Data from: The music of silence. Part I: Responses to musical imagery encode melodic expectations and acoustics

Musical imagery is the voluntary internal hearing of music in the mind without the need for physical action or external stimulation. Numerous studies have already revealed brain areas activated during imagery. However, it remains unclear to what extent imagined music responses preserve the detailed temporal dynamics of the acoustic stimulus envelope and, crucially, whether melodic expectations play any role in modulating responses to imagined music, as they prominently do during listening. These modulations are important as they reflect aspects of the human musical experience, such as its acquisition, engagement, and enjoyment. This study explored the nature of these modulations in imagined music based on EEG recordings from 21 professional musicians (6 females and 15 males). Regression analyses were conducted to demonstrate that imagined neural signals can be predicted accurately, similarly to the listening task, and were sufficiently robust to allow for accurate identification of the imagined musical piece from the EEG. In doing so, our results indicate that imagery and listening tasks elicited an overlapping but distinctive topography of neural responses to sound acoustics, which is in line with previous fMRI literature. Melodic expectation, however, evoked very similar frontal spatial activation in both conditions, suggesting that they are supported by the same underlying mechanisms. Finally, neural responses induced by imagery exhibited a specific transformation from the listening condition, which primarily included a relative delay and a polarity inversion of the response. This transformation demonstrates the top-down predictive nature of the expectation mechanisms arising during both listening and imagery.

Methods
  All participants were chosen to be very well-trained musicians and were all professionals or students at Conservatoire National Supérieur de Musique (CNSM) in Paris. They were given the musical score of the four stimuli in a one-page score and could practice on the piano for about 35 minutes. The experimenter checked their practice and verified that there were no mistakes in the execution. After practice, participants were asked to sing the four pieces in the booth with the tactile metronome, the sound was recorded in order to check their accuracy offline.

  The experiment consisted of a single session with 88 trials. For each condition (listening and imagery) each of the four melodies was repeated 11 times. The trial order was shuffled both in terms of musical pieces and conditions. In the listening condition, participants were asked to passively listen to the stimuli while reading the musical score. For the imagery condition, they were asked to imagine the melody in sync with the tactile metronome as precisely as they could.

    Four melodies from the corpus of Bach chorals were selected for this study (BWV 349, BWV 291, BWV354, BWV 271). All chorals use similar compositional principles: the composer takes a well-known melody from a Lutheran hymn (cantus firmus) and harmonizes three lower parts (alto, tenor, and bass) accompanying the initial melody on soprano, these cantus firmi were usually written during the Renaissance era. Our analysis only uses monophonic melodies, we therefore only use these cantus firmi as stimuli for our experiment, original keys were kept. The chosen melodies follow the same grammatical structures and show very similar melodic and rhythmic patterns. Participants were asked to listen to and imagine these stimuli at 100 bpm (about 30 seconds each). The audio versions were synthesized using a Fender Rhodes simulation software (Neo-Soul Keys). The onset times and pitch values of the notes were extracted from the midi files that were precisely aligned with the audio versions presented during the experiment.

The pre-processing has been made with filtering between 0.1Hz and 30Hz using Butterworth zero-phase filters and down-sampled to 64 Hz. Data were re-referenced to the average of all 64 channels and bad channels were removed and interpolated using spherical spline interpolation. The stimuli onsets that were closer than 500ms from a metronome beat were removes.

音乐意象（Musical imagery）指个体无需肢体动作或外部刺激，在脑海中主动内化聆听音乐的体验。过往诸多研究已明确了音乐意象过程中激活的脑区，但目前仍存在两个关键待解问题：其一，被唤起的音乐神经响应在多大程度上保留了声学刺激包络的精细时间动态特性；其二，正如音乐聆听过程中显著存在的那样，旋律预期是否会调节音乐意象的神经响应。此类调节机制具有重要研究价值，因其反映了人类音乐体验的多个维度，包括音乐习得、投入程度与愉悦感。本研究基于21名专业音乐家（6名女性、15名男性）的脑电图（Electroencephalogram, EEG）记录，探究了音乐意象过程中的此类调节机制。通过回归分析，本研究证实：与音乐聆听任务类似，音乐意象的神经信号可被精准预测；且该信号具备足够鲁棒性，能够从脑电图中准确识别出被意象的音乐作品。研究结果表明，音乐意象与音乐聆听任务所引发的神经响应在声学刺激的地形分布上存在重叠，但又各具特征，这与既往功能磁共振成像（functional magnetic resonance imaging, fMRI）相关研究结论一致。值得注意的是，两种任务条件下，旋律预期均诱发了极为相似的额叶空间激活模式，提示二者共享底层的神经机制。最后，音乐意象引发的神经响应相较于聆听任务存在特异性转换，主要体现为响应的相对延迟与极性反转。该转换特性证实了音乐聆听与意象过程中预期机制的自上而下预测本质。

### 方法
所有参与者均为训练精良的专业音乐人，均为巴黎国立高等音乐学院（Conservatoire National Supérieur de Musique, CNSM）的在校生或教职员工。实验人员为每名参与者提供了一页纸的4段实验刺激乐谱，参与者可在钢琴上练习约35分钟。实验人员会检查练习情况，并确认演奏无差错。练习结束后，参与者需在测试间配合触觉节拍器演唱这4段乐曲，演唱过程将被录制，以便后续离线核查演唱准确性。

本实验为单次实验会话，共包含88个试次。两种任务条件（音乐聆听与音乐意象）下，4段旋律各自重复呈现11次。试次顺序在乐曲类型与任务条件两方面均进行了随机打乱。在音乐聆听条件下，要求参与者在阅读乐谱的同时被动聆听刺激音频；在音乐意象条件下，要求参与者尽可能精准地配合触觉节拍器在脑海中意象该段旋律。

本研究选取了巴赫圣咏曲集中的4段旋律，分别为BWV 349、BWV 291、BWV 354、BWV 271。所有圣咏曲均遵循相似的创作原则：作曲家以文艺复兴时期创作的路德宗圣咏知名旋律（固定旋律，cantus firmus）为基础，为高音声部（女高音）的初始旋律配和声，创作3个低声部（女低音、男高音与男低音）。本分析仅使用单声部旋律，因此仅选取上述固定旋律作为实验刺激，且保留了原调。所选旋律遵循相同的结构规则，旋律与节奏模式均极为相似。参与者被要求以100拍每分钟（BPM）的速度聆听与意象这些刺激，每段时长约30秒。音频版本通过芬达·罗兹（Fender Rhodes）模拟软件「新灵魂键盘（Neo-Soul Keys）」合成。音符的起始时间与音高值均从乐器数字接口（Musical Instrument Digital Interface, MIDI）文件中提取，该文件与实验中呈现的音频版本精准对齐。

数据预处理环节采用巴特沃斯零相位滤波器进行0.1Hz至30Hz的带通滤波，并将采样率降采样至64Hz。数据以全部64个通道的平均值作为参考进行重参考，移除坏通道并使用球面样条插值法对坏通道进行插值补全。移除了距离节拍器节拍不足500ms的刺激起始点。