Dynamic organization of neural networks during cognition

Spontaneous brain activity measured with functional magnetic resonance imaging (fMRI) is organized in large-scale spatio-temporal structures or resting state networks (RSNs). The topography of RSNs resembles that of task networks (Smith et al., 2009) suggesting that RSNs may represent spatiotemporal "priors" for task activation (Raichle et al., 2011). However, recent fMRI and Magnetoencephalography (MEG) studies have shown that paying attention to location in space reorganizes RSNs (Spadone et al., 2015). Moreover, during natural vision (i.e., watching a movie), within-network band-limited power (BLP) time series correlation in the α band, typical of the resting state, is suppressed in parallel with an enhancement of between-network correlation in β, θ and γ bands (Betti et al., 2013). These findings suggest an alternative hypothesis: that RSNs reflect "idling" cortical circuits that must be suppressed to facilitate task specific synchronization. In a recent study (Betti et al., submitted), we used MEG to measure static and transient regional and network-level interaction in α and β-band limited power (BLP) in three conditions: visual fixation (rest), viewing of movie clips (natural vision), and time-scrambled versions of the same clips (scrambled vision). As compared to rest, we observed in both movie conditions a robust decrement of α-, but not β-BLP connectivity strength with a relative preservation of within-network topology. Graph analyses showed that the number of connections, especially between-networks, decreased in the α-band, but did not change in the β-band during natural vision. A core network of hub regions (i.e., regions that are strongly connected with other regions, and that is therefore 'central' in the network) remained stable during rest and movie conditions, especially in the β-band. At short time scales, these highly central nodes behave as temporally overlapping transient hubs that alternate periods of strong and weak centrality. While these joint hub activity fluctuations were altered in the α-band between rest and natural vision, they remained highly similar in the β-band. We conclude that the temporal and spatial configurations of cortical hubs in the β-band are highly similar between rest and natural stimulation, possibly reflecting temporal priors of the slow-varying temporal structure of the natural environment. These results support the hypothesis that intrinsic (beta rhythm) connectivity possibly provides advanced information about incoming stimuli. Finally, in order to test the hypothesis that the intrinsic brain connectivity preserves and maintains an internal model of the environment, a series of fMRI and MEG studies will be discussed in the context of a 5-years research project that aims to test how the synergic activity of the body and the environment shapes behavior and neural activity.