What did we do?
Results show patterns of connectivity occurring over different timescales for both rsMEG and rsfMRI, as we would expect. The four nodes depicted for clarity include the left and right posterior cingulate cortex (PCC), and the left and right Medial Frontal Gyrus (MFG). This is the complete rsfMRI supra-adjacency matrix for subject P022, which consists of 36 layers (each is 45s long):
As we may expect, the diagonal of this supra-adjacency matrix shows the highest correlations, as these are the within-window connectivity patterns of the four nodes. However, we may also observe a variety of stronger connectivity between activity patterns of a specific node across different windows (layers).
The same goes for the following rsMEG supra-adjacency matrix of the same subject, consisting of 22 layers (windows) of 12 seconds each.
However, these matrices are large and difficult to read – multilayer graph properties will be able to assess dynamics of these massive datasets in a data-driven way. For now, in order to preliminarily assess congruence between unimodal multilayers, the following figures contain only 3 layers per modality (rsfMRI on the left, rsMEG on the right) for the same four nodes in P022.
We may observe several commonalities, such has the relatively high connectivity between the LPCC and RPCC (both part of the Default Mode Network), or the general lowering of interlayer connectivity between layers 1 and 3 as compared to 1 and 2, likely since these windows are further apart in time. However, we also see marked differences between rsMEG and rsfMRI multilayers, particularly with respect to intralayer connectivity between the LMFG and RMFG, which is very high in rsfMRI and very low in rsMEG. These types of modality-dependent connectivities may strongly inform the multimodal multilayer properties that TIMNET aims to pick up on, while separate interpretation of these networks could yield contrasting and uninterpretable results.
Another examplar set of rsfMRI (left) and rsMEG (right) multilayers is shown for subject P033:
In order to begin to assess whether these multilayers indeed represent an individual signature of brain organization (and ultimately cognition), we calculated multilayer efficiency (i.e. the weight of all shortest paths in the network, taking both intralayer and interlayer connections into account) for each modality in each subject. When ranking these values, we notice that 3/5 subjects have the same rank for their rsMEG and rsfMRI multilayer.
Pilot part 3: Constructing a two-layer network of rsMEG and rsfMRI