The neural correlates of amplitude of low-frequency fluctuation: a multimodal resting-state MEG and fMRI–EEG study

Abstract The amplitude of low-frequency fluctuation (ALFF) describes the regional intensity of spontaneous blood-oxygen-level-dependent signal in resting-state functional magnetic resonance imaging (fMRI). How the fMRI–ALFF relates to the amplitude in electrophysiological signals remains unclear. We here aimed to investigate the neural correlates of fMRI–ALFF by comparing the spatial difference of amplitude between the eyes-closed (EC) and eyes-open (EO) states from fMRI and magnetoencephalography (MEG), respectively. By synthesizing MEG signal into amplitude-based envelope time course, we first investigated 2 types of amplitude in MEG, meaning the amplitude of neural activities from delta to gamma (i.e. MEG–amplitude) and the amplitude of their low-frequency modulation at the fMRI range (i.e. MEG–ALFF). We observed that the MEG–ALFF in EC was increased at parietal sensors, ranging from alpha to beta; whereas the MEG–amplitude in EC was increased at the occipital sensors in alpha. Source-level analysis revealed that the increased MEG–ALFF in the sensorimotor cortex overlapped with the most reliable EC–EO differences observed in fMRI at slow-3 (0.073–0.198 Hz), and these differences were more significant after global mean standardization. Taken together, our results support that (i) the amplitude at 2 timescales in MEG reflect distinct physiological information and that (ii) the fMRI–ALFF may relate to the ALFF in neural activity.