Although the scientific study of meditation goes back a good 40 years and includes thousands of papers, most research has focused on apparent health benefits and psychological effects, and only in the past decade has the underlying neurobiology come under close scrutiny. But recent progress in this area has been rapid, according to psychiatrist-researcher Charles Raison of University of Arizona.
“There are signs that meditation research is coming of age,” he says. “Until the last few years, the scientific community has been guilty of seeing meditation as a powerful black-box phenomenon,” paying little attention to what distinguishes one type of meditation from another, or what they have in common.
“The science is better now. Advances in neuroimaging technology have made it possible to look more closely at the brain at rest, and attempts to develop more rigorous control conditions are helping to identify meditation-specific effects and understand the mechanisms involved.”
Structure and function
Most investigations into structural brain effects associated with meditation have involved straightforward measurement of gray matter concentration in diverse regions, usually among highly experienced meditators compared with non-meditators. Recent studies have taken a more nuanced look at parameters like connectivity and complexity.
In a study published in the Aug., 15, 2011, issue of NeuroImage, UCLA researchers using diffusion tensor imaging (DTI) found white matter changes suggesting enhanced connectivity among a number of brain regions in people who had been meditating for an average of nearly 25 years. A 2012 study by the same group found increased gyrification—more folds in brain tissue—particularly in areas where earlier researchers had seen thickening of gray matter. The longer the person had been meditating, the more pronounced the gyrification, they reported.
Interesting as these findings may be, such cross-sectional studies have inherent limitations, says Gaëlle Desbordes of Boston University. “If you see differences, it’s hard to isolate what caused them. Genetic or environmental factors may make some people more likely to become long-term meditators; they may have different brains to begin with.”
Their relevance is another question. “Most people will say, ‘Monks have different brains. That’s great, but how does this help me? I could never meditate that much,'” she says.
A longitudinal study that Desbordes and colleagues published in the November 2012 Frontiers of Neuroscience addresses some of these issues. Using fMRI, the researchers examined brain activity in volunteers before and after 8 weeks of training and home practice of two different kinds of meditation, compared with an “active control” group who participated in a health education program. The scans were taken while participants viewed images of people in emotionally positive, negative, or neutral situations.
The people who had been trained in “mindful attention”— a basic meditation technique that involves mental focus—showed reduced activation in the amygdala, a region associated with emotional regulation, while viewing all the images. This is consistent, Desbordes says, with reports of less emotional reactivity and greater stability in meditators, which may account for some of its stress-relieving effects.
In participants who whose meditation emphasized cultivating compassion, their amygdala response dropped in response to positive images, but rose when viewing people in pain, sorrow, or other distress. The implications are unclear: Amygdala activation is often linked to anxiety and other negative emotions, but in this group, Desbordes notes, the altered response was associated with reduced depression scores.
While earlier functional studies focused on the brain during meditation, this is perhaps the first to indicate that changes associated with modest practice (instructions called for just 20 minutes daily) might persist even while the subjects were not meditating—suggesting a biological basis for reports that psychological and behavioral effects of meditation carry over into ordinary life.
Probing the heart of meditation
In virtually all its diverse permutations, meditation demands mental focus. “[It] can be thought of as a training ground to cultivate attention,” said University of Wisconsin’s Richard Davidson, a premier researcher in the field, in a talk at the university several years ago. “Most practitioners and teachers will tell you that training attention is the substrate, the basic skill needed for all other kinds of meditation.”
This usually involves concentrating on an object— the breath, a word or phrase, an image, the whole field of bodily sensations or one’s own thoughts—becoming aware when attention drifts off, and bringing it back to the object.
The neurobiology of attention has long attracted research interest, but it was just a decade ago that investigators took a close look at its opposite, “mind wandering,” and identified a neural system activated when the mind was not involved in a task—the “default mode network” (DMN).
Since then, cognitive neuroscientists and psychologists have explored the consequences of mind wandering at length: the title of one widely-cited paper in Science in 2010 presents its conclusions succinctly: “A wandering mind is an unhappy mind.”
“’Why does the mind wander?’ is one of the big questions for meditation practitioners, but in meditation studies, mind-wandering is a recent interest,” says Judson Brewer, medical director of the Yale Therapeutic Neuroscience Clinic. In a study he published in the Dec. 13, 2011, issue of PNAS, Brewer and colleagues compared brain activity in 12 experienced meditators and 12 untrained controls, as they performed three different types of meditation.
They saw reduced activation in key areas of the DMN (the medial prefrontal and posterior cingulate cortex) in the experienced practitioners compared with the controls—suggesting that, as expected, their minds wandered less while meditating. The researchers also found greater functional connectivity between their DMN and a “task-positive network” that regulates attention, working memory, and planning, both during meditation and at baseline.
“Usually the DMN and task-positive networks are anticorrelated—when one is up, the other is down,” says Brewer. “In experienced meditators, these networks are talking to each other. This may be the neurobiological process underlying their ability to be aware when their minds are wandering, and to pull back quickly.”
The fact that these areas were communicating whether or not veteran practitioners were actually meditating suggests that after years of training, paying attention rather than mind-wandering had become their “default state,” Brewer conjectured.
In pilot studies, Brewer has analyzed fMRI data on meditators in a variety of traditions—Zen, Tibetan Buddhism, Christian contemplation—and found similar deactivation in DMN regions.
Because a number of pathological conditions have been linked to DMN dysfunction—ADHD, anxiety, schizophrenia, even Alzheimer’s disease—research such as his may have clinical implications, Brewer suggests in his paper.
More broadly, meditation is already an accepted form of treatment—for stress-linked and psychiatric disorders, pain, and substance problems—and a better understanding of its underlying neurobiology might refine its use, perhaps helping doctors tailor specific techniques to individuals in a kind of “personalized medicine” approach, says Charles Raison.
Beyond that, “there’s a deeper interest: using meditation as a probe to explicate basic biological and psychosocial processes,” he says. By studying veteran practitioners who can reliably report on their mental states, “we might begin to understand, from a scientific point of view, the underlying mechanics of consciousness.”
An fMRI study at Emory University reported in NeuroImage in 2012, for example, enlisted experienced meditators who signaled when they lost focus during meditation, to clarify what happens in the brain as the mind shifts between attention and mind-wandering. The circuitry of compassion, interactions between mind and body, even the fundamental processes of adult neuroplasticity—meditation may provide a window onto all these.
“Meditators have trained themselves to look at their moment-to-moment experience very carefully,” says Brewer. “They’re perfect cognitive neuroscience subjects.”