single-sided deafness
 
 

Single-sided deafness (SSD) is defined by deafness on one ear combined with good hearing on the other ear. A similar condition is found in binaural deafness and single-sided cochlear implantation. The impact is particularly extensive when SSD appear during childhood.


We investigated the effects of such a condition on the representation of both ears in the auditory cortex (Kral et al., 2013). In hearing animals, stimulation at the contralateral (opposite) ear results in earlier and stronger activity than stimulation at the ipsilateral (same-side) ear (figure right, top). In SSD, we found an extensive reorganization favoring the hearing ear (figure right, bottom). The effect was strongest if the onset of single-sided deafness was early in life - there was a sensitive period lasting ~4 months in cats, corresponding to 3 years in children. A hemispheric specificity of the reorganizations was observed (Kral et al., 2013b). Binaural processing was extensively degraded in congenital SSD (Kral et al., 2015; Tillein et al., 2016). While normally the binaural response is stronger than the sum of the monaural responses, this was not the case in SSD animals (Tillein et al., 2016). The hearing ear tended to dominate the response, with small and mainly inhibitory influence of the deaf ear (Hubka et al., 2024).

AURAL PREFERENCE SYNDROME:

Based on corresponding results for speech understanding in implanted children (Illg et al., 2013; 2019), we propose the existence of an “aural preference syndrome” in early single-sided hearing (Gordon et al., 2015). It is characterized by:

  1. 1.Absence of peripheral (auditory nerve) asymmetry

  2. 2.Early onset of single-sided deafness, typically not preceded by periods of bilateral hearing.

  3. 3.An asymmetry in behavioral (speech) performance after binaural restoration of hearing (e.g. with cochlear implants) favoring the previously better-hearing ear.

  4. 4.Severe deterioration of binaural fusion and auditory localization ability.

Subjects with aural preference syndrome prefer the previously better hearing ear by consciously attending to this ear in binaural hearing. In consequence, behavioral performance is weaker on the other ear, and learning is delayed. The behavioral cosequence is a “stronger”, preferred ear, and a “weaker”, non-preferred ear. Existing children data support such condition in congenital asymmetry lasting longer than 2-4 years (Gordon & Kral, 2019). Improvements in speech understanding were present but small and very slow (years) in in the late-implanted ear (Illg et al., 2019).


While the observed effects remind to the observations in the visual system following monocular deprivation (amblyopia), there are substantial differences: in the auditory system single-sided hearing after late restoration of hearing on the deaf ear does not document deafness on the previously-deprived ear, while it causes blindness on the deprived eye in the visual system (for direct quantitative comparison, see Gordon & Kral, 2019). This is due to differences in the architecture of these sensory systems (ibid.).


When investigating the underlying cellular process of binaural deficits, we found reorganized excitatory-inhibitory interactions with particular emphasis on increased excitation from the hearing ear and more abundant mild inhibition from the deaf ear, consistent with expected reorganization at the brainstem level inherited to the cortex (Hubka et al., 2024).


A clinical study by the team of Prof. E. Karltorp at Karolinska Institute could recently demonstrate that, consistent with the proposed aural preference syndrome, simultaneous implantations in pediatric populations provide better spatial hearing in adolescence than sequential implantations (Asp et al., 2026). This, among many other studies around the world, confirms the lasting effect of asymmetric hearing on speech performance and spatial localization and supports the existence of the aural preference syndrome.

Amplitude-latency functions of Pa components of the LFP along the recording position in field A1. In hearing controls, stimulation at the contralateral ear generates faster and larger responses. In single-sided deaf animals, the hearing ear generates faster and larger responses (Brain 2013).

Comparisons of onset latencies on the hemisphere ipsilateral and contralateral to the hearing ear for responses to stimulation of the hearing ear. If onset of single-sided deafness (following periods of bilateral deafness) is early, latencies at both hemispheres are small and not different. If onset is late (after 3.5 months of age) contralateral cortex shows smaller onset latencies. In consequence, early onset reorganizes both hemispheres towards the hearing ear, late onset affects mainly the contralateral cortex (Front Syst Neurosci 2013).