The aim of this study is to investigate the effect of varying additional factors on loudness summation in Neuro Zti Cochlear Implant users.

Primary objective:

- To assess the effect of the number and spatial distribution of active electrodes on the stimulation charge required to achieve the loudness associated with C and T levels (Experiment 2).

Secondary objectives:

• To assess how loudness is affected by changes in electrical charge of individual pulses made by adjusting the pulse amplitude versus adjusting pulse duration (Experiment 1).
• To assess how loudness elicited by a train of pulses is affected by the duration of that pulse train (Experiment 3).

Experiment 1:

The test will investigate the effect of loudness on changes in electrical charge of individual pulses, made by separately adjusting pulse amplitude or pulse width. The electrical charge required to elicit different loudness ("just-audible", "soft", "medium", "comfortable", "loud but comfortable") either when increasing the amplitude or the duration of the pulses.

Experiment 2:

The test will investigate the effect of the number of stimulating electrodes and their distribution within the cochlea on the stimulation levels required to achieve T-levels and C-levels and to elicit different loudness ("just-audible", "soft", "medium", "comfortable", "loud but comfortable").

Experiment 3:

The test will investigate the effect on loudness of trains of pulses elicited by modifications to the duration of those trains, when operating around T and C levels. Stimulation durations ranging from 10 to 600 ms will be tested. Pulse trains will be presented to either one or several electrodes to elicit different loudness ("just-audible", "soft", "medium", "comfortable", "loud but comfortable").

Inclusion Criteria:

• Neuro One or Neuro 2 user
• Unilateral or bilateral recipient
• 12 months CI experience to allow for acclimatization to the CI sound processing and pathological changes that could alter the electrode - tissue interface and therefore T- & C-levels.
• Having at least 7 step sizes on three electrodes in their current clinical map (for experimentation 1).
• Able and willing to provide reliable threshold and comfort level judgements

Exclusion Criteria:

• Medical conditions that contraindicate the study tests (e.g. tinnitus, dizziness)
• Unrealistic expectations from the candidate regarding the possible benefits, risks, and limitations that are inherent to the device
• Unwillingness or inability of the candidate to comply with all investigational requirements
• Vulnerable population