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Bilateral Cochlear Implantation

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Focus On: Bilateral Cochlear Implantation


Bilateral Cochlear Implantation

Unilateral cochlear implantation already provides many users with a high level of speech understanding. Nevertheless, the loss of binaural hearing generally results in a hampered perception in noisy environments and loss of sound localisation ability, resulting in an increased listening effort. Since 1996, when the first patient was implanted with two MED-EL implants with the objective of achieving binaural hearing, over 100 publications have investigated speech understanding in noise, sound localisation, and many other basic psychophysical properties of bilateral CI users. Study results briefly summarised hereafter support the benefit of bilateral cochlear implantation for both adults and children.

Speech understanding was measured with speech and noise sounds simultaneously presented from two or more directions to simulate real-world listening situations. In post-lingually deafened adults, three effects were found to be significant in different experimental setups: the head shadow effect, where the head acts as a barrier between speech and noise sources: binaural summation, when both sides receive redundant information; and the squelch effect, where the brain uses the ear with the more disturbed signal for speech understanding by evaluating temporal differences between ears. Adaptive sentence testing demonstrated significant head shadow, summation, and squelch effects in adult users who received implants sequentially. The head shadow effect was the dominant effect in a series of studies. Improvements in speech discrimination with two implants were also demonstrated for word understanding in quiet as well as for tonal languages.

In simultaneously bilaterally implanted users with a short duration of deafness, effects were studied over the first years of bilateral use. The benefit of the bilateral over the better unilateral condition was consistently and significantly obtained over time. The emergence of a significant squelch effect after only 12 months of bilateral implant use was demonstrated, whereas binaural summation and head shadow effect were evident after only 6 months.

As with adult CI users, bilaterally implanted children appear to improve with binaural hearing. Early studies investigating bilateral cochlear implantation in children showed improved communicative behavior, particularly in complex listening conditions such as classrooms. Children also had better monosyllable understanding with two implants and significantly better speech understanding in noise when using both implants.

The question of whether the ability to localise sound can be (re)learned with bilateral implants has been studied in different ways. The minimal audible angle, i.e. the smallest detectable displacement of a sound source in the horizontal plane, was measured for bilateral CI users. Spatial discrimination was found to be nearly as good in the implant users as in a normal hearing control group. Sound localisation experiments showed a significantly improved spatial orientation with bilateral CIs, as compared to unilateral CI users as well as bilateral CI users using one implant only.

Unilateral subjects with better-than-chance localisation learned to use subtle monaural and loudness cues which are difficult to use in real-world scenarios. Although sound localisation is more difficult to assess, especially in very young children, significant benefits from using two implants could be demonstrated. Observer-based measurements of minimal audible angles showed that most bilaterally implanted toddlers were able to lateralise sounds better than toddlers using one implant only. Older children could already make significant use of the two inputs, indicating a development of localisation abilities with bilateral implants.

One focus of recent research is the question of auditory development in children as well as adults and the related schedule of implantations. Currently, research suggests that early simultaneous bilateral implantation supports optimal integration of both sides. Further research is needed to identify the critical periods that should not be missed in children with bilateral deafness.

Interaural cues in acoustic signals, such as level and timing differences, can be used by a normal hearing person for several tasks, such as sound localisation, enhanced speech understanding through binaural unmasking, de-reverberation, etc. Recent research shows that CI users analyse interaural timing differences coded in the timing as well as in the envelopes of the stimulation pulses for lateralisation, bilateral release of masking, and echo-detection. One of MED-EL's contributions to bilateral implantation is our continuing work on signal processing concepts that will be able to transmit binaural information optimally, for the benefit of bilateral CI users.

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