Binaural Advantages for Children with a Cochlear Implant and a Hearing Aid in Opposite Ears and Children with Bilateral Cochlear Implants that were Implanted Sequentially

[[abstract]]兒童由電子耳和助聽器併用（雙模）轉換為雙側電子耳（雙電）的決策關鍵，在於掌握此兩用法相對的雙耳優勢。雙耳優勢源於噪音中的三項物理及神經生理效應，即頭影效應、雙耳加成及雙耳靜噪。在兒童期，此雙耳優勢可視為三階段限制下的產物，即（1）助聽輔具、（2）重度以上聽損後續聽神經系統退化及（3）兒童個別發展因素。本研究按此三階段回顧雙電和雙模可提供的上述三項效應，以分析兒童順序植入雙電是否有超越雙模的優勢。第一階段優勢所根據的是正常成人聆聽以電腦模擬雙電和雙模的語音輸出的表現，第二、三階段優勢則分別根據習語後失聰的成人和習語前失聰的兒童實際使用雙電或雙模的表現。所得結論如下：當語音信號在正前方時，（1）雙模與雙電的優勢幅度多隨限制階段的增加而減小；（2）雙電於三階段的優勢都以噪音與第二電子耳不同側時的頭影效應最明顯，其幅度不僅大於其他兩效應甚多，且效果穩定，也不受植入前的殘存聽力影響；（3）雙模於三階段的優勢結果分歧，部分研究以噪音在前時的雙耳加成幅度最大，甚至可達雙電所提供的頭影效應幅度；此雙耳加成優勢雖有助於語音韻律的區辨，但需有足夠的殘存聽力。有鑑於國内雙模兒童的聽損幾乎皆達極重度，在殘存聽力極為有限的情況下，選擇雙電應較能極大化雙耳優勢。有關兒童順序植入後的語音接收表現、兩耳的表現差異及相關的發展性因素，本文也一併討論。 The decision for bilaterally deafened children to transition from bimodal fitting (a cochlear implant (CI) in one ear and a hearing aid (HA) in the opposite ear) to bilateral cochlear implantation rests upon the binaural advantages that bimodal fitting and bilateral CI can each provide and their relative strengths. Three effects, head shadow, binaural summation and binaural squelch, are believed to underlie the binaural advantages. They are limited at three stages including (1) the hearing aids/CI technology, (2) damage to the auditory neural pathway following the peripheral severe-to-profound hearing impairment and (3) individual developmental characteristics of the deaf child. This review article delineated the bimodal/bilateral CI advantages measured at each of the three stages to see if children receiving bilateral CIs sequentially is likely to gain extra advantages over bimodal fitting. Analyses were based on the three effects measured from normal adults listening to acoustic simulations and bimodal/bilateral CI users who were either post-lingually deaf adults or pre-lingually deaf children. The results showed for both bimodal and bilateral CI users, the binaural advantages decreased as the stage advanced. Bilateral CIs in all three stages offered the largest and robust advantages mostly attributed to the head shadow effect obtained when the noise originated from the opposite side of the second CI. The bimodal advantages, by contrast, were inconsistent across stages and studies. A few studies reported large binaural summation that helped discriminate low frequency speech information but were only accessible to users with adequate residual hearing. In view of the profound losses prevalent among children currently using bimodal fitting, transitioning to bilateral implantation is more likely to maximize the binaural advantages they could enjoy. Factors related to binaural performance after the sequential implantation were also discussed