Nowadays, cochlear implants (CI) are widely used prostheses for deaf people. They are stimulating the auditory nerve cells electrically, circumventing the tympanic membrane and ossicles of the middle ear and the inner and outer hair cells of the inner ear. A single cochlear implant usually provides substantial benefit to recipients who were previously severe-to-profoundly deaf. In quiet listening situations, scores for the perception of sentences typically show a strong ceiling effect . However, in even moderately challenging listening situations, where speech needs to be understood when presented at lower levels or in the presence of competing noise, speech perception scores drop to below the level required to support practical communication. The speech reception threshold (SRT), the signal-to-noise ratio (SNR) required to understand 50% of words in sentences for normal hearing subjects listening in their first language, is around -6 decibel (dB). For CI recipients this can be +9 to +15 dB. These figures relate to a test situation where target speech comes from directly in front of the CI recipient. If sound comes from their un-implanted side, it will typically suffer a further 6 dB of attenuation, a figure in agreement with published literature, with the information rich higher frequencies being even more attenuated. This puts unilaterally implanted CI users in a very difficult position. Bilateral cochlear implantation has been shown to provide advantages although its main advantage is in dealing with the head shadow problem, rather than restoring true binaural hearing advantages such as spatial release from masking. Bilateral implantation is obviously expensive and not available to adults in many countries. In addition, some subjects cannot get a second implant due to medical reasons or they do not want to undergo a second surgery. These subjects will have similar limitations as single-sided deaf (SSD) patients: due to the head shadow effect, soft sounds from the direction of the non-implanted side will not be heard. To overcome these limitations, the contralateral routing of signals (CROS) by means of a contralateral receiver could be a very beneficial tool. This system has already been used in the hearing instrument field for a while and showed benefit to patients suffering from single-sided deafness. Some preliminary studies using experimental devices were also conducted with cochlear implants users showing similar benefits.

A comparable system, the Naída CROS device, developed by Advanced Bionics as contralateral receiver will be evaluated in this study. The Naída CROS device is placed on the contralateral ear similar to a small hearing aid. It captures the incoming sound and transmits it wirelessly to the CI sound processor. In the sound processor, signals from both the ipsilateral and the contralateral side are mixed together.

While some work has been done acutely with hearing instrument prototypes, this study will evaluate the Naída CI device with the Naída CROS device and involve take home experience. The study will evaluate the benefit of adding a CROS device to unilaterally implanted subjects both objectively and subjectively.

A microphone is placed at the unilateral CI recipient's non-implanted ear. This microphone can then detect sounds from the head-shadowed side and route them to the CI sound processor. In the hearing instrument field such systems have been commonly applied for cases of unilateral deafness and are referred to as contralateral routing of signal or CROS systems. The most recent Advanced Bionics sound processors, the Naída CI Q70 and Q90, have the ability to communicate wirelessly with another Naída CI processor, or with some Phonak hearing instrument models, including a CROS device, placed on the contralateral ear. The full microphone signal is transmitted wirelessly to the Naída CI sound processor and can be mixed with the microphone signal from the implanted side. The Naída CROS device is based on the Phonak CROS H20 device hardware. It contains essentially a microphone, radio transmitter and battery so it is small and discrete. The Naída CROS device is designed to be an out-of-the-box solution to work automatically with the Naída CI without the need for fitting.

The Naída CI processors will be programmed using the conventional fitting software SoundWave (version 3.1) provided by Advanced Bionics, to enable wireless communication between the CI and Naída CROS devices. The software is not yet CE approved, but is tested with respect to the company's internal standards. The CE approved CI system as a whole is still used in its intended way during the study, i.e. to "restore a level of auditory sensation". All basic safety features incorporated in the implant system remain active and are not affected by the study. The Naída CROS device also works in its intended way without the need for programming. However, the investigational Naída CROS device will be pre-configured with a research software to instantaneously communicate with the approved Naída CI processor when switched on.