Grantham D Wesley, Ashmead Daniel H, Ricketts Todd A, Haynes David S, Labadie Robert F
Vanderbilt Bill Wilkerson Center for Otolaryngology and Communication Sciences, Vanderbilt University Medical Center, Nashville, Tennessee 37232, USA.
Ear Hear. 2008 Jan;29(1):33-44. doi: 10.1097/AUD.0b013e31815d636f.
The main purpose of the study was to measure thresholds for interaural time differences (ITDs) and interaural level differences (ILDs) for acoustically presented noise signals in adults with bilateral cochlear implants (CIs). A secondary purpose was to assess the correlation between the ILD and ITD thresholds and error scores in a horizontal-plane localization task, to test the hypothesis that localization by individuals with bilateral implants is mediated by the processing of ILD cues.
Eleven adults, all postlingually deafened and all bilaterally fitted with MED-EL COMBI 40+ CIs, were tested in ITD and ILD discrimination tasks in which signals were presented acoustically through headphones that fit over their two devices. The stimulus was a 200-msec burst of Gaussian noise bandpass filtered from 100 to 4000 Hz. A two-interval forced-choice adaptive procedure was used in which the subject had to respond on each trial whether the lateral positions of the two sound images (with the interaural difference favoring the left and right sides in the two intervals) moved from left-to-right or right-to-left.
In agreement with previously reported data, ITD thresholds for the subjects with bilateral implants were poor. The best threshold was approximately 400 microsec, and only five of 11 subjects tested achieved thresholds <1000 microsec. In contrast, ILD thresholds were relatively good; mean threshold was 3.8 dB with the initial compression circuit on the implant devices activated and 1.9 dB with the compression deactivated. The ILD and ITD thresholds were higher than previously reported thresholds obtained with direct electrical stimulation (generally, <1.0 dB and 100 to 200 microsec, respectively). When the data from two outlying subjects were omitted, ILD thresholds were highly correlated with total error score in a horizontal-plane localization task, computed for sources near midline (r = 0.87, p < 0.01).
The higher ILD and ITD thresholds obtained in this study with acoustically presented signals (when compared with prior data with direct electrical stimulation) can be attributed-at least partially-to the signal processing carried out by the CI in the former case. The processing strategy effectively leaves only envelope information as a basis for ITD discrimination, which, for the acoustically presented noise stimuli, is mainly coded in the onset information. The operation of the compression circuit reduces the ILDs in the signal, leading to elevated ILD thresholds for the acoustically presented signals in this condition. The large magnitude of the ITD thresholds indicates that ITDs could not have contributed to the performance in the horizontal-plane localization task. Overall, the results suggest that for subjects using bilateral implants, localization of noise signals is mediated entirely by ILD cues, with little or no contribution from ITD information.
本研究的主要目的是测量双侧人工耳蜗植入（CI）成人对于声学呈现的噪声信号的双耳时间差（ITD）和双耳声级差（ILD）阈值。次要目的是评估ILD和ITD阈值与水平面定位任务中的误差分数之间的相关性，以检验双侧植入者的定位是由ILD线索处理介导的这一假设。
11名成人，均为语后聋且均双侧佩戴MED-EL COMBI 40+人工耳蜗，在ITD和ILD辨别任务中接受测试，其中信号通过适配其两个设备的头戴式耳机以声学方式呈现。刺激为一段200毫秒的高斯噪声突发，经100至4000赫兹带通滤波。采用双间隔强迫选择自适应程序，受试者必须在每次试验中回答两个声像的横向位置（在两个间隔中双耳差异分别偏向左侧和右侧）是从左到右还是从右到左移动。
与先前报道的数据一致，双侧植入受试者的ITD阈值较差。最佳阈值约为400微秒，在测试的11名受试者中只有5名达到阈值<1000微秒。相比之下，ILD阈值相对较好；在植入设备上的初始压缩电路激活时，平均阈值为3.8分贝，压缩关闭时为1.9分贝。ILD和ITD阈值高于先前通过直接电刺激获得的阈值（通常分别<1.0分贝和100至200微秒）。当剔除两名极端受试者的数据时，对于中线附近的声源，ILD阈值与水平面定位任务中的总误差分数高度相关（r = 0.87，p < 0.01）。
本研究中使用声学呈现信号获得的较高ILD和ITD阈值（与先前直接电刺激的数据相比）至少部分可归因于前一种情况下人工耳蜗进行的信号处理。该处理策略有效地仅留下包络信息作为ITD辨别的基础，对于声学呈现的噪声刺激，其主要编码在起始信息中。压缩电路的运行会降低信号中的ILD，导致在此条件下声学呈现信号的ILD阈值升高。ITD阈值的大幅升高表明ITD对水平面定位任务的表现没有贡献。总体而言，结果表明对于使用双侧植入的受试者，噪声信号的定位完全由ILD线索介导，ITD信息几乎没有贡献。
