Abstract
Exploration of transient Granger causal interactions in neural sources of electrophysiological activities provides deeper insights into brain information processing mechanisms. However, the underlying neural patterns are confounded by time-dependent dynamics, non-stationarity and observational noise contamination. Here we investigate transient Granger causal interactions using source time-series of somatosensory evoked magnetoencephalographic (MEG) elicited by air puff stimulation of right index finger and recorded using 306-channel MEG from 21 healthy subjects. A new time-varying connectivity approach, combining renormalised partial directed coherence with state space modelling, is employed to estimate fast changing information flow among the sources. Source analysis confirmed that somatosensory evoked MEG was mainly generated from the contralateral primary somatosensory cortex (SI) and bilateral secondary somatosensory cortices (SII). Transient Granger causality shows a serial processing of somatosensory information, 1) from contralateral SI to contralateral SII, 2) from contralateral SI to ipsilateral SII, 3) from contralateral SII to contralateral SI, and 4) from contralateral SII to ipsilateral SII. These results are consistent with established anatomical connectivity between somatosensory regions and previous source modeling results, thereby providing empirical validation of the time-varying connectivity analysis. We argue that the suggested approach provides novel information regarding transient cortical dynamic connectivity, which previous approaches could not assess.
Highlights
Among cortices involved in the somatosensory information processing is not well understood, especially the relationship between the contralateral somatosensory cortices and ipsilateral SII9,27
The source locations and single-trial waveforms are estimated by Minimum Norm Estimation (MNE) to mitigate the field spread in MEG recordings, which complicates the interpretation of cortical connectivity patterns at the sensor level
We found that the evoked response to right index finger stimulation is most prominent in left central channels consistent with activation from postcentral gyrus of the anterior parietal lobe, where primary and secondary somatosensory cortex (SI and SII) are located, and the averages over the other brain areas look like the non-phase locking activity in the background
Summary
Among cortices involved in the somatosensory information processing is not well understood, especially the relationship between the contralateral somatosensory cortices (both SI and SII) and ipsilateral SII9,27. Functional connectivity of neural networks is usually estimated by classical methods, such as correlation and coherence, based on time- or frequency domain analysis in sensor space. These functional connective methods do not provide a direction of information flow. The approach quantifies the direction and strength of time-dependent network connectivity without relying on any prior assumption about the nonlinear, non-stationary and stochastic brain signals[35,36]. This study demonstrates the effectiveness of the time-varying Granger causal connectivity to explore the brain effective connectivity in source space in MEG/EEG fields, which provides a new way to investigate the network interplay between stimulus-evoked cortical activations
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