基于脑磁图记录的学龄前儿童听皮层语音加工能力研究

Human auditory cortex develops dramatically from 4 to 6 years old (preschool age). It is important to learn the ability of preschoolers’ auditory cortex in processing speech object, which helps us to find their developmental stage of auditory cortex and also possible abnormality. The cortical neural development during this period is featured by improvement in the speed and precision of neural conduction based on the synaptic density increase and axonal myelination, which is manifested in the improvement in the precise representation of time-varied sound information and its selective processing. We are going to study the auditory cortical processing of sound temporal information in preschool children using children-customized Magnetoencephalography (MEG) recording system and also their speech recognition under noisy reverberant conditions (behavioral measurement). Our key hypothesis is the representation of preschoolers’ auditory cortex to sound temporal information enhances across the preschool age, which is not as precise as that of adults but still correlated with their ability of speech recognition. The auditory cortical processing of slowly varying temporal envelope, more rapid temporal fine structure and speech under noisy reverberant conditions (simulated) will be addressed in our study. The current project will reveal how the auditory cortex of preschool children is different from that of the adults in processing sound temporal information and speech signal. Moreover, the results of our project are valuable in studying the brain mechanism of neural developmental disorders, i.e. the specific language disorder and autism, which are related to auditory temporal processing deficits and also helping their early screening and intervention.

人类听皮层在学龄前期（4-6岁）处于快速发育的关键时期，认识学龄前儿童听皮层语音加工特点对了解其听皮层的神经发育水平和可能的异常发育状况有重要意义。这一时期，听皮层发育的典型特征为在轴突髓鞘化和突触密度增加基础上的神经传导速度和精确性的提高，在功能上表现为对随时间快速波动的声音信息精准表达和选择性加工能力的增强。本项目拟使用儿童脑磁图仪考察学龄前儿童听皮层精准表达能力的发展特点及其与嘈杂混响环境下语音识别能力（行为测查）之间的功能联系。核心假设是，学龄前儿童对声音时间信息的精准表达能力与语音识别能力密切相关且随年龄增长而显著增强。研究重点是，学龄前儿童听皮层对声音时间包络、精细结构信息和模拟嘈杂混响条件下语音客体的加工能力。本项目不仅能够揭示学龄前儿童在皮层发育关键时期的语音加工特点，而且对与听觉时间信息加工缺陷有关的语言障碍和自闭症等神经发育疾病的脑机制研究及其早期诊断和干预有应用价值。

嘈杂环境中，听者不仅需要准确加工目标语音，而且需要利用一些线索克服环境中噪声的干扰。这种能力要求中枢听觉系统对声音随时间波动信息进行精准表征。中枢听觉系统中，听觉丘脑到听皮层的神经投射及其神经纤维的髓鞘化成熟较晚，最终成熟要到12岁左右。对听皮层尚未成熟的儿童，听觉时间信息加工能力很可能弱于成年人并且表现为噪声下语音加工的困难。本项目面向4-9岁典型发展儿童，重点探索了噪声下的语音加工特征，并基于脑磁图对大脑听觉时间加工特征进行了系统考察。噪声下的语音识别阈限行为测量结果显示，儿童在噪声掩蔽条件下的阈限显著高于成年人，并且在语句噪声掩蔽下的阈限也没有与成年人一样显著的低于语谱噪声掩蔽下的阈限。这说明儿童可能无法像成年人一样很好的加工语音中的时间信息和利用掩蔽语句中的静音时间间隙增强目标语音的识别（实验一）。项目完成的听觉时间加工能力脑磁测量结果显示，当听到两个连续快速呈现的纯音时（例如，时间间隔250 ms，接近正常语音音节速率），儿童听皮层对落后纯音的反应峰值与成年人无显著差别，但峰值潜伏期显著长于成年人（实验二）。对成年人，与领先纯音的频率不同时落后纯音诱发的听皮层反应峰值幅度显著高于与领先纯音的频率相同时落后纯音诱发的反应。但是，儿童的这种反应增强现象明显弱于成年人，说明儿童对特征改变的落后纯音敏感性较低（实验三）。旨在考察时间精细结构加工的双耳不相关片段测量结果显示，儿童在三种时间延迟条件下都没有像成年人一样表现出双耳不相关片段诱发的显著听皮层反应（实验四）。因此，项目结果揭示了儿童大脑听觉时间加工能力的三个重要特征：对快速呈现声音的加工速度慢于成年人，对快速呈现声音特征变化的敏感性弱于成年人，对时间精细结构信息加工的能力弱于成年人。本项目的研究结果不但对揭示典型发展儿童语音加工的发展特征具有重要意义，而且对建立语言障碍高危儿童的早期预警神经指标具有重要的应用价值。