Abstract
The brain remains electrically and metabolically active during resting conditions. The low-frequency oscillations (LFO) of the blood oxygen level-dependent (BOLD) signal of functional magnetic resonance imaging (fMRI) coherent across distributed brain regions are known to exhibit features of this activity. However, these intrinsic oscillations may undergo dynamic changes in time scales of seconds to minutes during resting conditions. Here, using wavelet-transform based time-frequency analysis techniques, we investigated the dynamic nature of default-mode networks from intrinsic BOLD signals recorded from participants maintaining visual fixation during resting conditions. We focused on the default-mode network consisting of the posterior cingulate cortex (PCC), the medial prefrontal cortex (mPFC), left middle temporal cortex (LMTC) and left angular gyrus (LAG). The analysis of the spectral power and causal flow patterns revealed that the intrinsic LFO undergo significant dynamic changes over time. Dividing the frequency interval 0 to 0.25 Hz of LFO into four intervals slow-5 (0.01–0.027 Hz), slow-4 (0.027–0.073 Hz), slow-3 (0.073–0.198 Hz) and slow-2 (0.198–0.25 Hz), we further observed significant positive linear relationships of slow-4 in-out flow of network activity with slow-5 node activity, and slow-3 in-out flow of network activity with slow-4 node activity. The network activity associated with respiratory related frequency (slow-2) was found to have no relationship with the node activity in any of the frequency intervals. We found that the net causal flow towards a node in slow-3 band was correlated with the number of fibers, obtained from diffusion tensor imaging (DTI) data, from the other nodes connecting to that node. These findings imply that so-called resting state is not ‘entirely’ at rest, the higher frequency network activity flow can predict the lower frequency node activity, and the network activity flow can reflect underlying structural connectivity.
Highlights
The brain consists of a collection of anatomically distinct and functionally relevant networks of brain regions [1,2]
Without relying on the common temporal stationarity assumptions about blood oxygen level-dependent (BOLD) low-frequency oscillations (LFO), we investigated the temporal dynamics of several frequency bands of BOLD LFO in the default-mode network comprising of the posterior cingulate cortex (PCC), the medial prefrontal cortex, left middle temporal cortex (LMTC) and left angular gyrus (LAG)
We computed wavelet-based time-frequency spectral map as shown in figure 2. These results of wavelet power spectra confirm that the BOLD fluctuations occur at low frequency (,0.1 Hz) oscillations and reveal further that these fluctuations might be varying in their amplitudes over time
Summary
The brain consists of a collection of anatomically distinct and functionally relevant networks of brain regions [1,2]. It is a selforganizing dynamical system [3] with ongoing neural oscillations coherent across distributed brain regions during resting stateunder no explicit tasks or no external sensory stimulation [4,5,6]. The brain’s underlying structural connectivity determines the coherent neural activity. Recent neuroimaging studies provided evidence for the relationship between the underlying brain structural network (structure) and coherent oscillations (function) during resting conditions [7,8,9,10,11,12]. In this study, we evaluated how the frequency band-specific net information flow from a brain node correlates with the net anatomical connections to (or from) the node from BOLD fMRI and diffusion tensor imaging (DTI) data
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