Brain Activity at Rest: a Multi-scale Hierarchical Functional Organization. Running Head: Functional Brain Organization at Rest 3 4

23 Spontaneous brain activity was mapped with fMRI in a sample of 180 subjects while in a conscious 24 resting-state condition. Using independent component analysis (ICA) of each individual FMRI signal, 25 and classification of the ICA-defined components across subjects, a set of 23 resting-state networks 26 (RNs) was identified. Functional connectivity between each pair of RNs was assessed using temporal 27 correlation analyses in the 0.01-0.1 Hertz frequency band, and the corresponding set of correlation 28 coefficients was used to obtain a hierarchical clustering of the 23 RNs. At the highest hierarchical 29 level, we found two anti-correlated systems in charge of intrinsic and extrinsic processing, 30 respectively. At a lower level, the intrinsic system appears to be partitioned in three modules that 31 subserve generation of spontaneous thoughts (M1a, default mode), inner maintenance and 32 manipulation of information (M1b), and cognitive control and switching activity (M1c), respectively. The 33 extrinsic system was found to be made of two distinct modules one including primary somatosensory 34 and auditory areas and the dorsal attentional network (M2a), the other encompassing the visual areas 35 (M2b). Functional connectivity analyses revealed that M1b played a central role in the functioning of 36 the intrinsic system while M1c seems to mediate exchange of information between the intrinsic and 37 extrinsic systems. 38 39