Bilateral cochlear implants controlled by a single speech processor

This study aimed to assess, in one profoundly hearing impaired subject, potential benefits and limitations in placing bilaterally implanted scala tympani electrode arrays under control of a single speech processor.

All available stimulation sites in both ears were compared in studies of pitch discrimination and pitch ranking, identifying three bilateral pairs capable of supporting interaural comparisons with no perceptible difference in pitch. Using those pairs, the subject's ability to lateralize sound was studied as a function of interaural time delay and interaural amplitude difference. Consonant identification scores were obtained for continuous interleaved sampling processors using various unilateral and bilateral combinations of electrodes.

For loudness-matched stimuli composed of 50-msec bursts of 80-microsec/phase pulses at 480 pulses/sec, the subject was able to identify the ear receiving the earlier onset for interaural delays at least as brief as 150 microsec for all three matched pairs. For similar simultaneous stimuli, the subject could identify the ear receiving the louder signal for the smallest deviations from loudness-matched amplitudes available from the implanted electronics. The consonant studies found no evidence that bilateral stimulation per se degrades speech processor performance, even for arbitrary divisions of information between the two ears. Additional contralateral as well as ipsilateral channels were observed to improve speech processor performance.

The ability of this subject to lateralize sounds on the basis of interaural delay or loudness difference, combined with the consonant identification results, supports further use of coordinated binaural stimulation to improve cochlear implant users' ability to understand speech, especially in the presence of competing speech noise.