School of Engineering, Trinity Centre for Biomedical Engineering and Trinity College Institute of Neuroscience, Trinity College Dublin, Dublin, Ireland.
Del Monte Institute for Neuroscience, Department of Neuroscience, Department of Biomedical Engineering, University of Rochester, Rochester, New York, USA.
Eur J Neurosci. 2022 Oct;56(8):5201-5214. doi: 10.1111/ejn.15805. Epub 2022 Sep 15.
Speech comprehension relies on the ability to understand words within a coherent context. Recent studies have attempted to obtain electrophysiological indices of this process by modelling how brain activity is affected by a word's semantic dissimilarity to preceding words. Although the resulting indices appear robust and are strongly modulated by attention, it remains possible that, rather than capturing the contextual understanding of words, they may actually reflect word-to-word changes in semantic content without the need for a narrative-level understanding on the part of the listener. To test this, we recorded electroencephalography from subjects who listened to speech presented in either its original, narrative form, or after scrambling the word order by varying amounts. This manipulation affected the ability of subjects to comprehend the speech narrative but not the ability to recognise individual words. Neural indices of semantic understanding and low-level acoustic processing were derived for each scrambling condition using the temporal response function. Signatures of semantic processing were observed when speech was unscrambled or minimally scrambled and subjects understood the speech. The same markers were absent for higher scrambling levels as speech comprehension dropped. In contrast, word recognition remained high and neural measures related to envelope tracking did not vary significantly across scrambling conditions. This supports the previous claim that electrophysiological indices based on the semantic dissimilarity of words to their context reflect a listener's understanding of those words relative to that context. It also highlights the relative insensitivity of neural measures of low-level speech processing to speech comprehension.
言语理解依赖于在连贯的上下文中理解单词的能力。最近的研究试图通过模拟大脑活动如何受到单词与前一个单词语义相似度的影响来获得这个过程的电生理指标。虽然得到的指标似乎很可靠，并且受到注意力的强烈调节，但仍有可能，这些指标不是捕捉单词的上下文理解，而是实际上反映了单词与单词之间的语义内容变化，而不需要听众进行叙事层面的理解。为了验证这一点，我们记录了受试者在听以其原始叙事形式或通过改变词序的方式呈现的言语时的脑电图。这种操作会影响受试者理解言语叙述的能力，但不会影响识别单个单词的能力。对于每个打乱顺序的条件，我们使用时间响应函数从听觉处理和语义理解的神经指标中得出。当言语未被打乱或仅被轻微打乱且受试者理解言语时，观察到语义处理的特征。随着言语理解能力的下降，较高的打乱水平则没有这些特征。相比之下，单词识别仍然很高，与包络跟踪相关的神经测量在打乱条件之间没有显著变化。这支持了之前的观点，即基于单词与其上下文之间的语义相似度的电生理指标反映了听者相对于该上下文理解这些单词的程度。它还突出了低水平言语处理的神经测量对言语理解的相对不敏感性。
