The Carmelite nuns live a life of silent prayer, separated from the modern world by the high stone wall that surrounds their monastery in an industrial part of Montreal. Except for medical care, they rarely leave their sanctuary. But that changed late last month, when they began to make periodic visits to, of all places, a science lab.

The sisters arrive at the neuro-science laboratory in the University of Montreal's psychology department two at a time, wearing habits sewn from thick, dark cloth, high white collars and veils that frame their faces and flow down their backs. On their feet are sensible brown laceups that appear to have never seen the outdoors before.

They come to take part in an experiment that will probe a mystical and very private part of their lives. Sister Diane, the monastery's prioress, and Sister Teresa admit to being nervous as they peer curiously into a dark chamber about the size of a walk-in closet and equipped with an old barber's chair.

It is here that they have agreed to try to relive unio mystica, a religious experience so intense that Christians profess to sense their Lord as a physical presence. The nuns hope to help Montreal neuroscientist Mario Beauregard uncover just what happens in their brains when they feel the hand of God.

Their openness to scientific examination is a sign of a relatively recent rapprochement between science and religion, especially in the new field of neurotheology, which uses the tools of psychology and neuroscience to probe the neural underpinnings of religious experience.

This is only a dry run, but the formidable Sister Diane suddenly looks vulnerable as she takes off her veil and loosens her thick grey hair. Research assistant Vincent Paquette gently helps her put on what looks like a red bathing cap full of holes. Inside the cap, electrodes below each hole will be attached to her scalp to measure the electrical activity of her brain.

"This isn't," she says, "what we are used to."

Indeed, life inside the monastery has changed little since the Carmelites founded it in 1875. Sister Diane and her nuns rise at 5:20 for breakfast and an hour of silent prayer that they liken to meditation. The days are filled with chanting the Psalms, attending mass and more silent prayer.

When they aren't praying, they are working; cooking, gardening, baking hosts for communion, washing and sewing habits, making crafts to earn money. They are permitted to talk to each other only during two 20-minute recreation periods, after lunch and after supper. In the evening, they must write notes if they have something pressing to say.

"We are hermits, living in a community," Sister Diane explains. They even pray in separate wooden compartments.

There are 19 nuns now in the monastery and all plan to stay until they die.

Now, they have agreed not only to venture out of the cloister, but also to relive perhaps the most intimate moment of their lives while researchers watch what happens to their brains.

Sister Diane says unio mystica, the mystical union with God, is difficult to put into words. St. Teresa of Jesus, the Spanish nun who established the Carmelite order in 1526, described it as talking lovingly to God as though He were a friend and sharing a divine intimacy. The experience happens only once or twice in a lifetime, typically before a person turns 30. Sister Diane had it happen twice in the same year -- 1977, when she was 29 -- and not again since. She has never talked about it before, she says; it was too private, too intimate. She was at a religious retreat, praying silently and recalls entering an altered state, with an intense sense of God's physical presence. She lost herself in it.

"I don't know what happened. I don't know how much time had passed. It is like a treasure, and intimacy. It is very, very personal. It was in the centre of my being, but even deeper. It was a feeling of fullness, fullness, fullness."

Sister Teresa, 43, also experienced her unio mystica when in her 20s. "It is more than a feeling," she says. "It is more intense than feeling, but you sense God is physically there. It brings intense happiness, even bliss."

When Dr. Beauregard and Mr. Paquette, his doctoral student, first approached Sister Diane about using three of the most powerful brain-imaging tools available to learn more about unio mystica, she was intrigued. She had heard about other experiments investigating the biological basis of religious experience.

The researchers were hoping the nuns would have a mystical experience right in the lab. Sister Diane told them that this would be impossible -- God can't be summoned at will. "You can't search for it. The harder you search, the longer you will wait," she says.

So the scientists came back with an alternative: Would the nuns be able to remember what it felt like? Dr. Beauregard is certain that when they recall such an intense experience, their brains will operate the same way as when the nuns actually felt God's physical presence.

He says there is plenty of evidence that this is likely. When we think about doing something physical, such as hitting a forehand in tennis, the same parts of the brain are active as when we are actually make the shot.

Similarly, he has conducted experiments with actors and found that dramatizing a sad experience causes intense activity in the parts of the brain that process emotion.

This approach pleased the nuns, and so far six have agreed to participate in the experiments, which will take two years to complete.

The first step is to measure their brain waves, or electrical activity, using an electroencephalographic (EEG) recording device as they re- live unio mystica as best they can.

The second, using functional magnetic imaging, will provide a living picture of their brains at work by showing which regions of their brain are active and which aren't.

In the third experiment, the nuns will be injected with a low-level radioactive chemical so that the scientists can use positron emission tomography, better known as a PET scan, to measure levels of the neurotransmitter serotonin in different parts of the brain. Serotonin is involved in regulating a person's moods, and there is evidence that psychedelic drugs such as LSD mimic it to produce hallucinations.

Some cultures use hallucinogens to communicate with God, and Dr. Beauregard believes that serotonin may play a role in unio mystica. Not that he is trying to prove that unio mystica is all in the head. Every human experience occurs in the mind, he says. The "experience is real, but the manifestation is in the brain."

When the analysis of all three experiments is done, he hopes to have a clear biological picture of an experience that mystifies even those who have lived it. Ultimately, he would like to know enough about how it works to be able to offer the same experience to anybody seeking spiritual growth.

Sister Diane says she is certain that Dr. Beauregard will discover a biological basis for the Carmelites' spiritual experience, one she says is shared by all human beings. God equipped people with the brains they need for a spiritual life, she insists. "Our body has a spiritual component. To be a human being is to be a spiritual being. I'm convinced this will show in the results."

Sister Teresa seems less sure. "It will be up to God," she says.

Dr. Beauregard is not the only researcher probing the neurobiology of belief. In September, the Dalai Lama, the spiritual leader of Tibetan Buddhism, took part in a high-profile meeting at the Massachusetts Institute of Technology held to compare Buddhist and scientific views about how the mind works.

Buddhists believe that they can regulate their emotions through meditation, and studies conducted on Buddhist monks have shown intense activity in specific parts of their brains when they meditate. Which part of the brain appears to depend on the type of meditation -- whether the person is focusing on compassion or on the details of a mental image of Buddha.

The sold-out MIT session attracted many respected scientists, including a researcher from the Royal Ottawa Hospital who is interested in whether meditation may be useful in treating anxiety disorders.

The study of meditation is no longer considered the flaky fringe of science, thanks to researchers such as Richard Davidson, a neuroscientist at the University of Wisconsin who helped to organize the session with the Dalai Lama.

In 1992, he travelled to northern India equipped with electrical generators, computers and machines that could measure the electrical output of the brain. In the foothills of the Himalayas, he wired up monks to learn more about their brains.

New, more powerful brain-imaging equipment has drawn other researchers to the field, including scientists at Harvard, the University of California at San Francisco, the University of Pennsylvania and Dr. Beauregard at the University of Montreal.

They are trying to answer a number of intriguing questions: Are humans hard-wired to have religious or spiritual experiences, which are common to almost every culture on Earth? What happens in the brain when they do have them? Is it something that non-religious people might be able to replicate with the right stimulation? Is a transcendent Buddhist experience, often described as feeling connected to everyone and everything in the universe, the same as Christians' unio mystica? Can religion and spirituality make people healthier, as some studies suggest?

Work with the Buddhist monks shows that meditation results in decreased activity in the parietal lobes, which are located at the top and back of the brain, and help to orient a person in time and space. (For example, they tell you that your hands are on the steering wheel and you're driving to the store.)

The theory is that a lack of parietal activity reduces the sense of self, and makes a person feel there is no boundary between his or her body and the rest of the universe. As well, there appears to be increased activity in the limbic system, which helps to process emotion.

Dr. Beauregard says Christian mysticism may involve a different biological mechanism. His is the first study to use three techniques for monitoring the brain activity of religious subjects. The two-year, $100,000 (U.S.) project is financed by a foundation created by John Templeton, the mutual-fund titan who is now in his 90s and wants to know more about God.

Mr. Templeton is investing $16-million to $30-million (U.S.) a year in the scientific study of spirituality, everything from whether prayer can heal to how primates exhibit forgiveness.

Dr. Beauregard's goal is to understand the neurobiology of Christian mysticism, and he has won over the Catholic establishment. Cardinal Jean-Claude Turcotte, the Archbishop of Montreal, has written in support of the project in a publication read by Quebec's other contemplative orders. The researchers hope to attract as many as 15 volunteers from four other Carmelite communities in the province.

It is not clear, however, that God is on-side. Sister Diane and Sister Teresa arrived at the neuroscience lab for their EEG tests only to find that someone had broken in and stolen key pieces of equipment. Although frustrated, the researchers walked the nuns through the process so they would know what to expect. The more relaxed they are, Mr. Paquette says, the easier it is to monitor their brains. A week later, the researchers were ready to go, but Sister Diane called in sick at the last minute, prompting another delay.

But the two nuns already tested were moved by the experience. One in particular, Sister Nicole, seemed to come especially close to recapturing unio mystica while perched in the barber's chair (used because it is comfortable -- and solid, even more vital to the research results).

When Mr. Paquette opened the door to the soundproof chamber, she was surprised that 20 minutes had passed sp quickly. Asked what it was like, she began to describe the unio mystica she achieved as a child; the two experiences had become blurred in her mind. She also told him that she had heard music, Pachelbel's Canon.

In the tape Mr. Paquette made of their conversation, her voice sounds dreamy and content. "I have never felt so loved," she says.

It is far too early to draw many conclusions from the experiments, but the researchers say they already find the data intriguing. "We are seeing things we don't normally see," says Marc Pouliot, an engineer who is analyzing the EEG results.

The two nuns experienced intense bursts of alpha waves in the brains, common in a reflective and relaxed state such as meditation. They also had intense activity in the left occipital region at the back of the brain -- which is not what the scientists were expecting in the wake of research by Michael Persinger, a controversial researcher at Laurentian University in Sudbury who has developed the so-called God helmet. He uses the device to stimulate the right side of the brain, including the parietal lobe, with low-level electromagnetic radiation. In 80 per cent of subjects, this induces the sensation that there is a presence in the room. Many weep and say they feel God nearby.

However, the real "God experience" may be different, according to the nuns. Rather than crying, they say they felt intense joy and looked forward to the lab experience since there is little chance they will ever enjoy a true mystical union with God again.

This may seem sad, but Sister Diane compares her love for God to the way two people love each other. When they fall in love, they feel a physical rush. They blush. They feel tingly. That, she says, is the kind of love young nuns feel for God when they experience unio mystica. But over time, the love deepens and matures. It isn't as thrilling, she says. It becomes more of a day-to-day relationship.

This is an intriguing observation, because some researchers have speculated that the human capacity for mystical experiences may have co-evolved with the brain networks involved in sexual pleasure.

At 55, Sister Diane describes her relationship with God as more like a marriage, solid, secure, but without the rush. She says she knows God has been present by the peace he leaves behind, not from the excitement of a mystical union.

"That feeling of peace flowing through you -- pacification -- tells you He has been here."

Anne McIlroy is the Globe and Mail's science reporter.