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

Abstract

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.

Noise-induced damage to ribbon synapses without permanent threshold shifts in neonatal mice

“…A number of studies have suggested that hearing loss during early development (even if temporary) can greatly influence the development of the central auditory system (Xu et al, 2007;Hatano et al, 2012;Butler and Lomber, 2013;Polley et al, 2013). Interestingly, exposure to noise on P14d in rats, which was a time point used in the present study, has a huge impact on startle responses and frequency coding differing from the effects of noise exposure in adult animals Suta et al, 2015).…”

c‐Fos and Arc/Arg3.1 expression in auditory and visual cortices after hearing loss: Evidence of sensory crossmodal reorganization in adult rats

“…The gradual decrease in size of SGNs (Table ) indicates progressive degeneration of the auditory nerve with time after cochlear damage. Thus, our model reproduces an abrupt cessation of peripheral synaptic auditory input, accompanied by progressive structural damage of first order neurons and their central synapses (Butler & Lomber, ; Wang, O'Donohue, & Manis, ). Animal models of deafness induced by cochlear mechanical lesion have been previously used to investigate post‐lesion plasticity in the auditory pathway (Vale & Sanes, ; Illing, Kraus, & Meidinger, ; Rubio, ; Alvarado, Fuentes‐Santamaría, & Henkel, ).…”

The Neural Consequences of Age-Related Hearing Loss

“…(Interestingly, Kamal et al also found that several of the measured changes showed reversal after returning the young rats to a quiet environment.) Such studies suggest that many of the findings regarding noise-induced hearing loss have implications for normal aging [50-52]. …”