The spatial organization of nonlinear interactions between different brain regions
during the first NREM sleep stage is investigated. This is achieved via consideration
of four bipolar electrode derivations, Fp1F3, Fp2F4, O1P3, O2P4, which are used to compare
anterior and posterior interhemispheric interactions and left and right intrahemispheric
interactions. Nonlinear interdependence is detected via application of a previously written
algorithm, along with appropriately generated surrogate data sets. It is now well understood
that the output of neural systems does not scale linearly with inputs received and thus
the study of nonlinear interactions in EEG is crucial. This approach also offers significant
advantages over standard linear techniques, in that the strength, direction and topography
of the interdependencies can all be calculated and considered. Previous research has linked
delta activity during the first NREM sleep stage to performance on frontally-activating tasks
during wake. In the current paper, it is demonstrated that nonlinear mechanisms are the
driving force behind this delta activity. Furthermore, evidence is presented to suggest that
the ageing brain calls upon the right parietal region to assist the pre-frontal cortex. This is
highlighted by statistically significant differences in the rates of communication between the
left pre-frontal cortex and the right parietal region when comparing younger subjects (< 23
years) with older subjects (> 60 years). This assistance has been observed in brain imaging
studies of sleep deprived young adults, suggesting that similar mechanisms may play a role in
the event of healthy aging. Additionally, the contribution to the delta rhythm via nonlinear
mechanisms is observed to be greater in older subjects
