Hjortkjaer Jens, Märcher-Rørsted Jonatan, Fuglsang Søren A, Dau Torsten
Hearing Systems Group, Department of Electrical Engineering, Technical University of Denmark, Ørsteds Plads, Building 352, Kgs., Lyngby, Denmark.
Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Hvidovre, Hvidovre, Denmark.
Eur J Neurosci. 2020 Mar;51(5):1279-1289. doi: 10.1111/ejn.13855. Epub 2018 Feb 16.
Neuronal oscillations are thought to play an important role in working memory (WM) and speech processing. Listening to speech in real-life situations is often cognitively demanding but it is unknown whether WM load influences how auditory cortical activity synchronizes to speech features. Here, we developed an auditory n-back paradigm to investigate cortical entrainment to speech envelope fluctuations under different degrees of WM load. We measured the electroencephalogram, pupil dilations and behavioural performance from 22 subjects listening to continuous speech with an embedded n-back task. The speech stimuli consisted of long spoken number sequences created to match natural speech in terms of sentence intonation, syllabic rate and phonetic content. To burden different WM functions during speech processing, listeners performed an n-back task on the speech sequences in different levels of background noise. Increasing WM load at higher n-back levels was associated with a decrease in posterior alpha power as well as increased pupil dilations. Frontal theta power increased at the start of the trial and increased additionally with higher n-back level. The observed alpha-theta power changes are consistent with visual n-back paradigms suggesting general oscillatory correlates of WM processing load. Speech entrainment was measured as a linear mapping between the envelope of the speech signal and low-frequency cortical activity (< 13 Hz). We found that increases in both types of WM load (background noise and n-back level) decreased cortical speech envelope entrainment. Although entrainment persisted under high load, our results suggest a top-down influence of WM processing on cortical speech entrainment.
神经元振荡被认为在工作记忆（WM）和言语处理中发挥着重要作用。在现实生活情境中听言语通常对认知要求较高，但目前尚不清楚工作记忆负荷是否会影响听觉皮层活动与言语特征的同步方式。在此，我们开发了一种听觉n-back范式，以研究在不同程度的工作记忆负荷下，皮层对言语包络波动的同步情况。我们测量了22名受试者在执行嵌入n-back任务的同时听连续言语时的脑电图、瞳孔扩张和行为表现。言语刺激由长的口语数字序列组成，这些序列在句子语调、音节速率和语音内容方面被设计成与自然言语相匹配。为了在言语处理过程中给不同的工作记忆功能增加负担，受试者在不同背景噪声水平下对言语序列执行n-back任务。随着n-back水平升高，工作记忆负荷增加，这与后部α波功率降低以及瞳孔扩张增加有关。额叶θ波功率在试验开始时增加，并随着n-back水平升高而进一步增加。观察到的α-θ波功率变化与视觉n-back范式一致，表明工作记忆处理负荷存在一般的振荡相关性。言语同步被测量为言语信号包络与低频皮层活动（<13Hz）之间的线性映射。我们发现，两种类型的工作记忆负荷（背景噪声和n-back水平）增加均会降低皮层对言语包络的同步。尽管在高负荷下同步仍然存在，但我们的结果表明工作记忆处理对皮层言语同步存在自上而下的影响。
