Graduation Semester and Year




Document Type


Degree Name

Doctor of Philosophy in Biomedical Engineering



First Advisor

Hanli Liu


Despite breakthroughs in the field of neuroimaging, there remain many unanswered questions about brain functions and activities and their underlying physiological processes. In my dissertation research, I examined brain functions while human subjects experienced four vigilance states: (1) resting state with eyes open, (2) resting awake state with eyes closed, (3) sleep stage 1 and (4) sleep stage 2. Simultaneous dual modality measurements using electroencephalography (EEG) and functional near infrared spectroscopy (fNIRS) were acquired during the aforementioned vigilance states. Specifically, I examined brain processes from two different perspectives: brain hemodynamics (blood flow) and electrophysiology (brain waves). My dissertation had three aims, leading to the following results. In Aim 1, I explored a state-of-the-art wavelet transform coherence (WTC) method to analyze neurovascular coupling (NVC), or the changes in brain hemodynamics corresponding to changes in brain activity in four vigilance states, based on dual-mode EEG-fNIRS simultaneous measurements. I had to reduce the EEG frequency range and obtain corresponding envelop functions in order to match the fNIRS frequency range for within-frequency analysis. With WTC, I was able to detect distinct in-phase and anti-phase coherence between hemodynamic and electrophysiological signals at four vigilance states. In specific, in-phase NVC was significantly higher in sleep stage 2 than other vigilance states in delta, theta and alpha bands of EEG versus endogenic band of fNIRS (p-value<0.05). However, anti-phase NVC was significantly higher in eyes closed state than other vigilance states in delta, theta and alpha bands of EEG versus endogenic band of fNIRS (p-value<0.05). These observations might reveal/suggest (i) activation of inhibitory pathways in the eyes-closed resting state in order to transition to sleep stages, and (ii) memory consolidation process at sleep stage two. In Aim 2, I confronted one of the challenges with dual modality measurements: fNIRS, by nature, acquires a slow hemodynamic signal, while EEG has a broad range of frequencies that are significantly higher than fNIRS. I explored phase-amplitude coupling (PAC) as a novel method for analyzing simultaneous EEG-fNIRS since PAC facilitates the relationship between low-frequency phase signals and high-frequency amplitude signals. My unique implementation of PAC on EEG-fNIRS data enabled me to investigate the modulation of brain activity by brain hemodynamics, so called vasculo-neuronal coupling (VNC), for each vigilance state. A significantly higher VNC was observed during sleep stage 1 than both eyes open and eyes closed in three EEG-fNIRS frequency band pairs: delta-endogenic, theta-endogenic and beta-endogenic (p-value<0.1). In addition, VNC observations can be inferred as working memory process and maintenance activities at resting state and early stages of the sleep. In Aim 3, I investigated the interaction between different EEG frequency bands by means of PAC as a cross-frequency coupling (CFC) method. I studied how different EEG frequency bands were coupled as the brain undergoes different vigilance states, so the communication between cortical and sub-cortical/deeper regions in the brain may be revealed by CFC between slow and fast oscillations of EEG. Sleep stage 1 has significantly stronger delta-gamma and theta-gamma coupling than the eyes closed and sleep stage 2 states (p-value<0.1). Additionally, in all vigilance states delta-gamma coupling is significantly stronger than theta-gamma coupling (p-value<0.1). These three frequency bands and their origins are the three key components of memory processes. The observed strong PAC between these frequency bands was another confirmation for involvement of the brain in working memory maintenance and memory consolidation during resting state and its transition to sleep stages. In summary, my dissertation project investigated the interplay between slow hemodynamic or vascular oscillations versus fast neurophysiological rhythms, as well as the communications between different regions of the brain through CFC seen in the EEG signals, during awake-to-sleep transitions in the human brain from healthy human subjects. My scientific contributions include (i) novel application of WTC to analyze NVC, (ii) innovative development and implementation of PAC enabling to map/observe VNC first time, a direction opposite to that of classic neurovascular coupling, and (iii) complementary findings suggesting that CFC between slow and fast oscillations of EEG facilitates communications between cortical and sub-cortical regions.


Wavelet coherence analysis, Phase amplitude coupling, Neurovascular coupling, Vasculo-neuronal coupling, Dual-mode EEG-fNIRS measurements, Vigilance states


Biomedical Engineering and Bioengineering | Engineering


Degree granted by The University of Texas at Arlington