Neonatal cochlear hearing loss results in developmental abnormalities of the central auditory pathways

Area-dependent change of response in the rat's inferior colliculus to intracochlear electrical stimulation following neonatal cochlear damage

To understand brain changes caused by auditory sensory deprivation, we recorded local-field potentials in the inferior colliculus of young adult rats with neonatal cochlear damage produced by systemic injections of amikacin. The responses were elicited by electrical stimulation of the entire cochlea and recorded at various locations along a dorsolateral-ventromedial axis of the inferior colliculus. We found that hair cells were completely destroyed and spiral ganglion neurons were severely damaged in the basal cochleae of amikacin-treated animals. Hair cells as well as spiral ganglion neurons were damaged also in the middle and apical areas of the cochlea, with the damage being greater in the middle than the apical area. Amplitudes of local-field potentials were reduced in the ventromedial inferior colliculus, but enhanced in the dorsolateral inferior colliculus. Latencies of responses were increased over the entire structure. The enhancement of responses in the dorsolateral inferior colliculus was in contrast with the damage of hair cells and spiral ganglion cells in the apical part of the cochlea. This contrast along with the overall increase of latencies suggests that early cochlear damage can alter neural mechanisms within the inferior colliculus and/or the inputs to this midbrain structure.

Fetal Central Auditory System Metabolic Response to Cochlear Implant Stimulation

OBJECTIVES: The purpose of this study was to examine the effects of profound auditory deprivation and its treatment by cochlear implantation and stimulation on the metabolic activity of the central auditory system in fetal sheep.

METHODS: Six ovine fetuses at 85% to 90% gestation were bilaterally deafened by kanamycin perfusion and unilaterally implanted with cochlear electrode arrays. Half of the implanted animals were stimulated with an extrauterine sound processor, and half were not. Four animals served as hearing controls. One week postoperatively, central nervous system metabolic activity was evaluated in ambient laboratory noise by quantitative autoradiography using 14C-deoxyglucose.

RESULTS: Kanamycin perfusion deafened all treated animals as verified by auditory brainstem response and scanning electron microscopy. Glucose utilization in the inferior colliculus was markedly lower in deafened and unstimulated animals relative to hearing controls. Glucose utilization in implanted-stimulated animals was similar to normal controls.

CONCLUSIONS: Changes in central auditory system metabolic activity associated with congenital deafness may be minimized by prompt auditory habilitation.

Evidence for Competition for Target Innervation in the Medial Prefrontal Cortex

Inputs to sensory cortices are known to compete for target innervation through an activity-dependent mechanism during critical periods. To investigate whether this principle also applies to association cortices such as the medial prefrontal cortex (mPFC), we produced a bilateral lesion during early development to the ventral hippocampus (vHC), an input to the mPFC, and analyzed the intensity of the projection from another input, the basolateral amgydala (BLA). We found that axons from the BLA had a higher density of "en passant" boutons in the mPFC of lesioned animals. Furthermore, the density of neurons labeled with retrograde tracers was increased, and neurons projecting from the BLA to the mPFC showed increased expression of FosB. Since neonatal ventral hippocampal lesion has been used as an animal model of schizophrenia, we investigated its effects on behavior and found a negative correlation between the density of retrogradely labeled neurons in the BLA and the reduction of the startle response in the prepulse inhibition test. Our results not only indicate that the inputs from the BLA and the vHC compete for target innervation in the mPFC during postnatal development but also that subsequent abnormal rewiring might underlie the pathophysiology of neuropsychiatric disorders such as schizophrenia.

Developmental hearing loss impairs signal detection in noise: putative central mechanisms

Listeners with hearing loss have difficulty processing sounds in noisy environments. This is most noticeable for speech perception, but is reflected in a basic auditory processing task: detecting a tonal signal in a noise background, i.e., simultaneous masking. It is unresolved whether the mechanisms underlying simultaneous masking arise from the auditory periphery or from the central auditory system. Poor detection in listeners with sensorineural hearing loss (SNHL) is attributed to cochlear hair cell damage. However, hearing loss alters neural processing in the central auditory system. Additionally, both psychophysical and neurophysiological data from normally hearing and impaired listeners suggest that there are additional contributions to simultaneous masking that arise centrally. With SNHL, it is difficult to separate peripheral from central contributions to signal detection deficits. We have thus excluded peripheral contributions by using an animal model of early conductive hearing loss (CHL) that provides auditory deprivation but does not induce cochlear damage. When tested as adults, animals raised with CHL had increased thresholds for detecting tones in simultaneous noise. Furthermore, intracellular in vivo recordings in control animals revealed a cortical correlate of simultaneous masking: local cortical processing reduced tone-evoked responses in the presence of noise. This raises the possibility that altered cortical responses which occur with early CHL can influence even simple signal detection in noise.

Cross-modal plasticity results in increased inhibition in primary auditory cortical areas

Loss of sensory input from peripheral organ damage, sensory deprivation, or brain damage can result in adaptive or maladaptive changes in sensory cortex. In previous research, we found that auditory cortical tuning and tonotopy were impaired by cross-modal invasion of visual inputs. Sensory deprivation is typically associated with a loss of inhibition. To determine whether inhibitory plasticity is responsible for this process, we measured pre- and postsynaptic changes in inhibitory connectivity in ferret auditory cortex (AC) after cross-modal plasticity. We found that blocking GABAA receptors increased responsiveness and broadened sound frequency tuning in the cross-modal group more than in the normal group. Furthermore, expression levels of glutamic acid decarboxylase (GAD) protein were increased in the cross-modal group. We also found that blocking inhibition unmasked visual responses of some auditory neurons in cross-modal AC. Overall, our data suggest a role for increased inhibition in reducing the effectiveness of the abnormal visual inputs and argue that decreased inhibition is not responsible for compromised auditory cortical function after cross-modal invasion. Our findings imply that inhibitory plasticity may play a role in reorganizing sensory cortex after cross-modal invasion, suggesting clinical strategies for recovery after brain injury or sensory deprivation.

Compromise of auditory cortical tuning and topography after cross-modal invasion by visual inputs

Brain damage resulting in loss of sensory stimulation can induce reorganization of sensory maps in cerebral cortex. Previous research on recovery from brain damage has focused primarily on adaptive plasticity within the affected modality. Less attention has been paid to maladaptive plasticity that may arise as a result of ectopic innervation from other modalities. Using ferrets in which neonatal midbrain damage results in diversion of retinal projections to the auditory thalamus, we investigated how auditory cortical function is impacted by the resulting ectopic visual activation. We found that, although auditory neurons in cross-modal auditory cortex (XMAC) retained sound frequency tuning, their thresholds were increased, their tuning was broader, and tonotopic order in their frequency maps was disturbed. Multisensory neurons in XMAC also exhibited frequency tuning, but they had longer latencies than normal auditory neurons, suggesting they arise from multisynaptic, non-geniculocortical sources. In a control group of animals with neonatal deafferentation of auditory thalamus but without redirection of retinal axons, tonotopic order and sharp tuning curves were seen, indicating that this aspect of auditory function had developed normally. This result shows that the compromised auditory function in XMAC results from invasion by ectopic visual inputs and not from deafferentation. These findings suggest that the cross-modal plasticity that commonly occurs after loss of sensory input can significantly interfere with recovery from brain damage and that mitigation of maladaptive effects is critical to maximizing the potential for recovery.

Severe and extensive neonatal hearing loss in cats results in auditory cortex plasticity that differentiates into two regions

We examined the response characteristics of primary auditory cortex (A1) neurons in adult cats partially but extensively deafened by ototoxic drugs 2-8 days after birth. The damage evoked extensive A1 topographic map reorganization as also found by others, but a novel finding was that in the majority of cats with low-frequency edges to the cochlear lesion, the area of reorganization segregated into two areas expressing the same novel frequency inputs but differentiated by neuronal sensitivity and responsiveness. Immediately adjacent to normal A1 is an approximately 1.2-mm-wide area of reorganization in which sensitivity and responsiveness to sound are similar to that in normal A1 in the same animals and in unlesioned adult animals. Extending further into deprived A1 is a more extensive area of reorganization where neurons have poorer sensitivity and responsiveness to new inputs. These two areas did not differ in response-area bandwidth and response latency. We interpret these novel changes as the cortical consequences of severe receptor organ lesions extending to low-frequency cochlear regions. We speculate that the two areas of A1 reorganization may reflect differences in the transcortical spatial distribution of thalamo-cortical and horizontal intracortical connections. Qualitatively similar changes in response properties have been seen after retinal lesions producing large areas of visual cortical reorganization, suggesting they might be a general consequence of receptor lesions that deprive large regions of cortex of normal input. These effects may have perceptual implications for the use of cochlear implants in patients with residual low-frequency hearing.

Intensive training in adults refines A1 representations degraded in an early postnatal critical period

The spectral, temporal, and intensive selectivity of neurons in the adult primary auditory cortex (A1) is easily degraded in early postnatal life by raising rat pups in the presence of pulsed noise. The nonselective frequency tuning recorded in these rats substantially endures into adulthood. Here we demonstrate that perceptual training applied in these developmentally degraded postcritical-period rats results in the recovery of normal representational fidelity. By using a modified go/no-go training strategy, structured noise-reared rats were trained to identify target auditory stimuli of specific frequency from a set of distractors varying in frequency. Target stimuli changed daily on a random schedule. Consistent with earlier findings, structured noise exposure within the critical period resulted in disrupted tonotopicity within A1 and in degraded frequency-response selectivity for A1 neurons. Tonotopicity and frequency-response selectivity were normalized by perceptual training. Changes induced by training endured without loss for at least 2 months after training cessation. The results further demonstrate the potential utility of perceptual learning as a strategy for normalizing deteriorated auditory representations in older (postcritical-period) children and adults.

Critical period window for spectral tuning defined in the primary auditory cortex (A1) in the rat

Experience-dependent plasticity during development results in the emergence of highly adapted representations of the external world in the adult brain. Previous studies have convincingly shown that the primary auditory cortex (A1) of the rat possesses a postnatal period of sensory input-driven plasticity but its precise timing (onset, duration, end) has not been defined. In the present study, we examined the effects of pure-tone exposure on the auditory cortex of developing rat pups at different postnatal ages with a high temporal resolution. We found that pure-tone exposure resulted in profound, persistent alterations in sound representations in A1 only if the exposure occurred during a brief period extending from postnatal day 11 (P11) to P13. We also found that postnatal sound exposure in this epoch led to striking alterations in the cortical representation of sound intensity.

Development of functional organization of the pallid bat auditory cortex

The primary auditory cortex is characterized by a tonotopic map and a clustered organization of binaural properties. The factors involved in the development of overlain representation of these two properties are unclear. We addressed this issue in the auditory cortex of the pallid bat. The adult pallid bat cortex contains a systematic relationship between best frequency (BF) and binaural properties. Most neurons with BF<30 kHz are binaurally inhibited (EO/I), while most neurons with BF>30 kHz are monaural (EO). As in other species, binaural properties are clustered. The EO/I cluster contains a systematic map of interaural intensity difference (IID) sensitivity. We asked if these properties are present at the time the bat acquires its full audible range (postnatal day [P] 15). Tonotopy, relationship between BF and binaural properties, and the map of IID sensitivity are adult-like at P15. However, binaural facilitation is only observed in pups older than P25. Frequency selectivity shows a BF-dependent sharpening during development. Thus, overlain representation of binaural properties and tonotopy in the pallid bat cortex is remarkably adult-like at an age when the full audible range is first present, suggesting an experience-independent development of overlapping feature maps.

Transcranial fluorescence imaging of auditory cortical plasticity regulated by acoustic environments in mice

Functional brain imaging using endogenous fluorescence of mitochondrial flavoprotein is useful for investigating mouse cortical activities via the intact skull, which is thin and sufficiently transparent in mice. We applied this method to investigate auditory cortical plasticity regulated by acoustic environments. Normal mice of the C57BL/6 strain, reared in various acoustic environments for at least 4 weeks after birth, were anaesthetized with urethane (1.7 g/kg, i.p.). Auditory cortical images of endogenous green fluorescence in blue light were recorded by a cooled CCD camera via the intact skull. Cortical responses elicited by tonal stimuli (5, 10 and 20 kHz) exhibited mirror-symmetrical tonotopic maps in the primary auditory cortex (AI) and anterior auditory field (AAF). Depression of auditory cortical responses regarding response duration was observed in sound-deprived mice compared with naïve mice reared in a normal acoustic environment. When mice were exposed to an environmental tonal stimulus at 10 kHz for more than 4 weeks after birth, the cortical responses were potentiated in a frequency-specific manner in respect to peak amplitude of the responses in AI, but not for the size of the responsive areas. Changes in AAF were less clear than those in AI. To determine the modified synapses by acoustic environments, neural responses in cortical slices were investigated with endogenous fluorescence imaging. The vertical thickness of responsive areas after supragranular electrical stimulation was significantly reduced in the slices obtained from sound-deprived mice. These results suggest that acoustic environments regulate the development of vertical intracortical circuits in the mouse auditory cortex.

Intra- and Extrauterine Maturation of Amplitude-Integrated Electroencephalographic Activity in Preterm Infants Younger than 30 Weeks of Gestation

OBJECTIVE

To prospectively investigate the longitudinal changes of amplitude-integrated electroencephalographic (aEEG) activity in preterm infants <30 weeks gestational age (GA).

METHODS

Infants (GA <30 weeks) without evidence of neurological abnormalities had weekly aEEG recordings performed. The relative duration of the three aEEG patterns (discontinuous low voltage, discontinuous high voltage and continuous) was determined and the influence of GA and postnatal age (PNA) on the occurrence of each pattern was assessed.

RESULTS

Ninety-eight infants (median GA 26 weeks; range 23-29 weeks) were studied. With higher GA (OR 1.68, 95% CI 1.33-2.13) and PNA (OR 1.91, 95% CI 1.53-2.38), the likelihood for the occurrence of continuous activity increased. The discontinuous low-voltage pattern was less likely to occur with increasing GA (OR 0.68, 95% CI 0.55-0.83) and PNA (OR 0.70, 95% CI 0.61-0.81).

CONCLUSION

Maturation of aEEG activity in preterm infants is influenced by both GA and PNA.

The auditory acclimatization effect in sensorineural hearing-impaired listeners: Evidence for functional plasticity

The present study provides new data on perceptual and physiological modifications associated with hearing aid (HA) fitting. Eight sensorineural hearing-impaired (SNHI) listeners participated. They had symmetrical hearing loss and were being fitted with binaural HAs for the first time. Perceptual performances were measured four times during auditory rehabilitation, using an intensity discrimination task and a loudness-scaling task. Pure tones of two different frequencies were used, one well amplified by HAs and the other weakly amplified. Two intensity levels were also tested, one rated 'soft' by SNHI listeners and the other 'loud'. Auditory brainstem responses (ABRs) to click stimulation were recorded. All measures were performed without HA. Results were consistent with the auditory acclimatization effect: most modifications induced by HA fitting were found at loud intensity levels and at high frequency, i.e., for acoustic information that was newly available to the listener. While both ears had similar hearing loss and aided gains, some differences between ears appeared in both perceptual tasks and in ABRs. In the right ear, a shortening of wave V latency paralleled perceptual modifications. The present results suggest that HA-fitting induces functional plasticity at the peripheral level of the auditory system.

Tone Frequency Maps and Receptive Fields in the Developing Chinchilla Auditory Cortex

Single-unit responses to tone pip stimuli were isolated from numerous microelectrode penetrations of auditory cortex (under ketamine anesthesia) in the developing chinchilla ( laniger), a precocious mammal. Results are reported at postnatal day 3 (P3), P15, and P30, and from adult animals. Hearing sensitivity and spike firing rates were mature in the youngest group. The topographic representation of sound frequency (tonotopic map) in primary and secondary auditory cortex was also well ordered and sharply tuned by P3. The spectral-temporal complexity of cortical receptive fields, on the other hand, increased progressively (past P30) to adulthood. The (purported) refinement of initially diffuse tonotopic projections to cortex thus seems to occur in utero in the chinchilla, where external (and maternal) sounds are considerably attenuated and might not contribute to the mechanism(s) involved. This compares well with recent studies of vision, suggesting that the refinement of the retinotopic map does not require external light, but rather waves of (correlated) spontaneous activity on the retina. In contrast, it is most probable that selectivity for more complex sound features, such as frequency stacks and glides, develops under the influence of the postnatal acoustic environment and that inadequate sound stimulation in early development (e.g., due to chronic middle ear disease) impairs the formation of the requisite intracortical (and/or subcortical) circuitry.

Postnatal Cortical Development in Congenital Auditory Deprivation

The study investigates early postnatal development of local field potentials (LFPs) in the primary auditory cortex of hearing and congenitally deaf cats. In hearing cats, LFPs elicited by electrical intracochlear stimulation demonstrated developmental changes in mid-latency range, including reductions in peak and onset latencies of individual waves and a maturation of their shape and latencies during the first 2 months of life. In long latency range (>80 ms), the P(1)/N(1) response appeared after the fourth week of life and further increased in amplitude and decreased in latency, reaching mature shapes between the fourth and sixth months after birth (p.n.). Cortical activated areas became increasingly smaller during the first 3 months of life, reaching mature values at the fourth month p.n. The layer-specific pattern of synaptic activity matured 4 months p.n. In congenitally deaf cats, the developmental pattern was different. The lowest cortical LFP thresholds were significantly smaller than in hearing controls, demonstrating a "hypersensitivity" to sensory inputs. The development of N(b) waves was delayed and altered and the long latency responses became smaller than in controls at the second and third months. The activated areas remained smaller than in controls until the third month, then they increased rapidly and exceeded the activated areas of age-matched controls. From the fourth month on, the activated areas decreased again and smaller synaptic currents were found in deaf cats than in controls. The presented data demonstrate that functional development of the auditory cortex critically depends on auditory experience.

Specialization of primary auditory cortex processing by sound exposure in the "critical period"

Environmental acoustic exposure to a complex tone sequence within the critical period in infant rats resulted in the emergence of large-scale, selective changes that radically altered primary auditory cortex (A1) organization. In the sound exposure-revised A1, responses were segregated into two explicit zones representing spectrally and temporally separated lower and higher frequency tone sequence progressions. Cortical neurons between these two A1 zones were poorly driven by sound stimuli. Stimulus sequence-specific ("combination-selective") responses emerged in the A1 of exposed rats. These selective representational changes induced in the critical period persisted into adulthood. These results show that the temporal order and pace of early, repetitive postnatal auditory inputs strongly affect the emergent and enduring functional organization of A1.

Canadian Association of Neuroscience Review: development and plasticity of the auditory cortex

The functions of the cerebral cortex are predominantly established during the critical period of development. One obvious developmental feature is its division into different functional areas that systematically represent different environmental information. This is the result of interactions between intrinsic (genetic) factors and extrinsic (environmental) factors. Following this critical period, the cerebral cortex attains its adult form but it will continue to adapt to environmental changes. Thus, the cerebral cortex is constantly adapting to the environment (plasticity) from its embryonic stages to the last minute of life. This review details important factors that contribute to the development and plasticity of the auditory cortex. The instructive role of thalamocortical innervation, the regulatory role of cholinergic projection of the basal forebrain and the potential role of the corticofugal modulation are presented.

Effects of sensorineural hearing loss on cortical event-related potential and behavioral measures of speech-sound processing

OBJECTIVE

To investigate systematically the effects of sensorineural hearing loss on cortical event-related potentials (ERPs) N1, MMN, N2 and P3 and their associated behavioral measures (d' sensitivity and reaction time) to the speech sounds /ba/ and /da/ presented at 65 and 80 dB ppe SPL.

DESIGN

Cortical ERPs were recorded to /ba/ and /da/ speech stimuli presented at 65 and 80 dB ppe SPL from 20 normal-hearing adults and 20 adults who are hearing impaired. The degree of sensorineural impairments at 1000 to 2000 Hz ranged from mild losses (defined as 25 to 49 dB HL) to severe/profound losses (75 to 120 dB HL). The speech stimuli were presented in an oddball paradigm and the cortical ERPs were recorded in both active and passive listening conditions for each stimulus intensity.

RESULTS

Both ERP amplitudes and behavioral discrimination (d') scores were lower for listeners with sensorineural hearing loss than for those with normal hearing. However, these differences in response strength were evident only for those listeners whose average hearing loss at 1000 to 2000 Hz exceeded 60 dB HL for the lower intensity stimuli and exceeded 75 dB HL for the higher intensity stimuli. In contrast, prolongations in the ERP and behavioral latencies, relative to responses from normal-hearing subjects, began with even mild (25 to 49 dB HL) threshold elevations. The amplitude and latency response changes that occurred with sensorineural hearing loss were significantly greater for the later ERP peaks (N2/P3) and behavioral discrimination measures (d' and RT) in comparison with earlier (N1, MMN) responses.

CONCLUSIONS

The results indicate that latency measures are more sensitive indicators of the early effects of decreased audibility than are response strength (amplitude, d' or percent correct) measures. Sensorineural hearing loss has a greater impact on higher level or "nonsensory" cortical processing in comparison with lower level or "sensory" cortical processing. Possible physiologic mechanisms within the cortex that may be responsible for these response changes are presented. Lastly, the possible clinical significance of these ERP and behavioral findings is discussed.

Cochlear implants in children--a review

Over the past two decades, cochlear implantation has become a widely accepted treatment of deafness in children. Over 20,000 children have received cochlear implants worldwide. Hearing, language and social development outcomes have been positive. We review current issues in cochlear implantation, candidacy, evaluation, surgery, habilitation, ethics and outcomes.

Auditory brain stem and midbrain development after cochlear implantation in children

Input to the central auditory system through a cochlear implant promotes psychophysical improvement of auditory skills. However, the developmental changes along the pathways have never been characterized in children with hearing loss who use implants. We aimed to measure auditory development in such children by using the electrically evoked auditory brain stem response (EABR). We made repeated measures of the EABR in 41 nonsedated children with implants before chronic stimulation and after 2, 6, and 12 months of consistent implant use. The results show that EABRs were present in all of the children even before chronic auditory stimulation, and that EABR wave latencies decreased from the time of initial activation throughout the first year of cochlear implant use. These findings reflect auditory development to the level of the midbrain as a result of the cochlear implant. The decreasing latencies likely reflect decreased neural conduction times at this level, in part because of increased synaptic efficacy.

Persistent and specific influences of early acoustic environments on primary auditory cortex

This study demonstrates that the adult form of 'tonotopic maps' of sound frequency in the rat primary auditory cortex (A1) arises from parallel developmental processes involving two cortical zones: the progressive differentiation and refinement of selectively tone-responsive receptive fields within an initially broadly-tuned posterior zone, and the progressive loss of tone-evoked, short-latency response over an initially large, very broadly tuned anterior zone. The formation of tonotopic maps in A1 was specifically influenced by a rat pup's early acoustic environments. Exposure to pulsed tones resulted in accelerated emergence and an expansion of A1 representations of those specific tone frequencies, as well as a deteriorated tonotopicity and broader-than-normal receptive fields. Thus, auditory experiences during early postnatal development are important in shaping the functional development of auditory cortical representations of specific acoustic environments.

Cochlear implants in children

Cochlear implants are electronic prostheses that provide a high quality sense of hearing to severely and profoundly deaf children and adults. As improvements in surgical technique and device performance have occurred, indications for implantation have expanded.

Plasticity in the adult human central auditory system: evidence from late-onset profound unilateral deafness

Experience-related changes in central nervous system (CNS) activity have been observed in the adult brain of many mammalian species, including humans. In humans, late-onset profound unilateral deafness creates an opportunity to study plasticity in the adult CNS consequent to monaural auditory deprivation. CNS activity was assessed by measuring long-latency auditory evoked potentials (AEPs) recorded from teens and adults with late-onset (post-childhood) profound unilateral deafness. Compared to monaurally stimulated normal-hearing subjects, the AEPs recorded from central electrode sites located over auditory cortical areas showed significant increases in inter-hemispheric waveform cross-correlation coefficients, and in inter-hemispheric AEP peak amplitude correlations. These increases provide evidence of substantial changes from the normal pattern of asymmetrical (contralateral > ipsilateral amplitude) and asynchronous (contralateral earlier than ipsilateral) central auditory system activation in the normal-hearing population to a much more symmetrical and synchronous activation in the unilaterally deaf. These cross-sectional analyses of AEP data recorded from the unilaterally deaf also suggest that the changes in cortical activity occur gradually and continue for at least 2 years after the onset of hearing loss. Analyses of peak amplitude correlations suggest that the increased inter-hemispheric symmetry may be a consequence of changes in the generators producing the N (approximately 100 ms peak latency) potential. These experience-related changes in central auditory system activity following late-onset profound unilateral deafness thus provide evidence of the presence and the time course of auditory system plasticity in the adult brain.

Environmental factors and disturbances of brain development

Foetal and neonatal brain is under the influence of environmental factors from maternal and extra-maternal origin. Based on the available data, these environmental factors can be classified into three arbitrary groups: (i) factors and maternal status with a demonstrated deleterious effect on the foetal brain (i.e. ethanol, cocaine, some drugs including anticonvulsants, some viral infections, maternal diabetes, untreated maternal phenylketonuria); (ii) factors highly suspected to interfere with foetal brain development (i.e. lead and other heavy metals, some drugs like benzodiazepines, nicotine); (iii) factors which have been shown to be safe for the developing brain in the available studies (i.e. low to moderate doses of caffeine, methadone). However, most of these studies do not address the potential risk of environmental factors on minimal to moderate cognitive and behavioural disturbances. Finally, the impact of the neonatal environment on brain development in very pre-term infants is probably underestimated.

Projections from the medial geniculate body to primary auditory cortex in neonatally deafened cats

In the present study, anatomical projections from the medial geniculate body (MGB) to primary auditory cortex (AI) were investigated in normal adult cats and in animals that were neonatally deafened with the ototoxic drug amikacin. Cochleotopic/tonotopic maps in AI (based on neural response characteristic frequency) were obtained with microelectrode recording techniques, and single or multiple injections of retrograde tracers (horseradish peroxidase and fluorescent dyes) were introduced into AI. The AI maps of the amikacin-treated cats had an abnormal cochleotopic organization, such that deprived cortical areas exhibited an expanded representation of intact regions of the damaged cochlea. However, retrograde tracer injections into different regions of AI produced a normal pattern of labeling in the ventral division of the medial geniculate body (MGBv). In both experimental and control animals, the main mass of labeled thalamic cells was found in the MGBv. Different isofrequency contours in AI receive input from different portions of the MGBv. Thus, cell arrays labeled by anterior AI injections were situated medially in MGBv, and injections into posterior AI labeled MGBv more laterally. Furthermore, the deafened cats did not develop a more divergent thalamocortical projection compared with normal control animals, indicating that an abnormal spread of the thalamocortical afferents across the frequency domain in AI (anterior-posterior axis) is not responsible for the altered cochleotopic map in these neonatally deafened animals. The relatively normal thalamocortical projection pattern suggests that, after neonatal cochlear lesions, the major reorganization of cochleotopic maps occurs at subthalamic levels.

Moderate noise trauma in juvenile cats results in profound cortical topographic map changes in adulthood

Cortical topographic map changes have been reported after profound drug-induced hearing loss in neonates, after progressive high-frequency hearing loss, and after mechanically induced lesions in the cochlea of adult animals. The present study demonstrates that exposure of 5-week-old kittens to a loud 6 kHz tone, producing mild to moderate high-frequency hearing loss, induces a profound reorganization of the frequency map in auditory cortex. In the reorganized cortical region, the frequency-tuning curves were of normal sharpness with near normal thresholds. Inhibitory tuning curve bandwidths were similar to those in control animals. Spontaneous activity in the reorganized part of the cortex was significantly increased. In contrast, the strength of the cross-correlation of the spontaneous activity of units recorded on different electrodes was the same in the normal and reorganized part. Minimum first-spike latency was significantly increased in trauma cats, largely for units at the dorsal side of the sampled region. Because most other neural response properties are normal in the reorganized part of cortex, sub-cortical topographic map changes are likely involved in producing the altered cortical topographic maps.

Cisplatin-induced increases in spontaneous neural activity in the dorsal cochlear nucleus and associated outer hair cell loss

Tinnitus is one of the consequences of cisplatin chemotherapy, but its underlying mechanisms are not well understood. Since it has been shown that cisplatin causes outer hair cell loss, it is possible that loss of these cells might induce tinnitus by increasing spontaneous activity in the central auditory system. To test this possibility, the present study examined the effects of cisplatin treatment on cochlear hair cells and on spontaneous neural activity in the dorsal cochlear nucleus of hamsters. Recordings, carried out approximately 1 month after cisplatin treatment, demonstrated significant increases in spontaneous activity across broad regions of the dorsal cochlear nucleus relative to levels in saline-treated controls. Histological results showed that cisplatin-treated animals also displayed dramatic loss of outer hair cells over most of the basal turn of the cochlea. Inner hair cells remained intact, although some evidence of damage to their stereocilia was evident. These findings indicate that cisplatin treatment causes abnormalities in spontaneous activity in the dorsal cochlear nucleus that are associated with widespread damage to outer hair cells. However, since some damage to inner hair cells was also observed, the role of inner hair cell injury in contributing to higher spontaneous activity cannot be ruled out.

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.

Chronic electrical stimulation by a cochlear implant promotes survival of spiral ganglion neurons after neonatal deafness

This investigation examined the consequences of neonatal deafness and chronic intracochlear electrical stimulation delivered by a cochlear implant during maturation. Kittens were bilaterally deafened by an ototoxic drug administered daily for 2 weeks immediately after birth. Unilateral electrical stimulation was initiated at 7-10 weeks of age and continued over periods of 22-47 weeks (4 hours/day; 5 days/week). Bipolar intracochlear electrodes delivered one of several different electrical signals designed to be temporally challenging to the central auditory system. Morphometric evaluation of spiral ganglion (SG) cell somata within Rosenthal's canal demonstrated a mean of approximately 50% of normal cell density maintained in the chronically stimulated ears, compared with approximately 30% on the control deafened side. This 20% difference in density was highly significant and was greater than differences reported in earlier studies using 30 pps stimulation delivered by either intracochlear bipolar or round window monopolar electrodes. However, the duration of stimulation was also longer in the present study, so it is unclear to what extent the nature of the temporally challenging stimulation vs. its duration contributed to the marked increase in survival. Measurements of the SG cell somata revealed a pronounced decrease in cell diameter in neonatally deafened cats studied about 1 year after deafening, and an additional decrease after long-term deafness (2.5-6.5 years). Furthermore, in the cochlear regions with the greatest stimulation-induced differences in SG cell density, direct measurements of cross-sectional soma area of the largest cells revealed that cells were significantly larger in the stimulated ears. Thus, in addition to the marked increase in the number of surviving SG cells, larger soma area contributed modestly to the pronounced increase in neural density following chronic electrical stimulation.

Central nervous system metabolic activity after cochlear implantation in the feline neonatal model

OBJECTIVE

To determine the effects of deafening and cochlear implant stimulation on central nervous system (CNS) metabolic activity in the feline neonate model.

BACKGROUND

Deafening of fetal animals has been shown to result in acute, profound depression of CNS glucose metabolism, in both auditory structures and the cerebral hemispheres. Preliminary studies have suggested that electrical stimulation of the auditory system may increase central nervous metabolic activity after deafening. The purpose of this study was to investigate this possibility.

METHODS

This was a prospective, randomized, blinded, and controlled animal study of 13 random-source newborn kittens. It was set in an animal research facility for otologic disorders.

OUTCOME

Deoxyglucose metabolism (assessed with autoradiograph densitometry) of brain cross-sections of normal, deafened, and deafened and cochlear-implanted animals after 6 weeks of auditory stimulation or deprivation.

RESULTS

Chronic deafening did not result in a profound reduction in CNS metabolic activity. Cochlear implantation and electrical stimulation did not significantly raise the level of CNS metabolic activity within either auditory pathways or the cerebral hemispheres.

CONCLUSIONS

Deafening is not associated with significant chronic reduction in CNS metabolic activity. Other parameters of CNS activity and maturation may be necessary to assess the effects of cochlear implantation and stimulation in animal models.

Short-term plasticity of the human auditory cortex

Magnetoencephalographic measurements (MEG) were used to examine the effect on the human auditory cortex of removing specific frequencies from the acoustic environment. Subjects listened for 3 h on three consecutive days to music "notched" by removal of a narrow frequency band centered on 1 kHz. Immediately after listening to the notched music, the neural representation for a 1-kHz test stimulus centered on the notch was found to be significantly diminished compared to the neural representation for a 0.5-kHz control stimulus centered one octave below the region of notching. The diminished neural representation for 1 kHz reversed to baseline between the successive listening sessions. These results suggest that rapid changes can occur in the tuning of neurons in the adult human auditory cortex following manipulation of the acoustic environment. A dynamic form of neural plasticity may underlie the phenomenon observed here.

Monaural response properties of single neurons in the chinchilla inferior colliculus

The responses of 274 inferior colliculus (IC) central nucleus neurons from 20 chinchillas were studied. Characteristic frequency (CF) increased as the IC was traversed in the dorsal-ventral direction. Most units had little or no spontaneous activity, with a mean threshold for response of about 30 dB SPL across all units. Tuning curve width varied between units, with a significant increase in Q20, with increasing CF. Peri-stimulus time histogram (PSTH) types were similar to those reported for cat inferior colliculus units. Transient, sustained, pauser, and buildup types were observed, with transient responses predominating. Response area (RA) types were also similar to those of cat IC units, with most units displaying stable best frequencies across a range of stimulus intensity levels. For a few units, excitatory RA regions were surrounded by inhibitory sidebands. Nonmonotonic discharge rate vs. stimulus intensity level functions were common in all CF ranges and for all PSTH and RA types. Mean first spike latencies, however, differed across PSTH groups, owing to the temporal definitions of these PSTH shapes. Latencies of sustained units were significantly longer than those of transient units, and buildup PSTHs showed significantly longer latencies than any other group.

The case for early identification of hearing loss in children. Auditory system development, experimental auditory deprivation, and development of speech perception and hearing

Human infants spend the first year of life learning about their environment through experience. Although it is not visible to observers, infants with hearing are learning to process speech and understand language and are quite linguistically sophisticated by 1 year of age. At this same time, the neurons in the auditory brain stem are maturing, and billions of major neural connections are being formed. During this time, the auditory brain stem and thalamus are just beginning to connect to the auditory cortex. When sensory input to the auditory nervous system is interrupted, especially during early development, the morphology and functional properties of neurons in the central auditory system can break down. In some instances, these deleterious effects of lack of sound input can be ameliorated by reintroduction of stimulation, but critical periods may exist for intervention. Hearing loss in newborn infants can go undetected until as late as 2 years of age without specialized testing. When hearing loss is detected in the newborn period, infants can benefit from amplification (hearing aids) and intervention to facilitate speech and language development. All evidence regarding neural development supports such early intervention for maximum development of communication ability and hearing in infants.

Implications of developmental plasticity for the language acquisition of deaf children with cochlear implants

The study of language acquisition in profoundly deaf children with cochlear implants informs us about the developmental plasticity of the auditory system. Sensory activity leads to neural development, and the sustained effects of sensory inactivity can lead to a loss of responsiveness. These effects may be reversed by the subsequent provision of sensory stimulation, such as that delivered by cochlear implants. Behavioral and electrophysiological research on the effects of speech deprivation on language acquisition shows that the age and modality of language acquisition is an important determinant of adult linguistic performance. Studies on profoundly deaf children deprived of speech stimulation, and then provided with a cochlear implant giving them access to the speech frequencies, shows that congenitally deaf children implanted under the age of around 5 years are likely to perform better on speech perception and speech production tasks than children implanted at an older age. Further investigation is required to understand why these large individual differences exist. In addition, other key issues for research are the effects of compensatory visual and somatosensory development prior to implantation, whether there is a maturational delay that approximates to the period of speech deprivation prior to implantation, and whether there are a number of sensitive periods that together describe the cascade of processes that underlies language acquisition.

Perceptual consequences of peripheral hearing loss: do edge effects exist for abrupt cochlear lesions?

There is a growing body of research that shows evidence of central neural reorganization in response to lesions in the auditory periphery, even if the lesions occur in maturity. This reorganization consists of an increased neural representation of frequencies corresponding to the edge frequency of the lesion. Data were collected to determine whether this over-representation might have consequences for human perception. The hypothesis was that increased central representation might increase acuity on some psychophysical tasks performed at the edge frequency. Tasks included frequency sweep detection (for tones), intensity discrimination (for 100-Hz-wide bands of noise and tones), gap detection and gap discrimination (both for 100-Hz-wide bands of noise). Results from observers with steeply sloping hearing losses were compared with results from normal-hearing observers performing these tasks with masking noise generated to simulate steeply sloping hearing loss. None of these data provide compelling evidence for the hypothesized edge effect. A 40-Hz following response to tone bursts was collected from a subset of the hearing-impaired observers in an attempt to confirm the animal physiology findings of neural over-representation of the edge frequency. No edge-frequency effect was noted in the results, though there was a non-significant tendency for one of the hearing-impaired observers to show shorter latency of response.

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

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

Faster recovery in central than in peripheral auditory system following a reversible cochlear deafferentation

Included among the exciting findings in auditory neuroscience are (i) central plasticity after peripheral injury and (ii) regeneration of auditory nerve fibres following excitotoxic damage. The present study extends our understanding of auditory system plasticity by examining changes in peripheral and central physiology as the cochlea recovers from temporary deafferentation due to excitotoxicity. Application of kainic acid (60 mM) to the round window membrane substantially depressed responses from both auditory nerve and brain stem (inferior colliculus), without affecting distortion-product otoacoustic emissions from the inner ear. The auditory nerve input/output functions recovered over a 30-day period whereas recovery of brainstem response amplitudes occurred within five days. In contrast to amplitudes, thresholds at both peripheral and central levels recovered simultaneously, within five days after kainic acid application. The results indicate that (i) cochlear afferent neurons can recover after excitotoxic damage; (ii) response threshold itself, either central or peripheral, is not sufficient to assess the integrity of the auditory periphery; (iii) the central auditory system can recover more rapidly than the periphery; and (iv) the system can maintain its function in the normal range as peripheral function continues to improve.

Effects of inner hair cell loss on inferior colliculus evoked potential thresholds, amplitudes and forward masking functions in chinchillas

The effects of outer hair cell (OHC) loss on evoked potential (EVP) thresholds, amplitudes and forward masking (FWM) functions have been fairly well characterized. In contrast, the effects of inner hair cell (IHC) losses are largely unknown, primarily due to the difficulty of producing selective IHC lesions. Recent studies have shown that IHCs of the chinchilla are preferentially damaged by the anticancer drug, carboplatin. In this study, we administered a single 100 mg/kg dose of carboplatin to four chinchillas, to examine the effects of IHC lesions on EVPs measured from the inferior colliculus (IC-EVPs). Thresholds and amplitude functions were measured for 0.25-16 kHz tone bursts, and FWM functions were measured at 1, 2 and 4 kHz, using masker probe intervals of 2, 5, 10, 20, 40 and 80 ms, before and 1-2 months after carboplatin treatment. Histology revealed IHC lesions ranging from approximately 15 to 90%, with virtually no loss of OHCs. Surprisingly, even massive IHC lesions were not associated with elevations of IC-EVP thresholds. IC-EVP amplitudes at suprathreshold levels were sometimes depressed, sometimes enhanced, and in some cases unchanged. IHC lesions increased susceptibility to FWM, particularly at intermediate (10-20 ms) masker-probe intervals, without significantly changing the overall time course of FWM. The results provide new perspectives on the contribution of IHCs to FWM, and on the ability of the central auditory system to adapt to a significant reduction of neural input from the cochlea.

Afferent projection patterns in the auditory brainstem in normal and congenitally deaf white cats

Cochlear implantation in congenitally deaf children is developing to a successful medical tool. Little is known, however, on morphology and pathophysiology of the central auditory system in these auditory deprived children. One form of congenital hearing loss, that seen in the deaf white cat, was investigated to see if there are differences in the afferent pathways from the cochlear nuclei to the inferior colliculus. The retrogradely transported fluorescent tracer diamidino yellow (DY) was injected into different parts of the central nucleus of the inferior colliculus (ICC) of normal cats and deaf white cats. It was found that the main afferent projection patterns in deaf white cats were unchanged in spite of congenital auditory deprivation; minor differences were seen.

Differential frequency conditioning enhances spectral contrast sensitivity of units in auditory cortex (field Al) of the alert Mongolian gerbil

Differential aversive auditory conditioning in the awake Mongolian gerbil was performed during single- and multi-unit recording in field Al of the primary auditory cortex. Presentations of pure tone stimuli of a given frequency (reinforced conditioned stimulus; CS+) paired with electrocutaneous stimulation (unconditioned stimulus) were combined with several other non-reinforced tone stimuli (non-reinforced conditioned stimulus; CS-). Stimulus presentation during training and testing was optimized for constancy of the probability of occurrence of both the CS+ and the CS- stimulus. The paradigm led to a reorganization of both the spectral and temporal response characteristics of auditory cortical neurons with the following basic results. First, tone-evoked responses of Al neurons recorded after multiple acoustic stimulation under these conditions varied statistically around a mean value (stationarity). Conditioning produced a shift in mean values of evoked responses. The altered tone responses were also stationary (stability of the plastic effects). Second, the frequency-receptive fields (FRFs) of neurons were reorganized in a frequency-specific way such that the CS+ frequency became located in a local minimum of the FRF after training. This resulted from a training-induced increase in the responses to frequencies adjacent to the CS+ frequency in the FRF relative to the CS+ response. The effect can be interpreted as an enhancement of the 'spectral contrast' sensitivity of the unit in the CS+ neighbourhood. Third, apart from this frequency-specific plastic effect, responses to other frequencies also underwent changes during training. The non-frequency-specific changes were not generally predictable but the post-trial responses were stationary. Fourth, the analysis of the long-term behaviour of FRF reorganization revealed the stability of plastic effects under retention training and the gradual re-establishment of the pretrial FRF during extinction training. Fifth, not only the spectral characteristics but also the temporal structure of the tone-evoked responses could be affected by the training. In most cases the training-induced changes measured within the first tens of milliseconds of the response corresponded to the response changes obtained by integration over the total response period. There were some cases, however, in which the direction of the response change varied with time, indicating that excitatory and inhibitory influences on the temporal response pattern were differently affected by training.

Middle latency responses to acoustical and electrical stimulation of the cochlea in cats

The middle latency responses (MLR) to acoustical stimulation (A-MLR) as well as to electrical stimulation (E-MLR) of the inner ear were recorded in pentobarbital-anaesthetised cats. Monopolar and bipolar MLR recordings were performed with electrodes located at different places on the primary auditory cortex (AI). The cochlea was electrically stimulated (ES) through a single round-window electrode or through a multichannel intracochlear implant. The slope of amplitude-intensity functions of the A-MLR was steeper when the stimulus frequency of the acoustical stimuli corresponded to the tonotopical recording place on the auditory cortex. Other response properties (waveshape, thresholds and latencies) were related to the recording site and stimulus frequency in only two-thirds of animals. Parameters of E-MLRs evoked by high-frequency ( > 4 kHz) and low-intensity ES in hearing cats, which produced an electrophonic effect, were similar to parameters of acoustically evoked MLRs. In deafened cats, the properties of responses to extracochlear ES were different from those recorded to acoustical stimulation and they were almost uniform in all cortical places. Variations in thresholds, in latencies and in the slope of the amplitude-intensity functions of the E-MLRs recorded in individual tonotopical cortical places were observed when the auditory nerve was stimulated with different configurations of electrodes through a multichannel intracochlear implant.

Consequences of chronic extracochlear electrical stimulation in neonatally deafened cats

This investigation examined the consequences of neonatal deafness and chronic electrical stimulation of the cochlea in the developing auditory system. Cats were bilaterally deafened by daily ototoxic drug administration for two weeks after birth. Electrical stimulation was initiated at 6-9 weeks of age and continued for up to 6 months, using monopolar round window electrodes that synchronously excited auditory neurons throughout the cochlea. Morphometric evaluation of the density of spiral ganglion cell somata within Rosenthal's canal demonstrated that chronic stimulation induced an increase of about 6% in neuronal survival. Although this difference was statistically significant, extracochlear stimulation in these cats was less effective in preventing neural degeneration than lower intensity, more restricted intracochlear stimulation that was shown in a previous study to induce an average increase of about 13% in neuronal survival. Electrophysiological recording experiments conducted in the inferior colliculus in these animals indicated that monopolar extracochlear stimulation can induce profound alterations in the spatial (frequency) selectivity of the auditory midbrain. On average, results were similar to those previously reported for bipolar intracochlear stimulation, showing about a two-fold expansion of the central representation of chronically stimulated electrodes. However, results with extracochlear stimulation showed much greater variability among individual animals. The results presented suggest that it is problematic to effect consistent 'whole' nerve stimulation using monopolar round window electrodes. Moreover, this mode of stimulation can induce profound functional alterations in the central nervous system and is substantially less effective in forestalling the degeneration of auditory neurons than intracochlear stimulation. Both these results contraindicate the implantation of such electrodes in young children for the purpose of maintaining the integrity of the auditory system for later application of a multichannel cochlear implant.

Central Auditory System Plasticity Associated with Speech Discrimination Training

A passively elicited cortical potential that reflects the brain's discrimination of small acoustic contrasts was measured in response to two slightly different speech stimuli in adult human subjects. Behavioral training in the discrimination of those speech stimuli resulted in a significant change in the duration and magnitude of the cortical potential. The results demonstrate that listening training can change the neurophysiologic responses of the central auditory system to just-perceptible differences in speech.