The Music of Silence. Part I: Responses to Musical Imagery Encode Melodic Expectations and Acoustics

Abstract

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.

Accurate Decoding of Imagined and Heard Melodies

“…Twenty-one healthy individuals (6 female, aged between 17 and 35, median = 25) participated in the EEG experiment, which was conducted as part of a previous study (Di Liberto et al, 2021a;Marion et al, 2021). All participants were highly trained musicians with music degree.…”

“…EEG signals were digitally filtered between 0.1 and 30 Hz using a Butterworth zero-phase filter (low-and high-pass filters both with order 2 and implemented with the function filtfilt), and down-sampled to 64 Hz. The analyses were also conducted on EEG data filtered between 1 and 30 Hz (Butterworth zero-phase filters with order 2) to assess the impact of low-frequency EEG < 1 Hz, which was previously suggested to encode a relevant portion of imagery EEG signals (Marion et al, 2021). EEG channels with a variance exceeding three times that of the surrounding ones were replaced by an estimate calculated using spherical spline interpolation.…”

“…The linear decoding model takes this into account by including multiple time-lags between stimulus and neural signal, providing us with model weights that include both space (scalp topographies) and time (music-EEG latencies) dimensions. A time-lag window of 0-350 ms was selected as such latencies were expected to capture the relevant TRF response based on previous work (e.g., Freitas et al, 2018;Jagiello et al, 2019;Di Liberto et al, 2020, 2021aMarion et al, 2021).…”

“…Recent work demonstrated that EEG signals recorded during music listening (Di Liberto et al, 2020) and imagery (Marion et al, 2021) encode both the sound envelope and higher-order neural activity reflecting melodic predictions. Here we devised a framework to capture this rich spectrum of signals including both low-and higher-level neural processes, with the goal of accurately decoding melodies from EEG.…”

“…Here we devised a framework to capture this rich spectrum of signals including both low-and higher-level neural processes, with the goal of accurately decoding melodies from EEG. We present a re-analysis of a publicly available dataset previously published by our team (Di Liberto et al, 2021a;Marion et al, 2021). EEG signals were recorded as participants listened or imagined Bach melodies.…”