Animal and human studies on developmental monaural hearing loss.

Asymmetric hearing has been the focus of many studies on brain plasticity in the past. Recently, the topic has gained clinical importance in cases with sequential cochlear implantation or in cases with deafness in one ear and preserved hearing in the other ear. Convergent evidence from animal experiments and from hearing impaired children suggest that asymmetric hearing during early development can reorganize the central auditory representation of the two ears with the consequence of a "stronger" representation of the better hearing ear with a "weaker" representation of the other, more poorly hearing, ear. These changes lead to a persistent aural preference for one ear, demonstrated by asymmetric speech comprehension when each ear is tested separately. Further, binaural integration is compromised as shown by reduced binaural fusion and reduced sensitivity for binaural cues. Finally, the data demonstrate a significant difference in cortical plasticity in response to juvenile monocular deprivation in the visual system and juvenile monaural deafness in the auditory system. The topic represents a unique example of translational research whereby mechanisms explored in animal models are combined with data from children to understand the clinical consequences of asymmetric hearing in early development.