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

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      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.

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      The period of susceptibility to the physiological effects of unilateral eye closure in kittens.

       T Wiesel,  D H Hubel (1970)
      1. Kittens were visually deprived by suturing the lids of the right eye for various periods of time at different ages. Recordings were subsequently made from the striate cortex, and responses from the two eyes compared. As previously reported, monocular eye closure during the first few months of life causes a sharp decline in the number of cells that can be influenced by the previously closed eye.2. Susceptibility to the effects of eye closure begins suddenly near the start of the fourth week, remains high until some time between the sixth and eighth weeks, and then declines, disappearing finally around the end of the third month. Monocular closure for over a year in an adult cat produces no detectable effects.3. During the period of high susceptibility in the fourth and fifth weeks eye closure for as little as 3-4 days leads to a sharp decline in the number of cells that can be driven from both eyes, as well as an over-all decline in the relative influence of the previously closed eye. A 6-day closure is enough to give a reduction in the number of cells that can be driven by the closed eye to a fraction of the normal. The physiological picture is similar to that following a 3-month monocular deprivation from birth, in which the proportion of cells the eye can influence drops from 85 to about 7%.4. Cells of the lateral geniculate receiving input from a deprived eye are noticeably smaller and paler to Nissl stain following 3 or 6 days' deprivation during the fourth week.5. Following 3 months of monocular deprivation, opening the eye for up to 5 yr produces only a very limited recovery in the cortical physiology, and no obvious recovery of the geniculate atrophy, even though behaviourally there is some return of vision in the deprived eye. Closing the normal eye, though necessary for behavioural recovery, has no detectable effect on the cortical physiology. The amount of possible recovery in the striate cortex is probably no greater if the period of eye closure is limited to weeks, but after a 5-week closure there is a definite enhancement of the recovery, even though it is far from complete.
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        Rapid task-related plasticity of spectrotemporal receptive fields in primary auditory cortex.

        We investigated the hypothesis that task performance can rapidly and adaptively reshape cortical receptive field properties in accord with specific task demands and salient sensory cues. We recorded neuronal responses in the primary auditory cortex of behaving ferrets that were trained to detect a target tone of any frequency. Cortical plasticity was quantified by measuring focal changes in each cell's spectrotemporal response field (STRF) in a series of passive and active behavioral conditions. STRF measurements were made simultaneously with task performance, providing multiple snapshots of the dynamic STRF during ongoing behavior. Attending to a specific target frequency during the detection task consistently induced localized facilitative changes in STRF shape, which were swift in onset. Such modulatory changes may enhance overall cortical responsiveness to the target tone and increase the likelihood of 'capturing' the attended target during the detection task. Some receptive field changes persisted for hours after the task was over and hence may contribute to long-term sensory memory.
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          Anatomical evidence of multimodal integration in primate striate cortex.

          The primary visual cortex (area 17 or V1) is not thought to receive input from nonvisual extrastriate cortical areas. However, this has yet to be shown to be the case using sensitive tracers in the part of area 17 subserving the peripheral visual field. Here we show using retrograde tracers that peripheral area 17 subserving the visual field at an eccentricity of 10-20 degrees receives projections from the core and parabelt areas of the auditory cortex as well as from the polysensory area of the temporal lobe (STP). The relative strength of these projections was calculated for each injection by computing the proportions of retrogradely labeled neurons located in the auditory and STP areas with respect to number of labeled neurons constituting the established projection from the superior temporal sulci (STS) motion complex (middle temporal area, medial superior temporal, fundus of the superior temporal area). In peripheral area V1 the projection from auditory cortex corresponds to 9.5% of that of the STS motion complex and STP to 35% of that from the STS motion complex. Compared to peripheral area 17, central and paracentral area 17 showed considerably weaker inputs from auditory cortex (0.2-0.8%) but slightly more from STP cortex (3.5-6.1%). The present results show that the connectivity of area 17 is eccentricity dependent. Direct projections from auditory and STP cortex to peripheral area 17 have important consequences for higher visual functions of area 17, including multimodal integration at early stages of the visual cortical pathway.

            Author and article information

            1Cerebral Systems Laboratory, Department of Physiology and Pharmacology, Brain and Mind Institute, University of Western Ontario London, ON, Canada
            2Cerebral Systems Laboratory, Department of Physiology and Pharmacology and Department of Psychology, National Centre for Audiology, Brain and Mind Institute, University of Western Ontario London, ON, Canada
            Author notes

            Edited by: Jonathan E. Peelle, Washington University in St. Louis, USA

            Reviewed by: Shaowen Bao, Unviersity of California-Berkeley, USA; David R. Moore, University of Cincinnati College of Medicine, USA

            *Correspondence: Blake E. Butler, Cerebral Systems Laboratory, Natural Sciences Centre, Brain and Mind Institute, 1151 Richmond Street North, London, ON N6A 5B7, Canada e-mail: bbutler9@

            This article was submitted to the journal Frontiers in Systems Neuroscience.

            Front Syst Neurosci
            Front Syst Neurosci
            Front. Syst. Neurosci.
            Frontiers in Systems Neuroscience
            Frontiers Media S.A.
            26 November 2013
            : 7
            Copyright © 2013 Butler and Lomber.

            This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

            Figures: 3, Tables: 4, Equations: 0, References: 198, Pages: 17, Words: 15511
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