This descriptive and observational research project aims to characterize MMN as a neuronal marker of localization deficit in single-sided deafened subjects and subjects with bilateral profound deafness treated by a cochlear implant (CI). It includes several electro-physiological and psychoacoustic assessments performed on subjects with single-sided deafness and cochlear implanted subjects, with normal-hearing subjects as control:

• Evaluation of the characteristics of the MMN involved in sound localization by EEG.
• Evaluation of the spatial localization abilities for a sound source presented in the open field.
• Assessment of performance for speech recognition in noise. These evaluations are performed in subjects with symmetrical hearing, in a natural binaural condition and a monaural condition (with a plugged ear), in subjects with single-sided deafness and in subjects with unilateral CI

Sound localization is the primary function of binaural hearing and facilitates speech recognition in noise. In case of unilateral or asymmetrical hearing loss, speech recognition in noise requires a constant adaptation of the head position to compensate for deafness. There is a significant impact of these deficits for socio-professional integration with a globally reduced quality of life . A researcher has found a significant correlation between performance for speech recognition in noise and the level of quality of life level assessed by a Spatial and Qualitative Hearing Scale (SSQ).

Subjects with bilateral profound deafness treated with cochlear implant (CI) usually recover excellent scores for speech recognition in quiet conditions. Nevertheless, hearing restoration is only partial and binaural hearing includes the fields where hearing remains impaired, as illustrated by the studies on sound localization or speech recognition in noise.

Studies using functional MRI, MEG or EEG to investigate neuro-functional reorganization following single-sided deafness show a reduced asymmetry in the hemispherical processing of auditory information, documented by an increase of cortical activity ipsilateral to the better ear, whether it is lateralized to right or left. However, the neural modifications underlying spatial hearing processing have hardly not been explored.

The mismatch negativity (MMN) is a specific potential, evoked by an auditory stimulation using two sounds almost similar. The presence of this MMN indicates that the auditory system is able to discriminate these 2 sounds, regardless of the subjective feedback reported by the subject.

The use of MMN to describe neural reorganization following cochlear implantation is expanding. Stimuli as diverse as pure sounds, tonal bursts or phonemes may be used, but to the investigator's knowledge there is no study investigating localization processes

Inclusion Criteria:

For symmetrical hearing subjects:

• Affiliation to a social security
• Normal hearing, or mild to moderate symmetrical hearing loss controlled by tone audiometry performed before inclusion (in air conduction and bone conduction, frequencies 250 Hz to 4000 Hz).
• Signature of the consent form

For subjects with single-sided deafness:

• Affiliation to a social security
• Subject having a unilateral deafness controlled by pure-tone audiometry before inclusion (in air conduction and bone conduction, frequencies 250 Hz to 4000 Hz).
• Signature of the consent form

For subjects with Cochlear Implants:

• Affiliation to a social security
• Subject with symmetrical or asymmetrical profound deafness treated by unilateral cochlear implantation.
• Signature of the consent form

Exclusion Criteria:

• History of associated neurological disorder
• Taking psychotropic drugs
• Persons with complicated social or medical situations