Christof Koch describes a classic 2008 study that investigated the brains of Buddhist monks:
The cognitive scientists fitted skullcaps with 128 electroencephalographic (EEG) electrodes to the heads of eight long-term Buddhist practitioners and 10 student volunteers. The former were asked to attain a state of “unconditional loving-kindness and compassion” (a form of meditation that does not focus on a single object and is sometimes referred to as “pure compassion”), whereas the volunteers thought about somebody he or she deeply cared about and then tried to generalize these feelings to all sentient beings.
The onset of meditation in the monks coincided with an increase in high-frequency EEG electrical activity in the so-called gamma band (spanning 25 to 42 oscillations a second), which was synchronized across the frontal and parietal cortices. Such activity is thought to be the hallmark of highly active and spatially dispersed groups of neurons, typically associated with focusing attention. Indeed, gamma activity in these monks is the largest seen in nonpathological conditions and 30 times greater than in the novices. The more years the monks had been practicing meditation, the stronger the (normalized) power in the gamma band.
More important, even when the monks were not meditating, but simply quietly resting, their baseline brain activity was distinct from that of the students. That is, these techniques, practiced by Buddhists for millennia to quiet, focus and expand the mind—the interior aspect of the brain—had changed the brain that is the exterior aspect of the mind. And the more training they had, the bigger the effect.