Integrating temporal and spatial scales: human structural network motifs across age and region of interest size.

Human brain networks can be characterized at different temporal or spatial scales given by the age of the subject or the spatial resolution of the neuroimaging method. Integration of data across scales can only be successful if the combined networks show a similar architecture. One way to compare networks is to look at spatial features, based on fiber length, and topological features of individual nodes where outlier nodes form single node motifs whose frequency yields a fingerprint of the network. Here, we observe how characteristic single node motifs change over age (12-23 years) and network size (414, 813, and 1615 nodes) for diffusion tensor imaging structural connectivity in healthy human subjects. First, we find the number and diversity of motifs in a network to be strongly correlated. Second, comparing different scales, the number and diversity of motifs varied across the temporal (subject age) and spatial (network resolution) scale: certain motifs might only occur at one spatial scale or for a certain age range. Third, regions of interest which show one motif at a lower resolution may show a range of motifs at a higher resolution which may or may not include the original motif at the lower resolution. Therefore, both the type and localization of motifs differ for different spatial resolutions. Our results also indicate that spatial resolution has a higher effect on topological measures whereas spatial measures, based on fiber lengths, remain more comparable between resolutions. Therefore, spatial resolution is crucial when comparing characteristic node fingerprints given by topological and spatial network features. As node motifs are based on topological and spatial properties of brain connectivity networks, these conclusions are also relevant to other studies using connectome analysis.

Biological versus technical variability in 2-D DIGE experiments with environmental bacteria.

Two-dimensional difference gel electrophoresis (2-D DIGE) allows for reliable quantification of global protein abundance changes. The threshold of significance for protein abundance changes depends on the experimental variation (biological and technical). This study estimates biological, technical and total variation inherent to 2-D DIGE analysis of environmental bacteria, using the model organisms "Aromatoleum aromaticum" EbN1 and Phaeobacter gallaeciensis DSM 17395. Of both bacteria the soluble proteomes were analyzed from replicate cultures. For strains EbN1 and DSM 17395, respectively, CV revealed a total variation of below 19 and 15%, an average technical variation of 12 and 7%, and an average biological variation of 18 and 17%. Multivariate analysis of variance confirmed domination of biological over technical variance to be significant in most cases. To visualize variances, the complex protein data have been plotted with a multidimensional scaling technique. Furthermore, comparison of different treatment groups (different substrate conditions) demonstrated that variability within groups is significantly smaller than differences caused by treatment.

Automatic network fingerprinting through single-node motifs.

Complex networks have been characterised by their specific connectivity patterns (network motifs), but their building blocks can also be identified and described by node-motifs-a combination of local network features. One technique to identify single node-motifs has been presented by Costa et al. (L. D. F. Costa, F. A. Rodrigues, C. C. Hilgetag, and M. Kaiser, Europhys. Lett., 87, 1, 2009). Here, we first suggest improvements to the method including how its parameters can be determined automatically. Such automatic routines make high-throughput studies of many networks feasible. Second, the new routines are validated in different network-series. Third, we provide an example of how the method can be used to analyse network time-series. In conclusion, we provide a robust method for systematically discovering and classifying characteristic nodes of a network. In contrast to classical motif analysis, our approach can identify individual components (here: nodes) that are specific to a network. Such special nodes, as hubs before, might be found to play critical roles in real-world networks.

Causal pattern recovery from neural spike train data using the Snap Shot Score.

We present a new approach to learning directed information flow networks from multi-channel spike train data. A novel scoring function, the Snap Shot Score, is used to assess potential networks with respect to their quality of causal explanation for the data. Additionally, we suggest a generic concept of plausibility in order to assess network learning techniques under partial observability conditions. Examples demonstrate the assessment of networks with the Snap Shot Score, and neural network simulations show its performance in complex situations with partial observability. We discuss the application of the new score to real data and indicate how it can be modified to suit other neural data types.