Department of Head and Neck Surgery, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California, USA.
Ear Hear. 2020 Sep/Oct;41(5):1362-1371. doi: 10.1097/AUD.0000000000000865.
Due to interaural frequency mismatch, bilateral cochlear-implant (CI) users may be less able to take advantage of binaural cues that normal-hearing (NH) listeners use for spatial hearing, such as interaural time differences and interaural level differences. As such, bilateral CI users have difficulty segregating competing speech even when the target and competing talkers are spatially separated. The goal of this study was to evaluate the effects of spectral resolution, tonotopic mismatch (the frequency mismatch between the acoustic center frequency assigned to CI electrode within an implanted ear relative to the expected spiral ganglion characteristic frequency), and interaural mismatch (differences in the degree of tonotopic mismatch in each ear) on speech understanding and spatial release from masking (SRM) in the presence of competing talkers in NH subjects listening to bilateral vocoder simulations.
During testing, both target and masker speech were presented in five-word sentences that had the same syntax but were not necessarily meaningful. The sentences were composed of five categories in fixed order (Name, Verb, Number, Color, and Clothes), each of which had 10 items, such that multiple sentences could be generated by randomly selecting a word from each category. Speech reception thresholds (SRTs) for the target sentence presented in competing speech maskers were measured. The target speech was delivered to both ears and the two speech maskers were delivered to (1) both ears (diotic masker), or (2) different ears (dichotic masker: one delivered to the left ear and the other delivered to the right ear). Stimuli included the unprocessed speech and four 16-channel sine-vocoder simulations with different interaural mismatch (0, 1, and 2 mm). SRM was calculated as the difference between the diotic and dichotic listening conditions.
With unprocessed speech, SRTs were 0.3 and -18.0 dB for the diotic and dichotic maskers, respectively. For the spectrally degraded speech with mild tonotopic mismatch and no interaural mismatch, SRTs were 5.6 and -2.0 dB for the diotic and dichotic maskers, respectively. When the tonotopic mismatch increased in both ears, SRTs worsened to 8.9 and 2.4 dB for the diotic and dichotic maskers, respectively. When the two ears had different tonotopic mismatch (e.g., there was interaural mismatch), the performance drop in SRTs was much larger for the dichotic than for the diotic masker. The largest SRM was observed with unprocessed speech (18.3 dB). With the CI simulations, SRM was significantly reduced to 7.6 dB even with mild tonotopic mismatch but no interaural mismatch; SRM was further reduced with increasing interaural mismatch.
The results demonstrate that frequency resolution, tonotopic mismatch, and interaural mismatch have differential effects on speech understanding and SRM in simulation of bilateral CIs. Minimizing interaural mismatch may be critical to optimize binaural benefits and improve CI performance for competing speech, a typical listening environment. SRM (the difference in SRTs between diotic and dichotic maskers) may be a useful clinical tool to assess interaural frequency mismatch in bilateral CI users and to evaluate the benefits of optimization methods that minimize interaural mismatch.
由于两耳间频率不匹配，双侧人工耳蜗（CI）使用者可能无法充分利用正常听力（NH）听众用于空间听觉的双耳线索，例如两耳间时间差和两耳间强度差。因此，即使目标和竞争说话者在空间上分开，双侧 CI 用户也难以分离竞争语音。本研究的目的是评估频谱分辨率、音调失配（相对于预期的螺旋神经节特征频率，分配给植入耳内的 CI 电极的声中心频率的频率不匹配）和两耳间失配（每只耳中的音调失配程度的差异）对 NH 受试者在存在竞争说话者的情况下双侧声码器模拟中语音理解和空间掩蔽释放（SRM）的影响。
在测试过程中，目标和掩蔽语音均以具有相同语法但不一定有意义的五字句子呈现。句子由五个固定顺序的类别组成（名称、动词、数字、颜色和衣服），每个类别有 10 个项目，因此可以通过从每个类别中随机选择一个词来生成多个句子。测量了在竞争语音掩蔽中目标句子的语音接收阈值（SRT）。目标语音同时传递到双耳，两个语音掩蔽器分别传递到（1）双耳（同听掩蔽器）或（2）不同耳（双耳掩蔽器：一个传递到左耳，另一个传递到右耳）。刺激包括未处理的语音和四个具有不同两耳间失配（0、1 和 2 毫米）的 16 通道正弦声码器模拟。SRM 计算为同听和双耳听条件之间的差异。
使用未处理的语音，同听掩蔽器和双耳掩蔽器的 SRT 分别为 0.3 和-18.0 dB。对于具有轻度音调失配且无两耳间失配的频谱退化语音，同听掩蔽器和双耳掩蔽器的 SRT 分别为 5.6 和-2.0 dB。当双耳的音调失配增加时，同听掩蔽器和双耳掩蔽器的 SRT 分别恶化至 8.9 和 2.4 dB。当双耳具有不同的音调失配（例如存在两耳间失配）时，双耳掩蔽器的 SRT 下降幅度比同听掩蔽器大得多。未处理语音的最大 SRM 为 18.3 dB。使用 CI 模拟，即使在轻度音调失配但无两耳间失配的情况下，SRM 也显著降低至 7.6 dB；随着两耳间失配的增加，SRM 进一步降低。
结果表明，频率分辨率、音调失配和两耳间失配对双侧 CI 模拟中的语音理解和 SRM 具有不同的影响。最小化两耳间失配可能是优化双耳益处和改善竞争语音 CI 性能的关键，竞争语音是一种典型的听力环境。SRM（同听掩蔽器和双耳掩蔽器之间的 SRT 差异）可能是一种有用的临床工具，可用于评估双侧 CI 用户的两耳间频率失配，并评估最小化两耳间失配的优化方法的益处。
