Kegler Mikolaj, Weissbart Hugo, Reichenbach Tobias
Department of Bioengineering, Centre for Neurotechnology, Imperial College London, London, United Kingdom.
Donders Centre for Cognitive Neuroimaging, Institute for Brain, Cognition and Behaviour, Radboud University, Nijmegen, Netherlands.
Front Neurosci. 2022 Jul 22;16:915744. doi: 10.3389/fnins.2022.915744. eCollection 2022.
Spoken language comprehension requires rapid and continuous integration of information, from lower-level acoustic to higher-level linguistic features. Much of this processing occurs in the cerebral cortex. Its neural activity exhibits, for instance, correlates of predictive processing, emerging at delays of a few 100 ms. However, the auditory pathways are also characterized by extensive feedback loops from higher-level cortical areas to lower-level ones as well as to subcortical structures. Early neural activity can therefore be influenced by higher-level cognitive processes, but it remains unclear whether such feedback contributes to linguistic processing. Here, we investigated early speech-evoked neural activity that emerges at the fundamental frequency. We analyzed EEG recordings obtained when subjects listened to a story read by a single speaker. We identified a response tracking the speaker's fundamental frequency that occurred at a delay of 11 ms, while another response elicited by the high-frequency modulation of the envelope of higher harmonics exhibited a larger magnitude and longer latency of about 18 ms with an additional significant component at around 40 ms. Notably, while the earlier components of the response likely originate from the subcortical structures, the latter presumably involves contributions from cortical regions. Subsequently, we determined the magnitude of these early neural responses for each individual word in the story. We then quantified the context-independent frequency of each word and used a language model to compute context-dependent word surprisal and precision. The word surprisal represented how predictable a word is, given the previous context, and the word precision reflected the confidence about predicting the next word from the past context. We found that the word-level neural responses at the fundamental frequency were predominantly influenced by the acoustic features: the average fundamental frequency and its variability. Amongst the linguistic features, only context-independent word frequency showed a weak but significant modulation of the neural response to the high-frequency envelope modulation. Our results show that the early neural response at the fundamental frequency is already influenced by acoustic as well as linguistic information, suggesting top-down modulation of this neural response.
口语理解需要快速且持续地整合信息，从低级的声学特征到高级的语言特征。这种处理过程大部分发生在大脑皮层。例如，其神经活动表现出与预测处理相关，在几百毫秒的延迟后出现。然而，听觉通路的特征还包括从高级皮层区域到低级区域以及皮层下结构的广泛反馈回路。因此，早期神经活动可能会受到高级认知过程的影响，但尚不清楚这种反馈是否有助于语言处理。在此，我们研究了在基频处出现的早期言语诱发神经活动。我们分析了受试者聆听由单一说话者朗读故事时获得的脑电图记录。我们识别出一个跟踪说话者基频的反应，该反应在11毫秒的延迟时出现，而由高次谐波包络的高频调制引发的另一个反应具有更大的幅度和更长的潜伏期，约为18毫秒，在40毫秒左右还有一个额外的显著成分。值得注意的是，虽然反应的早期成分可能起源于皮层下结构，但后者大概涉及皮层区域的贡献。随后，我们确定了故事中每个单词的这些早期神经反应的幅度。然后，我们量化了每个单词的上下文无关频率，并使用语言模型来计算上下文相关的单词意外度和精度。单词意外度表示在给定先前上下文的情况下一个单词的可预测程度，而单词精度反映了根据过去上下文预测下一个单词的信心。我们发现，基频处的单词级神经反应主要受声学特征影响：平均基频及其变异性。在语言特征中，只有上下文无关单词频率对高频包络调制的神经反应表现出微弱但显著的调制。我们的结果表明，基频处的早期神经反应已经受到声学和语言信息的影响，这表明该神经反应存在自上而下的调制。
