Department of Hearing and Speech Sciences, University of Maryland, College Park, Maryland, USA.
Ear Hear. 2018 Jan/Feb;39(1):110-123. doi: 10.1097/AUD.0000000000000470.
Bilateral cochlear implants (BI-CIs) are intended to improve sound localization and speech understanding in the presence of interfering sounds. For normal-hearing listeners, improved speech understanding in the presence of interfering sounds can be achieved with monaural head shadow and binaural unmasking. While some BI-CI listeners experience binaural unmasking under certain testing conditions, others appear to not. This study tested a group of BI-CI users with hearing histories that have been linked to poor binaural processing-early onset of deafness or long duration of deafness in just one ear. We predicted that these listeners would experience the opposite of binaural unmasking (i.e., contralateral interference) when trying to understand speech in the presence of a competing talker.
Nine adult BI-CI users who were deafened early in life or had an asymmetric hearing history (e.g., a much longer duration of deafness in one ear) participated in this study. The coordinate response measure corpus was used to assess speech understanding for a male target talker in quiet or in the presence of one male competing talker. Experiment 1 measured binaural unmasking in a paradigm that provided no head-shadow component. The target was always presented monaurally, while the interferer was presented either monaurally or diotically. Experiment 2 measured spatial release from masking in a paradigm that included both a head-shadow component and possible binaural-unmasking component. Nonindividualized head-related transfer functions were used to simulate talker locations in the front or 90° to the left or right.
In experiment 1, all nine listeners experienced contralateral interference (9 dB on average). Four listeners demonstrated roughly symmetric contralateral interference; five listeners experienced asymmetrical contralateral interference. In experiment 2, the listeners experienced only 1 dB of spatial release from masking on average; this small amount was possibly a result of the contralateral interference observed in experiment 1. The results were best explained by individual differences in speech understanding in quiet, which significantly correlated with the duration of deafness in the ipsilateral ear. Specifically, instances of asymmetrical contralateral interference could correspond to asymmetrical hearing histories.
Bilateral cochlear implantation should provide a hearing benefit to the recipient. For the BI-CI listeners specifically recruited for this study, there seems to be a conflict with processing the auditory information across the two ears, which produced the opposite of the desired hearing benefit. This suggests that there may be a subset of potential BI-CI users for whom contralateral interference offsets much of the potential head-shadow benefit. If so, earlier implantation in the second implanted ear might have produced larger binaural benefits, which is important information for clinicians advising patients considering bilateral implantation.
双侧人工耳蜗植入（BI-CI）旨在改善存在干扰声音时的声音定位和言语理解能力。对于正常听力的听众，通过单耳头影和双耳掩蔽可以在存在干扰声音的情况下提高言语理解能力。虽然一些 BI-CI 听众在某些测试条件下表现出双耳掩蔽，但其他人似乎没有。本研究测试了一组听力史与较差的双耳处理相关的 BI-CI 用户-单侧耳聋或单侧耳聋持续时间较长。我们预测，当试图在竞争说话者存在的情况下理解言语时，这些听众会经历与双耳掩蔽相反的情况（即对侧干扰）。
本研究纳入了 9 名成年 BI-CI 用户，他们在生命早期失聪或听力史不对称（例如，单侧耳聋持续时间较长）。坐标响应测量语料库用于评估安静或存在一名男性竞争说话者时男性目标说话者的言语理解能力。实验 1 在没有头影分量的范式中测量双耳掩蔽。目标始终是单耳呈现，而干扰源可以是单耳或双耳呈现。实验 2 在包括头影分量和可能的双耳掩蔽分量的范式中测量空间掩蔽释放。未个体化的头相关传递函数用于模拟说话者位于前方或 90°到左侧或右侧的位置。
在实验 1 中，所有 9 名听众均经历了对侧干扰（平均 9dB）。4 名听众表现出大致对称的对侧干扰；5 名听众经历了不对称的对侧干扰。在实验 2 中，听众平均仅经历了 1dB 的空间掩蔽释放；这一小量可能是实验 1 中观察到的对侧干扰的结果。结果最好用安静时言语理解的个体差异来解释，这与同侧耳的耳聋持续时间显著相关。具体而言，不对称的对侧干扰情况可能对应于不对称的听力史。
双侧人工耳蜗植入应为接受者提供听力益处。对于专门为此研究招募的 BI-CI 听众，似乎存在跨两耳处理听觉信息的冲突，这产生了与预期听力益处相反的效果。这表明，对于潜在的 BI-CI 用户来说，可能存在一个子集，其中对侧干扰抵消了大部分潜在的头影益处。如果是这样，在第二只植入耳中更早植入可能会产生更大的双耳益处，这对向考虑双侧植入的患者提供建议的临床医生来说是重要信息。
