Interdependency between heart rate variability and sleep EEG: linear/non-linear?

Objective: To investigate whether the interdependency between heart rate variability (HRV) and sleep electroencephalogram (EEG) power spectra is linear or non-linear. Methods: Heart rate and sleep EEG signals were recorded in 8 healthy young men. Spectral analysis was applied to electrocardiogram and EEG sleep recordings. Synchronization likelihood was computed over the first 3 non-rapid eye movement–rapid eye movement sleep cycles between normalized high frequency of RR intervals (RRI) and all electroencephalographic frequency bands. Comparison to surrogate data of different types was used to attest statistical significance of the coupling between RRI and EEG power bands and its linear or non-linear character. Results: Synchronization likelihood values were statistically greater than univariate surrogate synchronization for all sleep bands both at the individual and the group levels. With reference to multivariate surrogates, synchronization values were statistically greater at the group level and, in a majority of cases, for individual comparison except for σ and β bands. Conclusions: While all electroencephalographic power bands are linked to normalized high frequency RRI band, this interdependency is non-linear for δ, θ and α bands. Significance: Non-linear description is required to capture the full interdependent dynamics of HRV and sleep EEG power bands.