Thanks to the Dalai Lama, lots of monks have lent Richard Davidson their brains. For almost 20 years Davidson, a neuropsychologist at the University of Wisconsin, Madison, and a long-time meditator himself, has been curious about how Buddhist meditation of the kind the monks practice might change their brains. He has lugged electronic equipment up into the hills above Dharamsala (the Dalai Lama's home in exile in northern India) to test the brains of yogis, lamas and monks living in primitive huts there, and persuaded other monks to visit his lab.

Over the years he has found that the brains of monks who are the most experienced meditators are indeed different from other brains. They have a much stronger "gamma" wave, a form of electrical activity in the brain that is associated with consciousness and pulling together information and perceptions from different regions of the brain. They also have much greater activity in the left than the right prefrontal cortex (just behind the forehead), a mark of well-being and happiness.

But all of these studies came with an asterisk. There was no way to tell if the monks' brains started out different. That is, maybe people with high gamma-wave activity and lopsided left-prefrontal activity were more likely to become Buddhist monks. If so, then their brain traits caused them to become expert meditators, rather than their years of meditation changing their brain.

Now Davidson has taken a big step toward showing that the causal arrow really does point from meditation to brain changes rather than from brain differences to a life of meditation. Specifically, meditation can change brain circuits linked to attention.

He and his colleagues taught volunteers a form of Buddhist meditation called Vipassana. In this form of attention meditation, you first focus on an object such as your breath. You then let your focus expand, cultivating "bare attention," in which you let thoughts or perceptions engage your attention, but keep yourself from reacting emotionally or judgmentally (that's the "bare" part) "it's like, ah, I see my pants leg there; okay, now, moving on . . . The goal is to improve attention and reduce distractability.

The volunteers practiced Vipassana meditation for three months, for 10 to 12 hours a day. Another group got only a quickie one-hour course, then practiced Vipassana for 20 minutes a day for a week. Before the intense training, Davidson and his team tested them all on one form of attention, called attentional blink. In this glitch, if you pay close attention to one thing it's hard to notice something that comes hard on its heels, typically within half a second. For instance, Davidson had the volunteers watch a screen where capital letters flashed, one at a time, for one-twentieth of a second. Once or twice in the rapid-fire stream of 15 or so letters, a number snuck in. At the end, the volunteers typed which number or numbers had snuck in.

In general, if a second number creeps in less than half a second after the first, you don't notice it. Your attention has been so consumed by detecting the first number, there's not enough left to detect the second. "The attention momentarily goes off-line," Davidson says. "Your attention gets stuck on the first target, then you miss the second one." But as he and colleagues report online today in the journal PLoS Biology, mental training in the form of Vipassana meditation can change that. The meditators significantly improved their ability to detect the second number amid the barrage of letters, even when it came less than half a second later (the period when paying attention to the first number ordinarily keeps you from noticing the second). In addition, the amount of brain activity associated with seeing the first target fell in the meditators "apparently, mental training allowed them to use fewer neural resources to detect the first number, thus leaving enough to notice the second.

"Their previous practice of meditation is influencing their performance on this task," Davidson says. "The conventional view is that attentional resources are limited. This shows that attention capabilities can be enhanced through learning."