Department of Communication Sciences and Disorders, Syracuse University, Syracuse, NY 13244, USA.
Ear Hear. 2011 Mar-Apr;32(2):168-80. doi: 10.1097/AUD.0b013e3181f534b5.
This experiment was designed to evaluate whether neural encoding of speech features at the brain stem level is altered in the aging auditory system. In addition, the effect of minimal peripheral hearing loss on the auditory brain stem response (ABR) evoked by speech stimuli and interactions with aging were examined.
Speech-evoked ABRs (S-ABRs) were recorded using a synthetic 40-msec /da/ stimulus from both ears of participants in two groups: normal-hearing younger adults (n = 19) and normal-hearing older adults (n = 18). Latencies and amplitude for S-ABR peaks representing neural responses to the onset and offset of the speech syllable as well as a sustained frequency following response to the vowel content were analyzed. The role of hearing threshold differences between groups and the reduced overall stimulus level on the S-ABR were also examined. In addition, click-evoked ABRs (C-ABRs) were obtained from all participants, and age-group differences in the neural response to both types of stimuli at the brain stem level were compared.
S-ABR latencies, amplitudes, and sustained response mean data were obtained for younger adults and older adults. Older adults were found to have significantly smaller C-ABRs with longer latencies, despite all latencies falling within normal limits. Older adults also had significantly smaller onset and offset responses for the S-ABR, with significantly delayed offset latencies in response to this synthetic consonant-vowel syllable. Many of the C-ABR and S-ABR variables were found to significantly correlate with high-frequency audiometric thresholds, and few of the group differences remained significant when this was taken into account. The remaining significant S-ABR effects were decreased amplitude at the onset and significantly delayed offset responses in the older group. These effects were different from those of simply decreasing the overall stimulus level, which caused significant shifts in latency across the entire S-ABR.
Results of this study partially supported the hypothesis of age-related differences in neural processing of speech at the brain stem level. There were significant delays in the timing of the offset portion of the S-ABR in older listeners compared with their younger counterparts, even after accounting for the differences in peripheral hearing threshold between groups. There were also significant reductions in amplitude of the S-ABR at the onset. These results are consistent with a reduction in neural synchrony in older adults to transient components of both speech and nonspeech sounds. However, sustained components of the S-ABR, which follow the harmonic components of the syllable, showed group differences but were not significant after adjusting for peripheral hearing loss, suggesting that they may be more affected by hearing sensitivity and other peripheral changes. These results support further investigation into the ability of the aging auditory system to encode temporal cues at the brain stem level, particularly the response to speech stimulus offset and its relationship to speech perception and temporal processing abilities.
本实验旨在评估在衰老的听觉系统中，脑干水平的言语特征神经编码是否发生改变。此外，还研究了最小外周听力损失对言语刺激诱发的听觉脑干反应（ABR）的影响以及与衰老的相互作用。
使用来自参与者双耳的合成 40 毫秒 /da/ 刺激记录言语诱发 ABR（S-ABR），参与者分为两组：听力正常的年轻成年人（n=19）和听力正常的老年成年人（n=18）。分析 S-ABR 峰值的潜伏期和幅度，这些峰值代表言语音节起始和结束时的神经反应以及元音内容的持续频率跟随反应。还检查了组间听力阈值差异以及刺激整体水平降低对 S-ABR 的作用。此外，从所有参与者中获得了点击诱发 ABR（C-ABR），并比较了两种类型的刺激在脑干水平对年轻和老年成年人的神经反应的年龄组差异。
为年轻成年人和老年成年人获得了 S-ABR 潜伏期、幅度和持续反应平均值数据。尽管所有潜伏期均在正常范围内，但老年成年人的 C-ABR 潜伏期明显较长，幅度明显较小。老年成年人的 S-ABR 起始和结束反应也明显较小，对这个合成辅音-元音音节的结束潜伏期明显延迟。许多 C-ABR 和 S-ABR 变量与高频听力阈值显著相关，当考虑到这一点时，组间差异中的许多仍然具有统计学意义。S-ABR 仍然具有统计学意义的显著影响是在老年组中起始时的幅度减小和结束时的潜伏期明显延迟。这些影响与仅仅降低整体刺激水平不同，后者导致整个 S-ABR 的潜伏期发生显著变化。
本研究的结果部分支持了在脑干水平上言语处理存在与年龄相关的神经加工差异的假设。与年轻组相比，老年组的 S-ABR 结束部分的时间明显延迟，即使考虑了组间外周听力阈值的差异也是如此。S-ABR 的起始时的幅度也明显减小。这些结果与老年人对言语和非言语声音的瞬态成分的神经同步性降低一致。然而，S-ABR 的持续成分，跟随音节的谐波成分，显示出组间差异，但在调整外周听力损失后没有统计学意义，这表明它们可能更受听力灵敏度和其他外周变化的影响。这些结果支持进一步研究衰老听觉系统在脑干水平上对时间线索的编码能力，特别是对言语刺激结束的反应及其与言语感知和时间处理能力的关系。
