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two first principal components (PCA) of the correlation matrix, emphasizing the
separation between the different groups. Following hypothermia we again see the
same three motifs of SBEs with the same spatio-temporal pattern. Since the
spatio-temporal patterns of SBEs are strongly related to the network connectivity,
this last result may indicate that the neuronal connections did not change
significantly during the 20 hours of hypothermia. We also observed that the
separations between the SBE sub-classes in the PCA plots became more
pronounced after hypothermia application, with a narrower statistical distribution
of each cluster around its mean. Similar findings were observed in the two other
cultures.
All the above results clearly demonstrate that the network survived
hypothermia with little if any detectable changes to the underlying neural
network connectivity, though there was evidence of some cell death. The strong
inhibition of our network activity during hypothermia may support and may help
explain a recent experiment in which cooling was used to arrest epileptic seizures
in both human and animal experiments (Yang
et al.
2003). This cooling of the
brain might have reduced the neuronal activity while leaving the general
connectivity intact, as seen in our model.
In addition this model may be of use in the study of the effect of hypothermia
on memory. Our findings show that the network maintained its spatio-temporal
patterns before and after hypothermia. Each spatio-temporal pattern could be
regarded as representing a different active "state" of the network. The
maintenance of these patterns suggests the possibility that memory in neuronal
networks may be retained even following strong activity inhibition (Rubinsky
et al
., in press).
12.11. Summary
We have shown here analysis results on the observed activity in homogeneous
and structured cultured neuronal networks, with emphasis on the investigation of
the characteristics modes of network activity and on the expression of repeating
motifs and their relation to the network structure. It is evident that cultured
networks exhibit a rich spontaneous dynamical activity. The networks show
complex patterns in all aspects: In the developmental process from a collection of
isolated cells to a fully connected and functional network; In the spontaneous
emergence of bio-electrical activity; In the formation and structure of the
network topology; In the long-term characteristics of the network activity under
"steady state" conditions; And in the appearance of synchronized bursts.
