| Περίληψη: | Extraction of physiological and clinical information hidden in biosignals, such as cardiac and neural signals, is an important and fascinating field of research. Noninvasive assessment of the physiological parameters of a patient enables to study the physiology and pathophysiology of the investigated system, with minimal interference and inconvenience. This approach may also help to assess noninvasively the clinical condition of the patient.
The primary focus of this study is therefore to extend the arsenal of research tools for the noninvasive investigation of the neural and cardiac systems.
The approaches developed in this work concern two major directions:
The first direction relies on the analysis of cardiac and neural responses during hypoxia. Hypoxia-ischemia remains a great challenge to the researchers, since it triggers complex responses at different levels in the organism. The functional recovery depends on a number of factors among which the state of autonomic nervous system (ANS) regulation plays an important role. Two different applications were considered in this framework. The first application studied the effect of global ischemic preconditioning on the heart rate variability (HRV) response to the asphyxia insult. Using linear (time and frequency domain) and nonlinear (approximate entropy and parameters of Poincare plots) measures, we evaluated the dynamic time course of the HRV response to the asphyxia insult and the effect of preconditioning on the autonomic neurocardiac control. Our results show for the first time that global ischemic preconditioning influences the HRV response to the asphyxia injury. The neuroprotective effect of preconditioning translates into a faster recovery of the basal HRV and the autonomic modulation of the heart. For the preconditioned group, at about 90 min after the asphyxic insult, the autonomic neural balance (measured by LF/HF ratio) appears fully recovered.
Another application addressed the problem of phase synchronization analysis of EEG signals during monitoring of recovery process following brain injury episode. The concept of phase synchronization offers a new perspective on the understanding and quantification of the dynamical interactions established among coupled systems. In this thesis, we present a new approach for the identification of the degree of interaction between two complex dynamical systems from experimental data analysis. We use the empirical-mode-decomposition (EMD) technique to decompose the output signals into a number of elementary orthogonal modes with well defined instantaneous attributes (IMFs).
The second direction addressed the problem of correlations between anticipatory pursuit eye movements and the neural response in the Supplementary Eye Fields (SEF) of the Macaque monkey. Anticipatory pursuit is a smooth movement of the eye occurring before the appearance of an expected moving target. The expectation of the subject is based on a subjective estimation of the probability that the target will move in a given direction. Recently, it has been suggested that the SEF could play a role in using past experience to guide anticipatory pursuit. This hypothesis is currently being tested at the single neuron level. In the behaving monkey, it has been shown that electrical microstimulation in the SEF can facilitate smooth pursuit initiation towards a moving target, suggesting that activation of the SEF might change the internal gain of the smooth pursuit pathway. In this study, we favored anticipatory responses in monkeys by using a cognitive cue, which produces a different anticipatory pursuit response than the one observed in previous studies, based on repetition.
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