student profile: Mr Zobaer Zobaer


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Thesis work

Thesis title: Physiology-Based Modeling of Sleep and Wake Phenomena in the Human Brain

Supervisors: Sveta POSTNOVA , Peter ROBINSON , Cliff KERR

Thesis abstract:

The human brain controls most of the physiological functions of the body, and yet the relevant mechanisms and dynamics are still not fully understood. The brain is a complex system with connections of interacting subsystems, feedback loops operating at different timescales. In this thesis, the focus is on the brain dynamics of sleep-wake cycles and related phenomena. In particular, physiology based mathematical modeling is used to explain two specific sleep-wake phenomena: (i) the electrical activity of the brain during sleep, and (ii) the effects of light on the daily (circadian) rhythms.

A general overview of the thesis is given in Chap.~1, to provide the physiological background of the relevant brain dynamics; the electrical activity of the brain during sleep such as the so-called K-complexes (KCs) with spindles and evoked response potentials (ERPs); the phenomena of the circadian rhythm maintained in daily rhythms of sleep and wake. Modeling of the relevant brain structures, including corticothalamic system and the master circadian clock in the suprachiasmatic nucleus (SCN) are presented. The corticothalamic neural field model (with the cortex and thalamus) successfully explains different sleep-wake stages and key electrophysiological phenomena using a three-dimensional parameter space which also fits with the electroencephalography measuring the electrical brain activity on the scalp. The circadian pacemaker model successfully describes the circadian rhythm modulated by light exposure.

Chapter 2 discusses two EEG phenomena: first, spontaneous K-complexes (KCs), which are large-amplitude transient waveforms observed during sleep stage 2 with superposed 12--15 Hz spindle oscillations. Second is the evoked response potential (ERP) which is an electric potential measured on the scalp following a stimulus. It is shown here that both KCs and ERPs can be unified within a common theoretical framework using an established neural field theory (NFT) of the corticothalamic system. It is thus proposed that KCs can be considered and modeled as impulse responses. It is also argued that this may lead to better constraints on model parameters and motivates future work involving the automated fitting of the model ERPs to data.

Chapter 3 presents the model ERPs to the stability zone with its three-dimensional parameter space for normal (near-critical) states, and far from the critical states using the physiology-based corticothalamic neural field model presented in Chap.~2. The spectral responses for the model ERPs in abnormal states are found to be different from ERPs in normal states. Both theoretical time series and wavelet transform are used to characterize model ERPs with corresponding frequency and duration.

Chapter 4 studies the circadian dynamics in response to light of different spectral properties. Light is the strongest stimulus to affect the phase of the internal biological clock, which controls the timing of the physiological processes. In this chapter, a basic circadian model is improved to account for the effects of the light spectrum with the sensitivity function of the eyes. The model is tested to produce the circadian responses for blue, green, and white lights. The model results also are shown to reproduce the circadian phase responses as found in the experiment with respect to changing the light illuminance and photon flux.

This thesis is summarized in Chap.~5 which suggests an overall framework for the two physiology-based models. The outcomes, applications, the possibilities for future work, and improvements of the models are also discussed.

This thesis also includes an appendix to define systemically the physical and physiological properties of lights and photoreceptors in the eyes. Those properties are unified in a common framework to minimize confusion and lack of the standardization in the literature with regard to definitions, symbols, units, and inter-conversions between the mathematical and biological models for circadian phase and light stimuli.

Selected publications

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Journals

  • Zobaer, M., Anderson, R., Kerr, C., Robinson, P., Wong, K., D'Rozario, A. (2017). K-complexes, spindles, and ERPs as impulse responses: unification via neural field theory. Biological Cybernetics, 111(2), 149-164. [More Information]

2017

  • Zobaer, M., Anderson, R., Kerr, C., Robinson, P., Wong, K., D'Rozario, A. (2017). K-complexes, spindles, and ERPs as impulse responses: unification via neural field theory. Biological Cybernetics, 111(2), 149-164. [More Information]

Note: This profile is for a student at the University of Sydney. Views presented here are not necessarily those of the University.