Three-month results with bilateral cochlear implants

To evaluate possible binaural listening advantages for speech in quiet, speech in noise, and for localization in a group of postlingually deafened adults with two cochlear implants functioning independently after 3 mo experience.

Nine postlingually deafened subjects who had received a Cochlear Corporation CI24M implant in each ear were evaluated on a number of tasks. The subjects all had audiometric or biographical (e.g., duration of deafness) differences between the ears. Word and sentence materials were presented to the subjects in quiet and in noise with the signal always in the front and the noise from the front or either side. Results are reported for each ear and for both ears with the noise on either side. This allowed evaluation of head shadow and squelch effects. Additionally, localization ability was assessed for broadband noise presented either to the right or left of center at 45 degrees azimuth. Localization was assessed for each ear and for both ears.

Results of speech testing in quiet showed a significant advantage for the binaural condition over the better ear in four subjects. In noise, with both signal and noise in front of the subject, a significant advantage of two ears over the better ear was found for four subjects. For noise to one side of the head, when the ear opposite the noise source was added to the ear ipsilateral to the noise, a significant advantage was demonstrated for seven of seven tested subjects. When the ear ipsilateral to the noise was added to the ear contralateral to the noise, a significant advantage was shown for only one of seven (noise on right) and three of seven (noise on left) tested subjects. The localization task showed that all seven tested subjects could discriminate 45 degrees left from 45 degrees right above chance with bilateral stimulation. Three subjects could perform the discrimination above chance with only one ear. However, performance with both ears was significantly better than performance with one ear for two of these latter subjects.

We conclude that bilateral cochlear implants can provide real advantages, particularly when it is possible to utilize the ear that is away from a noise source, thus taking advantage of the head shadow effect. In addition, localization ability was generally better with two implants than with one.