1. Introduction
1.3 Organization of the Dissertation
This dissertation is composed of six chapters. In chapter 1, we introduce the background and the aim of our research. In addition, motivation and background of designing four studies are described. Chapter 2 reports the results and our findings of BPW research. The beginning of this chapter is devoted to the BPW mechanism, the experimental setup, and the signal processing method. The results and statistical analysis are presented at the end of the chapter.
Chapter 3 is focused on the F-VEP study. The first part includes the review of researches related to our work and the motivation of conducting this study. Illustration of experimental procedure follows. Finally the inter-session and inter-group comparisons are conducted.
In chapter 4, we proposed a method to monitor the spatio-temporal distribution of alpha waves using wavelet and fuzzy c-means. Finally, inter-group difference is justified by statistical analysis.
Chapter 5 describes the scheme for real-time alpha-rhythm detection employed in the F-VEP study. This study was attempted to explore the meditation effects on visual perception.
The last chapter summarizes the findings of four studies, that is, BPW, F-VEPs, spatiotemporal alpha, and alpha-dependent F-VEPs. This chapter concludes with our
anticipation of future work.
Chapter 2
Variation analysis of
sphygmogram to assess the cardiovascular system under Zen meditation
In this chapter, we studied how meditation affects the characteristics of cardiovascular system, mainly based on the blood pressure waveform (BPW). Four parameters derived from the BPW include the rising slope (
1 1
t
h ), normalized height of
T wave (
h ), and normalized height of D wave
(
2.1 Background and motivation
The blood pressure waveform (BPW) of the systemic arterial tree is an important determinative of cardiovascular system performance. This signal originates in the
systole and diastole of the heart and conveys such information as the blood ejection ability of the heart, the elasticity of the artery wall, the peripheral resistance, etc.
(Milnor, 1989). In examinations of the clinical value of BPW, Han explored (Han, 2000) possible biophysical and pathological mechanisms of BPW from the viewpoint of hemodynamics. Research showed that BPW analysis is a highly reproducible method and easy to apply to clinical studies. This measure provides important information about arterial stiffness and cardio-vascular interactions (Wilkinson, 1998;
O'Rourke1, 2001). Abnormality in the blood pressure waveform is linked to various physiological or pathological states such as aging and hypertension (Cohn et al., 1995;
Mcveigh et al., 1999). Actually, the blood pressure waveform of radial artery detected at the wrist is the sphygmographical signal used in Traditional Chinese Medicine (TCM) (Tan, 2004). According to theory of the sphygmographical signal, the TCM clinician can identify the status of the human body and treat the patient.
As more clinical evidence supported the benefits of meditation for health, about fifty years ago researchers began investigating the physiological phenomena of the human body under meditation. Dillbeck et al. (1987) compared the physiological differences in two groups of subjects, one under transcendental meditation and the other at rest. Schneider et al. (1995) found that the training of transcendental meditation could significantly lower the systolic and diastolic blood pressure of
hypertensive persons (Barnes et al., 2004; Alexander et al., 1996; Castillo-Richmond et al., 2000). Meditation hereafter became a feasible method to improve the hypertension.
Hankey compared Tibetan Buddhist meditation with Transcendental Meditation (Hankey, 2006). He summarized how practicing different meditation techniques influenced hypertension and other physiological changes. Barnes et al. (1999) found that, under meditation, total peripheral resistance decreased, and they suggested that was why meditation could decrease or control hypertension. To investigate the meditation effects on the cardiovascular system, here we evaluated the variations in blood pressure waveform before and after meditation.
We measured the blood pressure waveforms of twenty Zen-meditation practitioners and twenty normal, healthy subjects in the same age range as the practitioners. According to the clinical experience of TCM professionals, we designed a set of parameters that quantify the waveform patterns of BPW.
2.2 Methods for BPW analysis
2.2.1 Mechanism and recording procedure
The BPW prototype of a healthy subject is shown in Fig. 2-1. The heart pumping mechanism correlating with BPW is illustrated as follows (see Fig. 2-2). The ejection of blood from the left ventricle into the aorta results in the first peak in BPW that is
called the Percussion wave (P wave). The height of P wave, , and the fast ejection time of the left ventricle, , are related to the ejection ability of heart and the compliance index of aorta. We define the rising slope of P wave as
h1
t1
1 1
t
h . A larger slope
indicates a better performance of the heart ejection function and aorta compliance (Fey, 2003). Thus it is used as a quantitative feature to evaluate the cardiovascular system.
P wave
T wave
D wave
h
1h
3h
5h
4V3
V1
t
1Figure 2-1: Prototype of a normal blood pressure waveform.
Figure 2-2 (a): When the ventricle contracts, the semilunar valve opens. Blood ejects into the aorta and arteries and makes them expanded. At the same time, the pressure stores in the elastic walls. This percussion produces the P wave.
Figure 2-2 (b): Isovolumic ventricular relaxation makes the semilunar valve shut.
Elastic recoil of arteries sends blood forward into the rest of circulatory system. This rebound makes the T wave and the closing of valve makes the D wave.
The second peak, called the Tidal wave (T wave), appears when blood hits the artery wall and rebounds. As a result, T wave is manifest if the artery possesses excellent elasticity that reflects low peripheral resistance of the circulatory system. On the other hand, an artery with a stiff wall makes the T wave propagate fast according to the Moens-Korteweg equation of wave velocity (Nichols et al., 1990; Khir et al., 2002). Accordingly, the T wave will merge with the P wave, which results in a wider P wave. The second parameter,
1 3
h
h , where represents the height of T wave, is
utilized to measure the effect of T wave. We thus expect a large h3
1 3
h
h for an arterial
system with better elasticity. The valley height reveals the level of peripheral
resistance (Fey, 2003; Milor, 1989). As the peripheral resistance increases (decreases), parameter increases (decreases) as well. The normalized parameter,
h4
employed to measure the drift of peripheral resistance. Finally, when the aortic valve is closed, the Dicrotic wave (D wave) is generated. is the magnitude of D wave and the normalized parameter,
h5
1 5
h
h , represents the effect of D wave on arterial system.
will decrease due to a stiff aorta or aortic regurgitation.
h5
Fig. 2-3 displays the instrument for recording BPW from the wrist with a piezoelectric sensor. The sensor was manufactured by Skylark Company, with 3dB cutoff frequency at 10 KHz. The output is linearly dependent of the input when BP is below 1000 mmHg. The subject sat with one forearm on the desk. The angle between
upper arm and desk is about 120°. Pressure of the piezoelectric sensor on the wrist was adjusted for sufficient sensitivity of the BPW activity. The range of amplitude depends on the measuring position. In this study, we measured the BPW at the ‘chun’
position (Fig. 2-4). A well experienced TCM (Traditional Chinese Medicine) expert assisted us in the experimental setup and recording. He searched the “chun” position by palpating the wrist of the subject. The piezoelectric sensor was then attached to the proper position carefully identified by the expert. The pressure was adjusted until the output amplitude reached maximum, from which the amplitude recorded was in the range of 15 to 27 mmHg (Fey, 2003).
Figure 2-3: The acquiring instrument of blood pressure waveform.
Figure 2-4: The measuring position on the wrist.
2.2.2 BPW parameters
The peak or valley positions in Figure 2-1 demonstrated by crosses are determined by the Matlab program for searching the local maximum or minimum. We first identify the P wave that is the most distinctive one. The starting point V1 of the whole period is about 0.1 sec prior to the P wave. The next step is to find the local minimum V3 at about 0.35 sec afterward from V1. Then the T wave between P wave and V3 is ready to be extracted. D wave comes after V3, that is identified by finding the null of waveform differentiation between V3 and the end of the BPW period. The time values of these specific points depend on the blood speed and the conditions of blood vessels. We referred to the paper by Xie (Xie et al., 2000) to check the standard time values of these parameters.
In this research, the above four parameters,
1
assess the status of cardiovascular system and corresponding parameters are defined
Variation percentage = after meditation before meditation 100%
before meditation
where η denotes one of the four parameters,
1
variations in recording system characteristics and physiological conditions of each subject, heights h3-h5 were normalized to within the range of 0 to 1. Normalization ensures an even comparison of BPW features in various recording sections. To avoid null denominator caused by the disappearance of before-meditation T wave (in the case, h3=0), the variation percentage of
1 3
h
h is defined to be zero if T wave is absent
before and after meditation. On the other hand, if T wave exists only after meditation, the variation percentage of
1 3
h
h will be assigned to 100% to avoid infinity.
2.3 Experiment and results
2.3.1 BPW signal processing
The blood pressure waveforms of radial artery non-invasively detected at wrist by piezo-electric transducer were recorded for 10 seconds and digitized at a sampling rate of 100 Hz. To reduce high frequency noise, a low-pass filter is designed with a 3dB cutoff frequency of 50 Hz. Because null baseline was required in the analysis, we removed the mean value of each BPW period in pre-processing. Accordingly, negative waveforms might appear. Note that non-constant, linear baseline drift sometimes interfered in the BPW (Fig. 2-5). The sources of interference include the body movement, heavy respiration, etc. The resulting artifacts might be false amplitude or position shift of peaks/valleys. Although removal of the baseline drift may help, we found it necessary to further correct the interference patterns. In that case, baseline removal involved a linear-regression method for baseline-drift detection.
Figure 2-5: The linear baseline drift in the BPW (upper) and the result after removing it (lower).
The BPW recording instrument can only continuously trace for up to 10 seconds.
BPW is a quasi-periodic signal with 10 to 16 complete cycles in a 10-second term.
Here we averaged out the 10-16 cycles to get the final BPW for further quantification of embedded features. We thus can obtain triple to quadruple SNR (signal-to-noise ratio) that is linearly dependent of the square root of the number of measurements.
The SNR level could fulfill our research requirements.
2.3.2 Subjects and recording paradigms
The participants were divided into two groups - 20 meditators and 20 normal, healthy people without any experience in meditation. In the experimental group, 13 females and 7 males with a mean age of 26.6±2.2 years participated. Their experiences in Zen-Buddhist practice span 6.9±3.3 years. The control group comprised 9 females and 11 males with a mean age of 25.2±1.8 years. All the meditation practitioners learned Zen-Buddhist meditation in the Taiwan Zen-Buddhist Association. Only experienced practitioners with more than 3 years of meditation experience were invited. The controls were graduate students of National Chiao-Tung University. No subjects had any cardiovascular disease in their medical histories.
Participants sat in an isolated space during the experiments. Blood pressure waves were recorded before and after the 40-minute main session (meditation or relaxation). In the main session, experimental group practiced Zen meditation, while controls sat in normal relaxed position with eyes closed.
2.4 BPW before and after meditation
Fig. 2-6 illustrates an example of the blood pressure waveform of meditator. The solid curve shows the before-meditation BPW, and the dashed one shows that after meditation. In Fig. 2-6, after-meditation P wave rises more steeply than the before-meditation one. For this particular subject, poor arterial elasticity makes the before-meditation T wave occur earlier and merge with P wave, resulting in a
broadened P wave. In after-meditation BPW, the T wave becomes evident and distinguishable from the P wave, inferring that enhanced arterial elasticity is a consequence of meditation practice. Moreover, the V3 valley descends (i.e., h4
decreases) after meditation, indicating a decrease in peripheral resistance. The D wave magnitude is also strengthened after meditation. A typical example of 10-second BPW is shown in Fig. 2-7. Note that the pulse rate is slower after meditation, resulting in a phase difference between the two waves. In sum, the after-meditation BPW reflects a more robust cardiovascular system that could have been tuned up by the Zen meditation.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -5
0 5 10 15 20
Time (Sec)
Blood Pressure Wave (mmHg)
Before meditation After meditation
Figure 2-6: The before-meditation (solid curve) and after-meditation (dashed curve) BPW for an experimental subject.
1 2 3 4 5 6 7 8 9 10 -15
-10 -5 0 5 10 15 20 25
Time (Sec)
Blood Pressure Wave (mmHg)
Before meditation After meditation
Figure 2-7: The ten-second typical BPW of one meditation practitioner.
2.5 Inter-group comparison
Table 2-1 shows the results of the four parameters measured in both groups. The variation percentage of each parameter in each group is calculated by the formula (2.6). The P-values in Table 1 are evaluated using t-test, which is used to check whether the variation percentages show statistical differences between the groups. In this preliminary investigation, we concentrated on intra-subject differences between various experimental sections because the inter-subject variations in BPW were too complicated to manipulate.
In comparison with the control group, Zen-meditation practitioners have higher ranges of variation percentages in all four parameters. The performance details of each parameter are as follows: When considering the rising slope of P wave,
1 1
t h ,
that reflects the ejection ability of left ventricle or aorta elasticity, the experimental group had a mean increase from 390.8 to 435.1, a variation percentage of 11.7%, that was 5% higher than the variation percentage of control group. The second parameter
1 3
h
h , measuring the effect of T wave, demonstrated distinct enhancement of arterial
elasticity in the experimental group (three times the increasing rate of the control group). We discovered that even though some experimental subjects had vague T wave before meditation, it was often boosted after meditation. On the other hand, T wave variation of was not as obvious in the control group. We observed that some
control subjects did not even have a T wave before and after relaxation. If T waves merged with P waves, the parameter
1 3
h
h was considered to be zero. Next, the high
decreasing rate of
1 4
h
h revealed reduced peripheral resistance after meditation. Finally,
the increasing
1 5
h
h infers that Zen meditation significantly improves the quality of
semilunar valves and arterial elasticity. The t-test results showed that all P-values were smaller than 0.05, further corroborating the significance of the improvement in the meditation group. In comparison with normal relaxation, Zen meditation may effectively improve the characteristics of cardiovascular system according to parameters extracted from blood pressure waves.
Table 2-1. The statistical results of four parameters (
variation percentages. P values are evaluated to show the statistical significance
of discrimination between two groups.
1
Chapter 3
F-VEPs in Zen meditation
Observation of the inner-light perception in deep Zen meditation (Lo et al., 2003) has aroused our attention. Based on the recording of F-VEPs (flash visual evoked potentials), this study was thus designed to investigate the characteristics of visual nervous pathway for the Zen-meditation practitioners (experimental group), in comparison with that for the normal, healthy subjects (control group). Flash stimuli were applied before, during and after meditation / relaxation in experimental / control subjects. We focused on the F-VEPs at the occipital site Oz, central site Cz and frontal site Fz.
3.1 Background and motivation
During the past decades, a number of papers have reported the benefits of meditation to the physiological and mental health, with particular emphasis on transcendental meditation, Yoga meditation, and Japanese-Zen meditation. It is the first attempt to investigate the electrophysiological signals of the orthodox Zen-Buddhist practitioners. The main doctrine of Zen-Buddhist practice is to transcend the physiological, mental, and subconscious states via Zen meditation that
enables the practitioners to reach a fully egoless, transcendental state (the Alaya state) (Lo et al., 2003).
Researchers have been probing into the physiological and psychological parameters during meditation for several decades (Jevning et. al., 1992; West, 1980).
Some important results include: Wallace (Wallace, 1970) claimed the emergence of theta waves in the frontal area in transcendental meditation, Banquet observed the slowdown of alpha frequency and the increase of alpha amplitude as well as the occurrence of the rhythmic theta trains (Banquet, 1973). Recently, Lutz and et al.
found that long-term Buddhist practitioners self-induced sustained high-amplitude, gamma-band EEG and phase-synchrony during meditation (Lutz and et al., 2004).
These patterns obviously occurred at the lateral fronto-parietal electrodes. Travis found several physiological markers including the decrease of respiration rate, higher respiratory sinus arrhythmia amplitudes, higher alpha coherence, etc (Travis, 2001).
In addition, ERPs (evoked response potential) used to explore the underlying neuron activities in meditation were investigated. Zhang claimed the increase of F-VEP amplitudes of Qigong practitioners under meditation (Zhang, 1993). In Xu et, al.’s research, increase of amplitude and decrease of latency were reported (Xu et al., 1998). Furthermore, auditory evoked potential (AEP) under meditation has also been studied to investigate the meditation effects on the brainstem auditory response
(McEvoy et al., 1980; Tells et al., 1994).
The paper survey given above shows a particular phenomenon of meditation, that is, its effects on the frontal cortex. The transcendental state of Zen meditation, in fact, reflects that the human life system turns off its physical and mental sensors, leaves off the message transmission from outside world, and keeps subconscious tranquil. When further attaining the deeper meditation state, practitioners often ignite their inner energy, accompanied with the experience of perceiving the inner light (Lo et al.,
2003). Base on the observation of frontal EEG and the common experiences of inner-light perception, we hypothesized that meditation would affect the visual neuron
pathway, and the effect might be modulated by the meditation effects in the frontal area of the cortex.
Accordingly, this study aimed to investigate the meditation effects on visual neuron pathway by quantitatively analyzing the visual evoked potentials (VEPs).
Owing to the limitation that meditators must close their eyes during meditation, we employed the flashed light as the visual stimulus and recorded the flash visual evoked potential (F-VEP), with particular emphasis on channels Oz, Cz, and Fz.
3.2 Experimental setup and procedure
This study involves 30 experimental subjects (meditators) and 30 control
subjects (normal, healthy people without any experience in meditation). In the experimental group, 15 females and 15 males at the mean age of 28.7±4.6 years participated. Their experiences in Zen-Buddhist practice span 6.6±4.1 years. The control group consists of 9 females and 21 males at the mean age of 24.1±1.6 years.
EEG and F-VEP were recorded within the frequency range from 0.15Hz to 50Hz. The sampling rate is 1000Hz. We applied the 30-channel recording montage with the ground at the forehead and the reference as the linked mastoids.
F-VEPs were recorded before, during and after the main session (meditation or relaxation), that is called the pre-, mid-, or post-session. Continuous, 100 flash stimuli were applied to the subject in each session. The flash light was 10 μs in duration and 1 Hz in frequency produced by a xenon lamp that was placed 60 cm in front of the subjects’ eyes. These parameters were referred to the standard procedures (Adrian and Mathews, 1934; Cigánek, 1961; Odom et al., 2004). We averaged these 100 trials to get the averaging F-VEP in each session. Subjects sat in a isolated space during the recording. Each recording lasted for about one hour. The course included 10min pre-session, 40min mid-session, and 10min post-session recording. In the mid-session period, experimental subjects practiced the Zen meditation, while control subjects sat in normal relaxed position with eyes closed. During the meditation, the subject sat, with eyes closed, in the full-lotus or half-lotus position. In the begining of meditation,