Auditory cortical activity entrains to speech rhythms and has been proposed as a mechanism for online speech processing. In particular, neural activity in the theta frequency band (4-8 Hz) tracks the onset of syllables which may aid the parsing of a speech stream. Similarly, cortical activity in the delta band (1-4 Hz) entrains to the onset of words in natural speech and has been found to encode both syntactic as well as semantic information. Such neural entrainment to speech rhythms is not merely an epiphenomenon of other neural processes, but plays a functional role in speech processing: modulating the neural entrainment through transcranial alternating current stimulation influences the speech-related neural activity and modulates the comprehension of degraded speech. However, the distinct functional contributions of the delta- and of the theta-band entrainment to the modulation of speech comprehension have not yet been investigated. Here we use transcranial alternating current stimulation with waveforms derived from the speech envelope and filtered in the delta and theta frequency bands to alter cortical entrainment in both bands separately. We find that transcranial alternating current stimulation in the theta band but not in the delta band impacts speech comprehension. Moreover, we find that transcranial alternating current stimulation with the theta-band portion of the speech envelope can improve speech-in-noise comprehension beyond sham stimulation. Our results show a distinct contribution of the theta- but not of the delta-band stimulation to the modulation of speech comprehension. In addition, our findings open up a potential avenue of enhancing the comprehension of speech in noise.

中文翻译：

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Theta 波段而非 delta 波段的经颅交流电刺激可调节噪声中自然语音的理解

听觉皮层活动会影响语音节奏，并已被提议作为在线语音处理的一种机制。特别是，theta 频带 (4-8 Hz) 中的神经活动会跟踪音节的开始，这可能有助于语音流的解析。类似地，delta 频带 (1-4 Hz) 中的皮层活动会导致自然语音中单词的出现，并且已发现对句法和语义信息进行编码。这种语音节律的神经夹带不仅是其他神经过程的附带现象，而且在语音处理中起着功能性作用：通过经颅交流电刺激调节神经夹带会影响与语音相关的神经活动，并调节对退化语音的理解。然而，尚未研究 delta 和 theta 波段夹带对语音理解调制的不同功能贡献。在这里，我们使用经颅交流电刺激，其波形源自语音包络并在 delta 和 theta 频带中过滤，以分别改变两个频带中的皮质夹带。我们发现经颅交流电刺激在 theta 波段而不是在 delta 波段会影响语音理解。此外，我们发现使用语音包络的 theta 波段部分的经颅交流电刺激可以改善噪音中的语音理解，而不是假刺激。我们的结果显示了θ带刺激对语音理解调制的明显贡献，但不是增量带刺激。此外，