Laboratoire de Cartographie fonctionnelle du Cerveau, UNI - ULB Neuroscience Institute, Université libre de Bruxelles (ULB), 808 Lennik Street, Brussels 1070, Belgium.
Sir Peter Mansfield Imaging Centre, School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom.
Neuroimage. 2021 Jun;233:117969. doi: 10.1016/j.neuroimage.2021.117969. Epub 2021 Mar 17.
During continuous speech listening, brain activity tracks speech rhythmicity at frequencies matching with the repetition rate of phrases (0.2-1.5 Hz), words (2-4 Hz) and syllables (4-8 Hz). Here, we evaluated the applicability of wearable MEG based on optically-pumped magnetometers (OPMs) to measure such cortical tracking of speech (CTS). Measuring CTS with OPMs is a priori challenging given the complications associated with OPM measurements at frequencies below 4 Hz, due to increased intrinsic interference and head movement artifacts. Still, this represents an important development as OPM-MEG provides lifespan compliance and substantially improved spatial resolution compared with classical MEG. In this study, four healthy right-handed adults listened to continuous speech for 9 min. The radial component of the magnetic field was recorded simultaneously with 45-46 OPMs evenly covering the scalp surface and fixed to an additively manufactured helmet which fitted all 4 participants. We estimated CTS with reconstruction accuracy and coherence, and determined the number of dominant principal components (PCs) to remove from the data (as a preprocessing step) for optimal estimation. We also identified the dominant source of CTS using a minimum norm estimate. CTS estimated with reconstruction accuracy and coherence was significant in all 4 participants at phrasal and word rates, and in 3 participants (reconstruction accuracy) or 2 (coherence) at syllabic rate. Overall, close-to-optimal CTS estimation was obtained when the 3 (reconstruction accuracy) or 10 (coherence) first PCs were removed from the data. Importantly, values of reconstruction accuracy (~0.4 for 0.2-1.5-Hz CTS and ~0.1 for 2-8-Hz CTS) were remarkably close to those previously reported in classical MEG studies. Finally, source reconstruction localized the main sources of CTS to bilateral auditory cortices. In conclusion, t his study demonstrates that OPMs can be used for the purpose of CTS assessment. This finding opens new research avenues to unravel the neural network involved in CTS across the lifespan and potential alterations in, e.g., language developmental disorders. Data also suggest that OPMs are generally suitable for recording neural activity at frequencies below 4 Hz provided PCA is used as a preprocessing step; 0.2-1.5-Hz being the lowest frequency range successfully investigated here.
在连续语音聆听过程中，大脑活动以与短语（0.2-1.5 Hz）、单词（2-4 Hz）和音节（4-8 Hz）重复率相匹配的频率跟踪语音的节奏。在这里，我们评估了基于光泵磁强计（OPM）的可穿戴式 MEG 测量这种皮质跟踪语音（CTS）的适用性。由于 OPM 测量在 4 Hz 以下时固有干扰和头部运动伪影增加，因此测量 CTS 从理论上具有挑战性。尽管如此，这仍然是一个重要的发展，因为与经典 MEG 相比，OPM-MEG 提供了寿命合规性和大大提高的空间分辨率。在这项研究中，四名健康的右利手成年人连续听了 9 分钟的语言。同时记录磁场的径向分量，使用 45-46 个 OPM 均匀覆盖头皮表面，并固定在一个添加制造的头盔上，该头盔适合所有 4 名参与者。我们使用重建准确性和相干性来估计 CTS，并确定要从数据中去除的主要成分（PC）的数量（作为预处理步骤）以进行最佳估计。我们还使用最小范数估计确定了 CTS 的主要来源。在所有 4 名参与者中，在短语和单词速率以及 3 名参与者（重建准确性）或 2 名（相干性）在音节速率下，均可以显著地估计 CTS。总体而言，当从数据中去除 3 个（重建准确性）或 10 个（相干性）第一 PC 时，几乎可以获得最佳的 CTS 估计。重要的是，重建准确性的值（0.2-1.5 Hz CTS 约为 0.4，2-8 Hz CTS 约为 0.1）与以前在经典 MEG 研究中报道的值非常接近。最后，源重建将 CTS 的主要源定位到双侧听觉皮层。总之，这项研究表明 OPM 可用于 CTS 评估。这一发现为解开涉及皮质跟踪语音的神经网络开辟了新的研究途径，并为语言发育障碍等潜在改变提供了新的研究途径。数据还表明，只要使用主成分分析（PCA）作为预处理步骤，OPM 通常适合记录 4 Hz 以下的神经活动；0.2-1.5 Hz 是这里成功研究的最低频率范围。
