single-sided deafness

Single-sided deafness is deafness on one ear combined with (normal) hearing on the other ear. This condition is found also in cochlear-implanted subjects after single-sided cochlear implantation.

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 more activity than stimulation at the ipsilateral (same-side) ear (figure right, top). We found an extensive reorganization favoring the hearing ear in single-sided deafness (figure right, bottom). The effect was strongest if the onset of single-sided deafness was early in life - there was a sensitive period for this reorganization. The results demonstrated a hemispheric specificity of the reorganizations (Kral et al., 2013b). Finally, binaural processing was extensively degraded in this condition (Kral et al., 2015; Tillein et al., 2016).



Based on corresponding results for speech understanding  in children (Illg et al., 2013; 2018), 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, 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 auditory localization ability and binaural fusion.

Subjects with aural preference syndrome will show the tendency to prefer the previously better hearing ear by consciously attending to this ear in binaural hearing. In consequence, behavioral performance will be weaker on the other ear, and learning will be delayed on the weaker ear. Thus, the behavioral cosequence will be a “stronger”, preferred ear, and a “weaker” ear.

Therapy should include periods of concentration on the “weaker” ear in absence of hearing on the preferred ear (e.g. by transient switching off of the implant on the preferred ear). Training of binaural hearing needs to complement this therapy. Finally, language progress should be the first priority in children, with hearing balance being second priority.

While the observed effects remind to the observations in the visual system following monocular deprivation. However, there are substantial differences: in the auditory system single-sided hearing does never lead to deafness on the deprived ear, while it causes blindness in the visual system. This is due to differences in the architecture of these sensory systems (Gordon & Kral, 2019).

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).