Developmental neuroplasticity after cochlear implantation

Progress in brain research, 2006

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...

Brain Research Reviews, 2007

Sensory and environmental manipulations affect the development of sensory systems.

International Journal of Pediatric Otorhinolaryngology, 1999

Previous animal research and clinical experiences in humans suggest the existence of an auditory critical period in language acquisition. We review the literature and present the changes within the cochlear nuclei in bilaterally deafferentated adult non-human primates. We also present and analyse the results of 98 prelingually deaf children and teenagers who underwent a cochlear implantation at the University of Navarra. Patients received a Nucleus 22 or 24 multichannel cochlear implant (CI). They were grouped in five categories according to their age at surgery. Performance is compared with a control group of 58 postlinguals. Only early-implanted prelingual children (before 6 years of age) achieved a complete open-set speech recognition, even with better performance than postlinguals. These results clearly demonstrate the existence of a period of high neural auditory plasticity within the first 6 years of life. The introduction of auditory stimulation with a CI can not restore the loss of neural plasticity out of this period. Prelingual children under 6 years of age should receive a CI as soon as there is a reliable diagnosis of bilateral sensorineural hearing loss.

International Journal of Audiology, 2007

Journal of Communication Disorders, 2009

A basic tenet of developmental neurobiology is that certain areas of the cortex will reorganize, if appropriate stimulation is withheld for long periods. Stimulation must be delivered to a sensory system within a narrow window of time (a sensitive period) if that system is to develop normally. In this article, we will describe age cutoffs for a sensitive period for central auditory development in children who receive cochlear implants. We will review de-coupling and reorganization of cortical areas, which are presumed to underlie the end of the sensitive period in congenitally deaf humans and cats. Finally, we present two clinical cases which demonstrate the use of the P1 cortical auditory evoked potential as a biomarker for central auditory system development and reorganization in congenitally deaf children fitted with cochlear implants. Learning outcomes-Readers of this article should be able to (i) describe the importance of the sensitive period as it relates to development of central auditory pathways in children with cochlear implants, (ii) discuss the hypothesis of decoupling of primary from higher order auditory cortex as it relates to the end of the sensitive period, (iii) discuss cross-modal reorganization which may occur after long periods of auditory deprivation, and (iv) understand the use of the P1 response as a biomarker for development of central auditory pathways.

Advances in oto-rhino-laryngology, 2006

The benefit of cochlear implantation crucially depends on the ability of the brain to learn to classify neural activity evoked by the cochlear implant. Brain plasticity is a complex property with massive developmental changes after birth. The present paper reviews the experimental work on auditory plasticity and focuses on the plasticity required for adaptation to cochlear implant stimulation. It reviews the data on developmental sensitive periods in auditory plasticity of hearing, hearing-impaired and deaf, cochlear-implanted, animals. Based on the analysis of the above findings in animals and comparable data from humans, a cochlear implantation within the first 2 years of age is recommended.

Hearing Research, 2008

Cochlear implants have been implanted in over 110,000 deaf adults and children worldwide and provide these patients with important auditory cues necessary for auditory awareness and speech perception via electrical stimulation of the auditory nerve (AN). In 1942 Woolsey & Walzl presented the first report of cortical responses to localised electrical stimulation of different sectors of the AN in normal hearing cats, and established the cochleotopic organization of the projections to primary auditory cortex. Subsequently, individual cortical neurons in normal hearing animals have been shown to have well characterized input-output functions for electrical stimulation and decreasing response latencies with increasing stimulus strength. However, the central auditory system is not immutable, and has a remarkable capacity for plastic change, even into adulthood, as a result of changes in afferent input. This capacity for change is likely to contribute to the ongoing clinical improvements observed in speech perception for cochlear implant users. This review examines the evidence for changes of the response properties of neurons in, and consequently the functional organization of, the central auditory system produced by chronic, behaviourally relevant, electrical stimulation of the AN in profoundly deaf humans and animals.

Cerebral Cortex, 2002

The congenitally deaf cat suffers from a degeneration of the inner ear. The organ of Corti bears no hair cells, yet the auditory afferents are preserved. Since these animals have no auditory experience, they were used as a model for congenital deafness. Kittens were equipped with a cochlear implant at different ages and electrostimulated over a period of 2.0-5.5 months using a monopolar single-channel compressed analogue stimulation strategy (VIENNAtype signal processor). Following a period of auditory experience, we investigated cortical field potentials in response to electrical biphasic pulses applied by means of the cochlear implant. In comparison to naive unstimulated deaf cats and normal hearing cats, the chronically stimulated animals showed larger cortical regions producing middle-latency responses at or above 300 µV amplitude at the contralateral as well as the ipsilateral auditory cortex. The cortex ipsilateral to the chronically stimulated ear did not show any signs of reduced responsiveness when stimulating the 'untrained' ear through a second cochlear implant inserted in the final experiment. With comparable duration of auditory training, the activated cortical area was substantially smaller if implantation had been performed at an older age of 5-6 months. The data emphasize that young sensory systems in cats have a higher capacity for plasticity than older ones and that there is a sensitive period for the cat's auditory system.

Audiology and Neuro-Otology, 2001

Congenitally deaf cats were used as a model for human inborn deafness and auditory deprivation. The deaf cats were supplied with a cochlear implant, chronically exposed to an acoustic environment and conditioned to acoustic stimuli. In case of early implantation the cats learned to make use of the newly gained auditory channel behaviourally. Neurophysiological and fMRI data showed that the central auditory system was recruited, if implantation took place within a sensitive period of ! 6 months.

The Journal of Comparative Neurology, 2009

Electrical stimulation of spiral ganglion neurons in deafened cochlea, via a cochlear implant, provides a means of investigating the effects of the removal and subsequent restoration of afferent input on the functional organization of the primary auditory cortex (AI). We neonatally deafened seventeen cats before the onset of hearing, thereby abolishing virtually all afferent input from the auditory periphery. In seven animals, the auditory pathway was chronically reactivated with environmentallyderived electrical stimuli presented via a multi-channel intracochlear electrode array implanted at eight weeks of age. Electrical stimulation was provided by a clinical cochlear implant that was used continuously for periods of up to seven months. In ten long-term deafened cats and three age-matched normal hearing controls, an intracochlear electrode array was implanted immediately prior to cortical recording. We recorded from a total of 812 single unit and multi-unit clusters in AI of all cats as adults, using a combination of single tungsten and multi-channel silicon electrode arrays. The absence of afferent activity in the long-term deafened animals had little effect on the basic response properties of AI neurons but resulted in complete loss of the normal cochleotopic organization of AI. This effect was almost completely reversed by chronic reactivation of the auditory pathway via the cochlear implant. We hypothesize that maintenance or re-establishment of a cochleotopically organized AI by activation of a restricted sector of the cochlea -as demonstrated in the present study -contributes to the remarkable clinical performance observed among human patients implanted at a young age.