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Fig. 12.7. Identifying clusters of SBEs with similar activation patterns. (A) Similarity matrix
between 300 consecutive bursts in a culture with an architecture of four interconnected clusters.
The resulting matrix shows no special features that depend on the temporal ordering of the SBEs.
The color-coding for (A), (B), and (C) is according to the scale on the far-right. (B) The same
matrix shown in (A), after re-ordering of the rows and columns according to a dendrogram
algorithm. The resulting new matrix reveals a clear division of the data into blocks of square areas
along the diagonal containing high similarity values, separated by lower values between blocks.
Five independent motifs were identified in the matrix, marked by leaning "U" symbols on the right
of the image. (C) Similarity values between 20 bursts of each motif and the mean representation of
each motif. The matrix clearly shows the high similarity within each motif, relative to the low
values of similarity between motifs. (D) Raster plot of the activation of each mean SBE
representing each of the 5 motifs identified in (B). The raster plots show only the mean relative
timing of the first spike of each neuron. Neurons are grouped according to their physical location in
the 4 clusters of the network architecture. (E) Raster plot of the SBEs according to the time of
appearance in the recording, and sorted according to the identified motifs. Each bar marks the
timing of each burst (bin=10 min.). Each of the 300 bursts was associated with one of the 5 motifs
identified in (B) according to its maximal similarity. According to the image, it appears that the
expression of the motifs is in a random, nearly uniform, temporal manner. Figure taken with
permission from Raichman and Ben-Jacob, 2008.
12.10. Network under Hypothermia
The fact that cultured networks exhibit repeating motifs may help us in
monitoring changes taking place within the network. For example, the number of
motifs is related to the large scale structure, and an elimination of a motif might
hint at some process of deterioration. Another factor is the activation path of each
motif, which may be affected as well by external or internal processes. As an
example of using network motifs as a tool for monitoring network reaction to
stimuli, we conducted several experiments in which hypothermia was applied on
the culture. Hypothermia of the brain is commonly used to reduce metabolism for
