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Physiological Aspects of Meditation

INTRODUCTION

Alternative medicines such as acupuncture, homeopathy, herbal treatments, folk remedies and massage have become increasingly popular in the last decade, both with patients and with doctors (NIH, 2000). A survey showed that the number of Americans who use such therapies rose from about 33 percent in 1990 to more than 42 percent in 1997 (Eisenberg, 1998). In addition, Americans spent over $27 billion on these therapies that year, more than was spent on out-of-pocket expenses for all U.S. hospitalizations. The U.S. government has also increased its financial interest in alternative medicines, increasing available funding for National Center for Complementary and Alternative Medicine (NCCAM) from $2 million dollars in 1993 to 68.7 million in 2000. This funding represents congress’s dedication to developing and supporting quality, scientific research in the safety and efficacy of these alternative medicines, and to provide the public with reliable information.

One of the alternative therapies that has been gaining in popularity is the use of meditation. It has been traditionally associated with spiritual practice, whether the tradition was practiced by early Jewish and Christian mystics, or Hindi guru and Buddhist monks. The goal of the meditative exercise, in a variety of spiritual paths, is to reach or achieve an absolute or sacred state of consciousness, wherein one transcends our tangible world, views reality with unfettered clarity, or develops a metaphysical connection to God. However, meditation need not be part of a spiritual path to reap rewards from the practice. By itself, it is an effective treatment for reducing stress (Benson, 1995; Calderon & Schneider, 1999; Kulkarni, 1998; Shapiro, et al., 1998; Walton, et al., 1995), which can lead to the more serious medical condition of hypertension. In addition, long-term practitioners report more positive moods and better quality of life (Baldwin, 1999; Gimbel, 1998; Shapiro, 1998). These factors may be instrumental in explaining the reports that meditation can improve outcomes of chronically ill patients (Anton, 1999; Davis, et al., 1998; Speca, et al., 2000; Vedanthan, et al., 1998; Young, 1999; Zaza, et al., 1999). One study even reported that psoriasis patients recovered faster when including a meditation regimen into treatment (Kabat-Zinn, et al., 1998). Meditation has also become a popular tool in psychotherapy as a relaxation technique (eg., Benson, 1978) and to stimulate insight (Shapiro, 1995).

Although some research of meditative techniques date back 50 years or more (Vakil, 1950), the early published reports were primarily anecdotal or based on case reports. Steadily, however, research on the health benefits of meditation on mind and body have been growing over the last 30 years, fueled by a growing acceptance of meditation as an effective tool in therapy. To better understand the usefulness of meditation, it is helpful to be familiar with specific ways in which the practice affects the body as well as the mind. There are several recent reviews of meditation as a therapeutic tool (Craven 1989; Gimble, 1998; Harmon & Myers, 1999), but few on the physiological effects (Jevning, et. al, 1992). For that reason, the goal of this paper is to present an overview of the physiological effects of meditation.

It is important to note that there is not just one meditative technique that is utilized in scientific research, but that an almost unlimited number of modalities are possible. However, there are four elements that are consistent: a quiet environment; a comfortable position; the focusing of attention to a rhythmic device, such as the breath or silently repeating a word or syllable; passively letting the mind wander without concentrating or dwelling on any particular thought. Some of the more popular techniques used in research are transcendental meditation (TM), Hatha yoga, and various forms of the method popularized by Herbert Benson in studying the "relaxation response" (Benson, 1975). Each method has the advantage of being easy to learn and replicable in the laboratory setting. Discussion of the differences in technique have been kept at a minimum in this paper, but have been mentioned if particularly relevant to the study being reviewed (see Woolfolk, 1975 for comparisons of techniques).

PHYSIOLOGICAL EFFECTS OF MEDITATION

Our bodies can adapt to changes in our mood or environment without our conscious effort: we shiver when we’re cold, our hearts beat faster when we run, and our hands get cold when we’re nervous. They are autonomic, or "automatic," in the senses that for an action to occur, we don’t have to think about it. However, with conscious effort, we can influence these automatic functions. One of the first to study this phenomena was Neil E. Miller (1969), who used conditioning techniques to train rats to alter involuntary functions with a system of rewards. For instance, under certain conditions, a rat could receive a food pellet only if it lowered its heart rate. The rats learned to control this ordinarily involuntary function in expectation of reward. This is the basis for biofeedback, the concept that people can change their heart-rate, blood pressure, skin temperature, and other metabolic functions if they are given information about those functions and consciously try to alter them. These techniques were studied largely in a search for better methods to reduce hypertension, a medical condition that can be a precursor to atherosclerosis, stroke, and heart disease.

Oxygen Metabolism and Respiration

During biofeedback studies in the early 1970s, Herbert Benson (1975) noted that subjects who were able to lower their systolic blood pressure during sessions reported employing relaxing thoughts to achieve the desired results. With this in mind, Benson decided to unburden himself of the rather cumbersome biofeedback equipment and develop methods to consciously reduce blood pressure. Benson looked into earlier research on the effects of meditation, such as B. K. Anand’s (1961) study of a yogi in India who could lower his oxygen metabolism at will, and similar reports of Zen monks in Japan who could reduce their oxygen consumption by 20% during meditation sessions (Jevning, 1992). Through his own studies, Benson found that Americans who practiced TM were able to lower their oxygen metabolism an average of 12%, a greater drop than during sleep. A later and more comprehensive study of TM (Farrow & Herbert, 1982) found an even greater 40% decline in oxygen consumption at a 50% decline in respiration rate. Reduced sensitivity to CO2 has also been observed during meditation (Kesterson & Clinch, 1989). These changes suggest that meditation is a hypometabolic state like sleep or rest, but from an analysis of other measures discussed below, it appears that meditative states have unique psychophysiological patterns.

Tissue Metabolism

The study of organ, tissue, and cellular changes during meditation have become more popular in recent years, primarily with long-term meditators in 30-40 minute sessions (Jevning, 1992). One common finding is a decline in arterial lactate levels (Jevning, et. al., 1978; Solberg, 2000; Wilson, et al., 1987). A 20% increase in phenylalanine concentrations was found in another study (Jevning, 1977), compared with insignificant blood level changes in 12 other amino acids. Although the reasons for metabolic the changes during meditation are often unclear, they seem to be related to a specific hypometabolic functioning unrelated to sleep or other rest states.

Blood Flow

Decreases in renal (kidney) and hepatic (liver) blood flow during meditation has been hypothesized as being due to increased muscle, skin, or brain blood-flow demands (Jevning, et al., 1978). Further research found that muscle and skin blood flow did not increase as renal and hepatic blood flow decreased, implicating an increase in blood flow to the brain (Jevning, et al., 1979; Jevning, 1992). Blood pressure has been a focus of study as well. Reductions in systolic blood pressure have been observed during experimental sessions (Barnes, et al., 1999; Castillo, et al., 2000; Lee, et al., 2000; Wallace, et al., 1971), as well as for long-term practitioners (Wallace, et al., 1983). Such reductions in blood pressure, as noted above, are associated with lowered risk of cardiovascular disease.

Autonomic functioning

In comparing meditation groups to resting controls, reports of increased galvanic skin resistance (GSR), and decreased spontaneous electrodermal response (EDR) are common (Cuthbert, 1981; Goleman & Schwartz, 1976; Holmes, Solomon, Cioppro & Greenberg, 1980; Orme-Johnson, 1973; Travis & Wallace, 1999; Wallace & Benson, 1972;Wallace, Benson & Wilson, 1971), as are reports of faster GSR recovery to baseline after a stressor is applied (Goleman & Schwartz, 1976). These measured effects of meditation on skin resistance suggest decreased sympathetic activity and a move towards a relaxed state (Jevning, 1992).

The effects of meditation on heart rate tend to be less consistent, however. Several studies have shown marked decreases in heart rate during meditation (Cuthbert, 1981; Holmes, et al, 1980; Orme-Johnson, 1973), suggesting relaxation. However, others have shown no change or an increase of heart rate during meditation (Heide & Borkovec, 1983; Ley, 1988; Morse, 1977; Wenger, 1998). There are several possible explanations for these inconsistencies in heart rate measures, including: the experience level of the mediators (Corby, Roth, et al., 1978), the meditators’ "states of consciousness" as reported by subjects during measurement (Farrow, 1982), the timing of measures (Jevning, 1992), and relaxation-induced anxiety (Heide & Borkovec, 1983; Ley, 1988; Wenger, 1998).

Endocrine, Hormones, and Neurotransmitters

There is a good deal of evidence for meditation’s effects on adrenocortical activity, during experimental sessions and after long term use, specifically in the reduction of cortisol and ACTH (Bevan, 1980; Jevning, et al., 1978; Kamei, et al., 2000; Michaels, et al., 1979; Subrahmanyam, et al., 1980; Sudsuang, et al., 1991). As well, there is recent evidence that cortisol, thyroid-stimulating hormone, and growth hormone secretion is reduced by meditation after experimentally-induced stress (McLean, et al., 1994). Levels of the urinary metabolite of serotonin 5-HIAA have also been measured before and after meditation sessions. Levels were found to increase significantly versus a control group (Bujatti, 1976), and were measured as higher in long-term meditators overall. Levels of arginine vasopressin, thought to play a role in memory and learning, are higher during meditative states (O’Halloran, 1985). In addition, recent research has explored meditation’s effects on levels of beta-endorphin and corticotropin-releasing hormone (Harte, et al., 1995), melatonin (Massion, et al., 1995; Tooley, et al.,2000), dehydroepiandrosterone sulfate (DHEA-S) (Glaser, et. Al., 1992), and hormonal changes mimicking gamma aminobutyric acid (GABA) (Elias, et al., 2000).

Electroencephalography (EEG)

Electroencephalography (EEG) is a technique that allows researchers to look at activation in regions of the brain by reading electrical activity emanating from regions of interest. Probes are placed on the scalp, and the changes in electrical activity over time for different regions are recorded and compared. Extensive research exists that used EEG to study meditative states. Such studies usually report that slow alpha activity is increased (mean square amplitude) in central and frontal regions, with high voltage theta burst activity in the frontal region (Banquet, 1972; Herbert & Lehmann, 1977;Kamei, et al., 2000; Khare, et al., 2000; Wallace, 1971). Beta and delta activity are typically decreased or do not change. Sleep, rest, and meditation have been compared on EEG, and like other physiological measures, and with few exceptions (Fenwick, et al., 1977), meditation has been found to have its own signature (Banquet, 1973; Farrow, 1982; Herbert & Lehmann, 1977; Jacobs, et al., 1996). For example, long-term meditators in meditative states have been found to produce theta-burst activity, whereas such a phenomenon has not been found in restful or sleeping states (Herbert & Lehmann, 1977).

A more telling use of EEG is to measure phase coherence or "synchrony," which measures the degree to which EEG amplitude increases or decreases in simultaneous comparison to another region of the brain (Badawi, et al, 1984; Farrow, & Herbert, 1982; Haynes, et al., 1976; Travis & Wallace, 1999). Phase coherence has been linked with clear or "pure" thinking (Badawi, et al, 1984; Farrow, & Herbert, 1982) and creativity (Haynes, et al., 1976; Orme-Johnson & Hayes, 1981). Significant increases in synchrony have been noted particulary for long-term practitioners on alpha wave activity (Badawi, et al., 1984; Bennett & Trinder, 1976; Farrow & Herbert, 1982; Haynes, et al., 1976). Studying phase coherence using EEG is a relatively easy and cost-effective method in learning about the relationship between meditation and its related psychological effects.

Cerebral Blood Flow and Neuroactivity

Only in the last decade have neuroimaging techniques such as positron emission tomography (PET), single photon emission tomography (SPECT), and functional magnetic resonance imaging (fMRI) been used to measure cerebral blood flow (CBF) and activity in the brain. More so than most of the modal shifts in previous research, these techniques are building upon data gathered through studies of hemodynamics, neurotransmitter activity, and measures of electrical activity in the brain using EEG. They are related, in that when an area of the brain is activated, it requires more energy, and therefore requires more blood to provide the fuel for metabolism in the form of sugars and oxygen. As well, areas of activation produce electrical signals which are detected by EEG. The advantage of imaging techniques over EEG are that they can measure regions of interest (ROI) with more specificity. Although imaging techniques may also serve other functions, the studies discussed below use them to measure CBF.

Earlier in this paper it was mentioned that nonhepatic, nonrenal blood flow decreases during meditation, with skeletal muscle and skin blood flow not changing. That being the case, Jevning (1996) hypothesized that cardiac output most likely would, then, increase blood flow to the brain. To measure cerebral blood flow (CBF), he used a method called rheoencephalography (REG). Rheoencephalography passes a small electrical current through the brain, using electrodes, which are influenced by changes in electrical impedance caused by blood flow. In this study, electrodes were placed to collect data on blood flow in the occipital and frontal areas, which are implicated by EEG as areas of activation during meditation. As expected, the meditation group was found to have greater CBF in the regions of interest in comparison to a control group whom were at rest. Since greater blood flow to an area infers more activity, this study provided strength for a theory that meditation is not similar to sleep, but is a state of restful alertness.

Positron emission tomography and fMRI are two other techniques that can be used to measure CBF, and therefore brain activity. Because of the expense and complexity of operation, they have not yet been used a great deal in researching meditation. The first published study using neuroimaging techniques to examine meditative states (Herzog, 1990) used PET to measure regional cerebral metabolic rate of glucose, a sign of increased activity. This study did not find a profound difference in regional or global CBF, but did find a slight increase in frontal areas compared to visual centers in the occipital regions. Not until almost ten years later was another PET study was conducted. This study (Lou, 1999) used a measure of oxygen metabolism, which is more sensitive than glucose when looking for changes in a relatively short period of time, to compare a meditation group with a resting control group. Although mean global CBF did not change for either of the groups, the meditation group showed more activity in regions associated with imagery tasks, and the resting control group (compared to the meditation group) showed more activity in areas associated with an executive attentional network. The authors concluded that the differences show differential patterns of brain activity for meditation and a resting state.

A final technique that has been used to measure CBF is fMRI. An advantage of this procedure over PET is that an fMRI scan of brain activity can be done in seconds, rather than several minutes. In that sense, changes in CBF can be measured with greater temporal clarity. A study published earlier this year was the first to use fMRI to study meditation (Lazar, et al., 2000). Interestingly, one of the authors was one of the first researchers in the field of meditation research, Herbert Benson, mentioned above. The results of this study showed activation in neural structures involving attention (lateral prefrontal and parietal regions) and arousal/autonomic control (pregenual anterior singulate, amygdala, midbrain and hypothalamus). Other areas of activating were identified in the putamen, precentral and postcentral gyri and the hippocampal/ parahippocampal structures. In addition, a global CBF reduction was seen across subjects during the meditation sessions. In comparison to the PET study by Lou (1999) mentioned above, there are consistencies in general regions of activation. However, some differences did exist, that the authors hypothesize was a result of variations in study designs, measurement differences, and differences in the types of meditation used.

CONCLUSIONS

Studies on meditation will likely continue into the future. Further research may be used to improve our knowledge of its ability to lower stress and hypertension, and improve well-being. Research on states of consciousness will continue to contrast meditative states from rest, sleep, and wakefulness. Furthermore, as science explores more into the ways in which the brain and body interact in response to stress, the more we will find out how the practice of meditation influences these processes. Other studies may produce data suggesting how meditation may become more effective in psychotherapy as a tool for gaining personal insight. Government funding is available now more than ever to research alternative medicines, and its acceptance by patients and the medical community is growing. In the future, we may even see meditation leave the moniker of "alternative" behind.

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The text on this page (c) December, 2000 by Scott McDonald.