• As the left and right hemisphere are specialized for different auditory tasks, the proposed study aims at demonstrating different consequences of right or left-sided deafness for the affected individual.
• Furthermore, the question should be answered if auditory deficits and plastic changes can be partially reversed by cochlear implantation of the deaf ear.
• Multicenter, prospective, open, non-randomized clinical trial with 5 patients with right-sided and 5 patients with left-sided sensineural deafness.
• Pre-operative: Audiometry, Sound Localization Audiometry, PET, EEG/MEG
• Comparison of pre-operative investigations with 10 healthy subjects (age and gender matched control group)
• Cochlea implantation
• Follow-up Visits at 3, 6, 9 and 12 month post-operative: Audiometry, Sound Localization Audiometry, PET, EEG, Questionnaires
• Trial with medical device

Postlingual single-sided deafness (SSD) is a type of hearing impairment with normal hearing in one ear and severely impaired hearing in the other ear. The condition induces multiple changes of neural plasticity in central auditory pathways. One manifestation reflects an increased common activation of the contralateral and ipsilateral pathways after stimulation of the normal hearing ear which is correlated with an increased activity between the contralateral and ipsilateral hemispheres. As the left and right hemisphere are specialized for different auditory tasks, the proposed study aims at demonstrating different consequences of right or left-sided deafness for the affected individual. Furthermore, the question should be answered if auditory deficits and plastic changes can be partially reversed by cochlear implantation of the deaf ear. It is assumed that some changes induced by SSD can be detected only by reversal through a cochlear implant.

Inclusion criteria: • Patients with acquired single sided sensorineural deafness due to cochlear damage.

• Age: 18-70 years old.
• Onset of SSD within 6 months to 10 years before Study inclusion.
• Normal hearing on the contralateral ear (Hearing thresholds 0.125-2 kHz 25 dB HL or better, 4-8 kHz 35 dB HL or better, 100% speech discrimination at 80 dB SPL).
• Regular middle ear function on the hearing ear.
• Regular structure of the cochlea and the cochlear nerve (as demonstrated by MRI, MRI scans are included in the basic assessment of SSD and are no study-specific investigations).
• Fluency in the German language.
• Subject is willing to comply with all study requirements.
• Impairment in daily life as a consequence of SSD (communication problems, annoyance by tinnitus perception).
• Subject is not participating in another ongoing research study related to the SSD.
• Subject does not have unrealistic expectations, regarding the outcome of the intervention.
• Subject has had a trial with conventional acoustic measures in SSD (CROSS, BAHA).

Exclusion criteria: • Uncertainty of correct diagnosis of SSD.

• Retrocochlear cause of SSD (Deafness due to lesions of the acoustic nerve or central auditory pathways).
• Active middle ear infections.
• Ossification of the cochlear that prevents electrode insertion.
• Tympanic membrane perforation.
• Psychiatric comorbidities such as depression or cognitive deficits.
• Severe coexisting illness with a medium survival of less than 5 years.
• Exposure to radiation with a cumulative effective dose of 5 mSV within the last 5 years (including the ongoing year).
• Increased risk profile for general anesthesia due to cardiovascular comorbidity.
• Metallic implants constituting an exclusion criterium for MEG procedures of the brain.
• Pregnancy (a pregnancy test will be performed in women in the reproductive age group before study enrolment and before postoperative PET scan) and lactation.