresholds in all of the cats (mean difference = 6.5 dB); and 4) the cumulative number of action potentials for a sample of ICC neurons increased monotonically as a function of the amplitude and the number of stimulating biphasic pulses. This physiological result suggests that the output from the ICC may be integrated spatially across neurons and temporally integrated across pulses when the auditory nerve array is stimulated with a train of biphasic current pulses. Because behavioral thresholds were lower and reaction times were faster at a pulse rate of 30 pps compared with a pulse rate of 2 pps, spatial-temporal integration in the central auditory system was presumably reflected in psychophysical performance.

Behavioral and neurophysiological thresholds for electrical cochlear stimulation in the deaf cat

Psychophysical detection thresholds for unmodulated electrical pulse trains or for sinusoidally amplitude-modulated (SAM) pulse trains were estimated in deaf juvenile cats using a conditioned avoidance paradigm. Biphasic current pulses (0.2 ms/phase) were delivered by scala tympani electrodes consisting of 4-8 electrode contacts driven as bipolar pairs. Electrical auditory brainstem response (EABR) thresholds were obtained periodically, and at the conclusion of behavioral training, response thresholds were obtained for neurons in the inferior colliculus (IC) and the primary auditory cortex (A1) in acute physiological experiments in the same animals. The results of the study include: (1) detection thresholds for unmodulated pulse trains and for SAM pulse trains were virtually identical; (2) EABR thresholds and behavioral thresholds were significantly correlated, although EABR thresholds consistently overestimated behavioral thresholds; (3) the lowest thresholds in the IC and the A1 were significantly correlated with behavioral thresholds, and (4) mean lowest thresholds in the IC and the A1 were essentially the same as the mean psychophysical detection threshold in the trained deaf cats.

Temporal properties of chronic cochlear electrical stimulation determine temporal resolution of neurons in cat inferior colliculus

As cochlear implants have become increasingly successful in the rehabilitation of adults with profound hearing impairment, the number of pediatric implant subjects has increased. We have developed an animal model of congenital deafness and investigated the effect of electrical stimulus frequency on the temporal resolution of central neurons in the developing auditory system of deaf cats. Maximum following frequencies (Fmax) and response latencies of isolated single neurons to intracochlear electrical pulse trains (charge balanced, constant current biphasic pulses) were recorded in the contralateral inferior colliculus (IC) of two groups of neonatally deafened, barbiturate-anesthetized cats: animals chronically stimulated with low-frequency signals (< or = 80 Hz) and animals receiving chronic high-frequency stimulation (> or = 300 pps). The results were compared with data from unstimulated, acutely deafened and implanted adult cats with previously normal hearing (controls). Characteristic differences were seen between the temporal response properties of neurons in the external nucleus (ICX; approximately 16% of the recordings) and neurons in the central nucleus (ICC; approximately 81% of all recordings) of the IC: 1) in all three experimental groups, neurons in the ICX had significantly lower Fmax and longer response latencies than those in the ICC. 2) Chronic electrical stimulation in neonatally deafened cats altered the temporal resolution of neurons exclusively in the ICC but not in the ICX. The magnitude of this effect was dependent on the frequency of the chronic stimulation. Specifically, low-frequency signals (30 pps, 80 pps) maintained the temporal resolution of ICC neurons, whereas higher-frequency stimuli significantly improved temporal resolution of ICC neurons (i.e., higher Fmax and shorter response latencies) compared with neurons in control cats. Furthermore, Fmax and latencies to electrical stimuli were not correlated with the tonotopic gradient of the ICC, and changes in temporal resolution following chronic electrical stimulation occurred uniformly throughout the entire ICC. In all three experimental groups, increasing Fmax was correlated with shorter response latencies. The results indicate that the temporal features of the chronically applied electrical signals critically influence temporal processing of neurons in the cochleotopically organized ICC. We suggest that such plastic changes in temporal processing of central auditory neurons may contribute to the intersubject variability and gradual improvements in speech recognition performance observed in clinical studies of deaf children using cochlear implants.