The purpose of this study is to understand the speech perception in noise and in quiet and localization benefits in a laboratory setting in listeners that receive a cochlear implant in an ear with severe to profound hearing loss and have normal, or near-normal hearing in the opposite ear. We will also evaluate the functional benefit of restoring hearing to the impaired ear via questionnaires that subjects will be answering while listening in their natural environment.

Inclusion Criteria:

1. Participants will be adults who utilize English as their primary language.
2. Unilateral severe to profound sensorineural hearing loss for a duration of at least 6 months (to ensure stability of hearing loss), but no greater than 10 years.
3. Contralateral ear with normal, or near-normal, hearing.
4. Poorer ear (ear to be implanted): severe to profound sensorineural hearing loss, defined as pure-tone thresholds 70 dB HL or greater for the frequencies 500, 1000, 2000, 3000, and 4000 Hz with monosyllabic word understanding, as measured using the CNC Word Test at 60 dBA under earphones, less than or equal to 20%
5. Better ear (contralateral ear): normal or near-normal hearing defined as pure-tone thresholds no poorer than 30 dB HL at 250, 500, 1000, 2000, and 3000 Hz and no poorer than 40 dB HL at 4000 Hz with monosyllabic word understanding, as measured using the CNC Word Test at 60 dBA under earphones, of greater than or equal to 85%.
6. Willingness to comply with all study requirements.
7. Patent cochlea and normal cochlear anatomy.

Exclusion Criteria:

1. Medical or psychological conditions that contraindicate undergoing surgery.
2. Ossification or any other cochlear anomaly that might prevent complete insertion of the electrode array.
3. Unrealistic expectations on the part of the candidate and/or candidate's family, regarding the possible benefits, risks, and limitations that are inherent to the surgical procedure(s) and prosthetic devices.
4. Unwillingness or inability of the candidate to comply with all investigational requirements.
5. Active middle ear infection.
6. Tinnitus primary motivation for implantation.

Participants will be adults 18 years of age or older who utilize English as their primary language. These adults will have unilateral severe to profound sensorineural hearing loss in one ear for a duration of at least 6 months (to ensure stability of hearing loss), but no greater than 10 years. The contralateral ear will present with with normal, or near-normal, hearing.