EINSTEIN FELT IT. It's what draws people to church, prayer, meditation, sacred dance and other rituals. Chances are you've felt something like it too--in the mountains, by the sea, or perhaps while listening to a piece of music that's especially close to your heart. In fact, more than half of people report having had some sort of mystical or religious experience. For some, the experience is so intense it changes their life forever.

But what is "it"? The presence of God? A glimpse of a higher plane of being? Or just the mystical equivalent of déjà vu, a trick the brain sometimes plays on your conscious self? At some level, of course, all our thoughts and sensations--however unusual--must involve the brain. Indeed, experiments on the brain have led neuroscientists to suggest that the capacity for religion may somehow be hardwired into us. If so, why do people's religious experiences differ so profoundly, moving some so deeply while leaving others cold?

Andrew Newberg, a neuroscientist at the University of Pennsylvania in Philadelphia, has been fascinated by the neurobiology of religion for more than a decade. He admits it's an awkward role for a scientist. "I always get concerned that people will say I'm a religious person who's trying to prove that God exists, or I'm a cynic who's trying to prove that God doesn't exist," he says. "But we try to approach it without bias." Earlier this month he published a book, which lays out the most complete theory to date of how mystical or religious experiences can be generated in the brain.

Together with the now deceased Eugene d'Aquili, a colleague from Penn, Newberg was keen to study the sensations that are unique to religious experiences but shared by people of all faiths. One of these is the sense of "oneness with the Universe" that enthralled Einstein. The other is the feeling of awe that accompanies such revelations and makes them stand out as more important, more highly charged, and in a way more real than our everyday lives.

But Newberg realised that rare, fleeting revelations would be almost impossible to study in the lab. It meant he had to ignore the one-off experiences that strike out of the blue and focus instead on meditation and prayer--sedate, but at least reproducible.

Through a colleague who practised Tibetan Buddhism, Newberg and d'Aquili managed to find eight skilled meditators who were willing to undergo brain imaging. The volunteers came to the lab one at a time, and a technician inserted an intravenous tube into one arm. Then the volunteer began to meditate as normal, focusing intently on a single image, usually a religious symbol. The goal was to feel their everyday sense of self begin to dissolve, so that they became one with the image. "It feels like a loss of boundary," says Michael Baime, one of the meditators and also a researcher in the study. "It's as if the film of your life broke and you were seeing the light that allowed the film to be projected."

Hidden in the next room, Newberg and d'Aquili waited. When the meditator felt the sense of oneness developing--usually after about an hour--they would tug on a string. This signalled the researchers to inject a radioactive tracer through the intravenous line. Within minutes the tracer bound fast to the brain in greater amounts where the blood flow, and hence brain activity, had been higher. Later a scanner would measure the distribution of the tracer to yield a snapshot of brain activity at the time of binding. The technique, called Single Photon Emission Computed Tomography, or SPECT, allowed the subjects to meditate in the relative peace of the lab rather than the claustrophobic whirr of a scanner. Once the tests were completed, Newberg and d'Aquili compared the activity of the subjects' brains during meditation with scans taken when they were simply at rest.

Perhaps unsurprisingly, the researchers found intense activity in the parts of the brain that regulate attention--a sign of the meditators' deep concentration. But they saw something else, too. During meditation, part of the parietal lobe, towards the top and rear of the brain, was much less active than when the volunteers were merely sitting still. With a thrill, Newberg and d'Aquili realised that this was the exact region of the brain where the distinction between self and other originates.

Broadly speaking, the left-hemisphere side of this region deals with the individual's sense of their own body image, while its right-hemisphere equivalent handles its context--the space and time inhabited by the self. Maybe, the researchers thought, as the meditators developed the feeling of oneness, they gradually cut these areas off from the usual touch and position signals that help create the body image.

"When you look at people in meditation, they really do turn off their sensations to the outside world. Sights and sounds don't disturb them any more. That may be why the parietal lobe gets no input," says Newberg. Deprived of their usual grist, these regions no longer function normally, and the person feels the boundary between self and other begin to dissolve. And as the spatial and temporal context also disappears, the person feels a sense of infinite space and eternity.

More recently, Newberg has repeated the experiment with Franciscan nuns in prayer. The nuns--whose prayer centres on words, rather than images--showed activation of the language areas of the brain. But they, too, shut down the same self regions of the brain that the meditators did as their sense of oneness reached its peak.

This sense of unity with the Universe isn't the only characteristic of intense religious experiences. They also carry a hefty emotional charge, a feeling of awe and deep significance. Neuroscientists generally agree that this sensation originates in a region of the brain distinct from the parietal lobe: the "emotional brain", or limbic system, lying deep within the temporal lobes on the sides of the brain.

The limbic system is a part of the brain that dates from way back in our evolution. Its function nowadays is to monitor our experiences and label especially significant events, such as the sight of your child's face, with emotional tags to say "this is important". During an intense religious experience, researchers believe that the limbic system becomes unusually active, tagging everything with special significance.

This could explain why people who have had such experiences find them so difficult to describe to others. "The contents of the experience--the visual components, the sensory components--are just the same as everyone experiences all the time," says Jeffrey Saver, a neurologist at the University of California, Los Angeles. "Instead, the temporolimbic system is stamping these moments as being intensely important to the individual, as being characterised by great joy and harmony. When the experience is reported to someone else, only the contents and the sense that it's different can be communicated. The visceral sensation can't."

Plenty of evidence supports the idea that the limbic system is important in religious experiences. Most famously, people who suffer epileptic seizures restricted to the limbic system, or the temporal lobes in general, sometimes report having profound experiences during their seizures. "This is similar to people undergoing religious conversion, who have a sense of seeing through their hollow selves or superficial reality to a deeper reality," says Saver. As a result, he says, epileptics have historically tended to be the people with the great mystical experiences.

The Russian novelist Fyodor Dostoevsky, for example, wrote of "touching God" during epileptic seizures. Other religious figures from the past who may have been epileptic include St Paul, Joan of Arc, St Theresa of Avila and Emanuel Swedenborg, the 18th-century founder of the New Jerusalem Church.

Similarly, neurosurgeons who stimulate the limbic system during open-brain surgery say their patients occasionally report experiencing religious sensations. And Alzheimer's disease, which is often marked by a loss of religious interest, tends to cripple the limbic system early on, says Saver.

The richness that limbic stimulation brings to experience may explain why religions rely so heavily on ritual, claims Newberg. The deliberate, stylised motions of ceremony differentiate them from everyday actions, he says, and help the brain flag them as significant. Music, too, can affect the limbic system, Japanese researchers reported in 1997, driving it towards either arousal or serene bliss. Chanting or ritual movements may do the same. Meditation has also been shown to induce both arousal and relaxation, often at the same time. "Sometimes people refer to it as an active bliss," says Newberg. That marriage of opposites, he thinks, adds to the intensity of the experience.

Even if these feelings of oneness and awe fall short of the personal experiences of God that many people report, anyone who still doubts the brain's ability to generate religious experiences need only visit neuroscientist Michael Persinger at Laurentian University in the bleak nickel-mining town of Sudbury, Ontario. He claims almost anyone can meet God, just by wearing his special helmet.

For several years, Persinger has been using a technique called transcranial magnetic stimulation to induce all sorts of surreal experiences in ordinary people (New Scientist, 19 November 1994, p 29). Through trial and error and a bit of educated guesswork, he's found that a weak magnetic field--1 microtesla, which is roughly that generated by a computer monitor--rotating anticlockwise in a complex pattern about the temporal lobes will cause four out of five people to feel a spectral presence in the room with them.

What people make of that presence depends on their own biases and beliefs. If a loved one has recently died, they may feel that person has returned to see them. Religious types often identify the presence as God. "This is all in the laboratory, so you can imagine what would happen if the person is alone in their bed at night or in a church, where the context is so important," he says. Persinger has donned the helmet himself and felt the presence, though he says the richness of the experience is diminished because he knows what's going on.

Not everyone accepts that Persinger's apparitions could equal what religious devotees experience. "That is quite detached from anything that's a genuine religious experience, in the same way that psychoactive drugs can affect mood, but not in a legitimate way," says Julian Shindler, a spokesman for the Chief Rabbi's office in London. "It's not the genuine article, somehow."

Whatever their validity, Persinger's experiments show that mystical experiences consist of not only what we perceive, but also how we interpret it. "We fit it into a niche, a pigeonhole," says Persinger. "The label that is then used to categorise the experience will influence how the person remembers it. And that will happen within a few seconds." There's a third aspect, too: the reinforcement that humans, as social animals, get from sharing religious rituals with others.

"Religion is all three of those, and all three are hardwired into the brain," says Persinger. "We are hardwired to have experiences from time to time that give us a sense of a presence, and as primates we're hardwired to categorise our experiences. And we crave social interaction and spatial proximity with others that are the same. What's not hardwired is the content. If you have a God experience and the belief is that you have to kill someone who doesn't believe as you do, you can see why the content from the culture is the really dangerous part."

So where does all this leave us? For whatever reason--natural or supernatural--our big, powerful brains clearly allow a novel sort of experience that we call religion. But it's difficult to say much more than that. "In a sense, biology evolving has discovered something new about the Universe," says Charles Harper, executive director of the Templeton Foundation, a private institution that explores the interaction between religion and science. "Almost all cultures have this religious sense," he says. "Does that offer any insight for understanding the grain of the Universe? That's a haunting question."

Sceptics of religion are quick to claim that the brain's hardwiring proves that God has no real existence, that it's all in the brain. "The real common denominator here is brain activity, not anything else," says Ron Barrier, a spokesman for American Atheists based in Cranford, New Jersey. "There is nothing to indicate that this is externally imposed or that you are somehow tapping into a divine entity."

But Newberg isn't so sure. "We can't say they're wrong," he says. "On the other hand, if you're a religious person, it makes sense that the brain can do this, because if there is a God, it makes sense to design the brain so that we can have some sort of interaction. And we can't say that's wrong, either. The problem is that all of our experiences are equal, in that they are all in the brain. Our experience of reality, our experience of science, our mystical experiences are all in the brain."

In fact, he goes on, practically the only way we can judge the reality of an experience is by how real it feels: "You can have a dream and it feels real at the time, but you wake up and it no longer feels as real. The problem is, when people have a mystical experience, they think that is more real than baseline reality--even when they come back to baseline reality. That turns everything around." To Newberg, it means that reductionist science, powerful as it is, has its limitations.

Religious experts agree. "You could say Shakespeare's sonnets are nothing but a combination of pencil lead and cellulose," says Harper. "But you could also say this is the outflow of a great soul, and that would also be true." He says there are different levels of explanation which are each true at their own level, but which don't offer a comprehensive explanation.

Just as physicists cannot fully understand the electron as either a particle or a wave, but only as both at once, says Newberg, so we need both science and a more subjective, spiritual understanding in order to grasp the full nature of reality.